OSI MODEL Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions. An open system is a set of protocols that allow two different systems to communicate regardless of their underlying architecture. Open Systems Interconnection model is fundamental to all communications between network devices It is now the theoretical model for how communication takes place between network devices. The OSI reference model is an arbitrary hierarchical stratification (layering) of computer networking functions. The stratification consists of seven layers. There are various ways of implementing the protocols at any given layer. OSI is a CONCEPTUAL model. OSI Model allows complete interoperability between incompatible systems. The communication is governed by an agreed upon series of rules and conventions called PROTOCOLS. One of the most widely used network architectures is the Open System Interconnection (OSI), developed by the International Standard Organization (ISO). The OSI is popular because architecture; modularity, simplicity, flexibility and achieve the primary goals of network architecture: modularity, simplicity, flexibility and openness. The various tasks associated with network communication are distributed among the server functional layers: each layer is responsible for a set of tasks and is guided by as set of protocols. The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network.

LAYERS IN THE OSI MODEL 1) Physical Layer The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur. The following figure shows the position of the physical layer with respect to the transmission medium and the data link layer.

The physical layer is also concerned with the following: Physical Characteristics of interfaces and medium. The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium such as copper wire or fiber optic cable. Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s or ls) with no interpretation. To be transmitted, bits must be encoded into signals - electrical or optical. The physical layer defines the type of encoding (how 0s and 1s are changed to signals). Transmission rate- Define the transmission rate of bits. This provides number of bits transmitted per second. Synchronization of bits. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized. Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices. Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies). Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication. In the half-duplex mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time. 2) Data Link Layer The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). The figure shows the relationship of the data link layer to the network and physical layers.

Responsibilities of the data link layer include the following: Framing – The physical layer delivers raw bits from source to destination. During transmission, the value of the bits can change. It is also possible that the number of bits received by the receiver may be different from the number of bits sent by the sender. To resolve this problem, the data link layer divides the stream of bits received from the network layer into manageable data units called frames. Physical addressing/Encapsulation. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one. Header is assigned when the frames are to be distributed to various systems within the network. These headers are called Protocol data unit (PDU) and the process of adding header is called encapsulation. Flow control. It may happen that the speed at which the sending and receiving nodes operate may differ. The sending node may transmit data at a faster but the receiving node may receive data at a slower rate. The rate of transmission between two nodes should be controlled to keep both the node is synchronized. This process is called flow control. Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame. Access control. When the devices in the network use the same link for communication at a time only one node can transmit data. The data link layer provides access to a particular device to use link at a given time. The figure illustrates hop-to-hop (node-to-node) delivery by the data link layer.

Communication at the data link layer occurs between two adjacent nodes. To send data from A to F, three partial deliveries are made. First, the data link layer at A sends a frame to the data link layer at B (a router). Second, the data link layer at B sends a new frame to the data link layer at E. Finally, the data link layer at E sends a new frame to the data link layer at F.

Network Layer The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination. If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-todestination delivery. The figure shows the relationship of the network layer to the data link and transport layers.

Other responsibilities of the network layer include the following: Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the data that includes the logical address (IP address) of the source to destination. It is 32 bit address that uniquely identifies the device connected. Handling congestion issueAny given network has a certain capacity to deliver or handle number of packet. When the packet exceeds the handling capacity then the network is unable to cope with them and results in excess accumulation of packet over the network. This is called congestion. This may occur due to slow link or slow processor. It is also the responsibility of network layer to control such congestion problems. Routing. – Routing is the process when a proper path is defined for the packet to reach the destination. Routing can be of two types static or dynamic routing. In static routing, the route to be followed by a packet is already set up by the network administrator. In dynamic routing, the route to be followed by a packet is decided by the time of transmission of the packet When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism. The figure illustrates end-to-end delivery by the network layer.

The network layer at A sends the packet to the network layer at B. When the packet arrives at router B, the router makes a decision based on the final destination (F) of the packet. As we will see in later chapters, router B uses its routing table to find that the next hop is router E. The network layer at B, therefore, sends the packet to the network layer at E. The network layer at E, in turn, sends the packet to the network layer at F. Transport Layer The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-todestination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-todestination level. The figure shows the relationship of the transport layer to the network and session layers.

Other responsibilities of the transport layer include the following: Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission. Connection control. The transport layer can be either connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated. Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link. Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission. The figure illustrates process-to-process delivery by the transport layer.

Session Layer The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems. Specific responsibilities of the session layer include the following: Dialog control. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half- duplex (one way at a time) or full-duplex (two ways at a time) mode. Synchronization. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent. The figure illustrates the relationship of the session layer to the transport and presentation layers.

Presentation Layer The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. The figure shows the relationship between the presentation layer and the application and session layers.

Specific responsibilities of the presentation layer include the following: Translation. The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format. Encryption. To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form. Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video. Application Layer The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. The figure shows the relationship of the application layer to the user and the presentation layer.

Specific services provided by the application layer include the following: Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on. File transfer, access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally. Mail services. This application provides the basis for e-mail forwarding and storage. Directory services. This application provides distributed database sources and access for global information about various objects and services.

Data unit

Host layers

Data

Segments

Layer

Function

7. Application

Network process to application

6. Presentation

Data representation, encryption and decryption, convert machine dependent data to machine independent data

5. Session

Inter-host communication, applications

4. Transport

Reliable delivery of packets between points on a network.

Packet/Datagram 3. Network

managing

sessions

between

Addressing, routing and (not necessarily reliable) delivery of datagram between points on a network.

Media layers Bit/Frame

2. Data link

A reliable direct point-to-point data connection.

Bit

1. Physical

A (not necessarily reliable) direct point-to-point data connection.

LAYER

Application 7

WHAT THE LAYER IS RESPONSIBLE FOR/SERVICES CORRESPONDING CORRESPONDING ETC. DEVICES PROTOCOLS 1. Interface between the user & the computer (applications & Gateways). Provides services · SNMP that directly support user applications, such as · FTP the USER INTERFACE, E-MAIL, FILE TRANSFER, · TELNET TERMINAL EMULATION, DATABASE ACCESS, Gateways · WWW etc. (can work at all · HTTP 2. API incorporated in this layer layers) · SMB 3. Allows applications to use the network. · NCP 4. Handles Network access, flow control & error · TCP recovery. · TFTP 5. Messages are sent between layers. · NFS

· SMTP

Presentation 6

1. Translation of data into understandable format for transmission (into a form usable by the application layer i.e. translates data between the formats the network requires and the computer expects). 2. Handles character encoding, bit order and byte order issues. Encodes and decodes data. 3. Data compression and encryption takes place Gateways at this layer. 4. Generally determines the structure of data 5. The redirector works at this layer. 6. Responsible for protocol conversion 7. Messages are sent between layers 8. Communicates through GATEWAYS and APPLICATION INTERFACES 9. SERVICES: Telnet, FTP use TCP, TFTP, NFS, SNMP, SMTP use TCP

· JPEG · MIDI · MPEG · All kinds of music, pictures & movie formats · NCP

Session 5

1. Responsible for opening, using and closing session. That is. It allows applications on connecting systems to establish a session (Establishes and maintains a connection). 2. Provides synchronization between communicating computers (nodes), messages are sent between layers (i.e. Manages upper layer errors). Gateways 3. Also places checkpoints in the data flow, so that if transmission fails, only the data after the last checkpoint needs to be retransmitted. 4. Handles remote procedure calls. 5. Communicates through Gateways & application interfaces. 6. SERVICES: Telnet, FTP use TCP, TFTP, NFS, SNMP, SMTP use TCP

· Network File System (NFS) · SQL · RPC

Transport 4

1. Responsible for PACKET HANDLING. Ensures error free delivery. Repackages messages, divides messages into smaller packets (Fragments and reassembles data), and handles error handling Gateways 2. Ensures proper sequencing and without loss and duplication. 3. Takes action to correct faulty transmissions 4. Controls flow of data 5. Acknowledges successful receipt of data

· TCP · UDP · SPX · NetBEUI

6. Sliding window is at this Layer -segments of message fragments are sent between layers 7. TCP/SPX - connection oriented communication for applications to ensure error free delivery. 8. UDP - connectionless communications and does not guarantee packet delivery between transfer points 9. Communicates through Gateway Services, routers & brouters.

Network 3

Data 2

1. Logical addressing - software addresses to hardware addresses are resolved (ARP/RARP). 2. Routing of message (Packets) between hosts & networks (IP/IPX). 3. Determining the best route (Makes routing decisions & forwards packets (a.k.a. DATAGRAMS) for devices that could be farther away than a single link. 4. Moves information to the correct address. 5. Sends messages and reports errors regarding packet delivery (ICMP) 6. Reports host group membership to local multicast routers (IGMP) 7. Communicates through GATEWAY SERVICES, ROUTERS & BROUTERS

1. Provides for flow of data over a single link from one device to another 2. Controls access to communication channel 3. Controls flow of data 4. Packets placed into frames at this layer (i.e. Organizes data into logical frames - logical units of information). 5. Identifies the specific computer on the network 6. CRC is added at this Layer (Error detection). Link 7. If CRC fails at the receiving computer, this layer will request re-transmission. 8. MAC addresses are resolved at this Layer (switches, brouters and bridges function on this layer using the MAC sub layer) 9. Sends data from network layer to physical layer. 10.Manages physical layer communications between connecting systems. 11.Data frames are sent between layers. 12.Ethernet, Token Ring & other communications occur here via frames. LLC -(802.2) manages

· Routers · Brouters

· IP · IPX · RIP · ICMP · ARP · RARP · OSPF · EGP · IGMP · NetBEUI · DLC · DecNET

· Brouters · Bridges · Switches

· HDLC (High-level Data Link Control) Supports asynchronous & synchronous transmissions. Uses LLC flow control. · SLIP · PPP

link control & defines SAP'S (Service Access Points). MAC- (802.3, 802.4, 802.5, 802.12) communicates with adapter card. 13.Communicates through: SWITCHES, BRIDGES & INTELLIGENT HUBS NOTE: The Data Link Layer contains two SUBLAYERS . LLC (Logical Link Control) - The upper sublayer, which establishes and maintains links between communicating devices. Also responsible for frame error correction and hardware addresses . MAC (Media Access Control) - The lower sub-layer, which controls how devices share a media channel. Either through CONTENTION or TOKEN PASSING

Physical 1

1. Data (BITS) is sent across physical media like wires and hubs. 2. Responsible for encoding scheme (like Manchester encoding) 3. Defines cables, cards and physical aspects. 4. Provides electrical and mechanical interfaces for a network. 5. Specifies how signals are transmitted on network 6. Communicates through: REPEATERS, HUBS, SWITCHES, CABLES, CONNECTORS, TRANSMITTERS, RECEIVERS, MULTIPLEXERS

· Hubs · Repeaters · Amplifiers · Transceivers · Multiplexers · Receivers · Transmitters · Connectors · Cables · Switches

None

At which level of OSI model does repeaters, bridges, routers and gateways operate? Repeater – Physical layer Bridges – Data link layer & Physical layer Routers – Network, data link & physical layer Gateways – Transport and upper layers Determine which OSI layers are responsible for the following? i) Providing end-end communication with reliable service. - Transport layer ii) Providing node-node communication with reliable service. - Data link layer What are the two types of error? Explain. Single bit error: Only one bit of the given data unit has been changed Burst error: Two or more bits in the given data unit have been changed. What is Piggybacking? Piggybacking is the method of combining the data frame and the acknowledgement of the previous frame in a single unit. Write the qualities of CRC. 1. It must have exactly one bit less than the divisor 2. Data string appended with the CRC must be exactly divisible by the divisor. Define flow control and error control. Flow control is the restriction on sender’s data rate so that the receiver buffer does not overflow. Error control is the protocol that deals with error detection and error correction. TRANSMISSION MEDIUM It can be defined as physical path between transmitter and receiver in a data communication system. Networking involves connection of computers, workstation and peripheral devices. Networking hardware involves the peripheral devices, interface cards and other equipment such as cables and wires that are used to establish network connection. We can connect these devices either by using physical transmission or wireless transmission medium.

Physical Transmission medium involves the use of wires and cables. Wireless Transmission involves the use of radio waves, infrared and microwave techniques. Characteristics of cable – Before selecting a cable for a network we may have to consider various cable characteristics. 1) Segment Length – It defines the length of a single wire. Every cable has a limitation to carry signals only to a particular distance after which the quality of the signal degrades and become corrupt. The capacity of the cable to carry signals to a certain distance till they can be regenerated using amplifiers or repeaters. 2) Attenuation – As the signal travels through copper wires they lose their strength. The loss of signal strength in a cable is called attenuation. Attenuation is measured in Decibel (DB) and expressed as negative values as it indicate a loss of signal. As the operating frequency and cable length increase, attenuation also increases. 3) Bandwidth – It is the amount of digital data that a cable can carry. Each type of cable can carry only a certain amount of information or data over a given period of time. This capacity is measured in terms of bandwidth. It is measured in terms of bits/bytes. 4) No. of segments – Define the maximum number of segments in a network. Every network cable is subject to a maximum number of segments it can have. The amount of time taken by a signal to traverse through a particular length of the cable is known as latency. Every cable is subject to latency. If the time taken to traverse is long, it is assumed that the signals will not reach other end. Therefore, cables are subject to maximum number of segments so that there is no delay in receiving the signals from one end to other end. 5) CostCables come in different quality. Depending on the length, the size of network we may need to choose the right type of cable .Copper cables are the cheapest form of cable whereas fiber optic cable are the most expensive. Installation cost of copper wire is cheap as compared to that of fiber optic cable. 6) Interference Susceptible Each type of cable is susceptible to some interference such as internal or external noise. External noise may consist of EMI (Electromagnetic interference) and RFI (radio frequency interference). Cables that consist of shielding are less susceptible to interference. Coaxial cable and Twisted pair cables are less susceptible while fiber optic cables are least susceptible for interference. 7) Crosstalk This is the concept of interference generated by when magnetic field of one cable affects electric current of another cable. Cable crosstalk can result into signal distortion which leads to network problems. It can be reduced by twisting and shielding the wire pair. In TP cable, the magnetic fields cancel each other to decrease the cross talk.

Types of Transmission/Communication media – There are two types of transmission media – 1) Guided form of transmission – Transmission capacity depends on the medium, length and whether the medium is point to point or multipoint. 2) Unguided form of transmission -

Coaxial Cable – It consists of solid copper wire surrounded by an insulator made up of Polyvinyl chloride (PVC) or Teflon. PVC is a plastic made from chemical vinyl chloride. It is surrounded by a mesh like ground wire called shielding or braiding and outer cover is known as a sheath. The insulator acts as a barrier between the shielding and the core. If the insulator is absent, the shield and the core may come in contact and cause a short circuit. The braiding protects the cable from external interference such as EMI and RFI. The various parts of co-axial cable1) Outer insulator or jacket – made up of plenum(Teflon) or non plenum (polyethylene or poly vinyl chloride) 2) Copper mesh – Some co-axial cables have a braid shield, some have a foil shield and other have both braid and foil shields. These shields supply ground for the wire and protect from EMI and RFI. 3) Insulator – it is non conductor around the carrier wire usually made up of polyethylene or Teflon 4) Copper wire – It is a wire conductor at the center of the cable and is made up of copper and may be either solid or stranded. Type Co-axial cable mainly divided into four categories – RG58, RG8, RG6 and RG59. Use – One of the most popular use in cable TV for the distribution of TV signal. Another important use in LAN. Connector The most common type of connector used with coaxial cable is the Bayone Neill Concelman(BNC) connector

Advantage – 1) Secured data transmission 2) Protect from crosstalk 3) Protect from EMI and RFI Disadvantage – 1) Most expensive 2) Difficult to install

Twisted Pair Cable – A pair of cables twisted around each other forms a twisted pair cable. A number of twisted wires are placed inside a protective sheath to form a cable. Whenever a signal passes through wire, an electromagnetic field is generated around it called as loop. When other wires come into the loop, the signal passes onto them. Thus signals of both the wire get jumbled with each other. This phenomenon is called as crosstalk. To avoid this, two wires are wounded across each other where one wire acts as a ground and absorb the electromagnetic field of other wire. This reduces the problem of crosstalk which is an internal noise.

There are two types of twisted pair cables1) UTP (Unshielded Twisted Pair) A pair of unshielded wires wound around each other is known as UTP. An UTP is a pair of copper wires wound by plastic insulator. UTP is the cheapest form of cable available for networking purpose. It is highly used in LAN. The installation cost is very cheap to install. ANSI

have defined standard for UTP cabling and the cable is divided into following categories like Cat 1,Cat 2,Cat 3, 4, 5, 5e, 6 and cat7. Connector used in UTP and STP – RJ45 (Registered Jack-8 pin connector) Advantage – Easy to handle and install. Disadvantage – 1. Highly prone to external interference like EMI and RFI 2. Highly prone to crosstalk 3. Unable to provide secured form of transmission.

2) STP(Shielded Twisted Pair) A pair of cable around each other and each pair is placed inside a protective foil wrap to protect it from crosstalk. These wires are placed in an outer copper braid to protect it from external interference as the shielding absorbs all the EMI and RFI signals. STP is cheaper than fiber optic cable but more expensive than UTP. Use of STP cables reduces the electrical noise within and outside the cable. STP cables are used for business installation. There are two types of STP cables – 1) Cat 5e 2) 150 Ohm shielded cable Advantage – 1. Better protection from crosstalk. 2. Easy to handle and install. 3. Better protection from external interference 4. Secured form of transmission. Disadvantage – 1. Costly than UTP. 2. Difficult to handle and install.

FIBER OPTIC CABLE A fiber optic cable consists of center glass of core through which signal passes. It is surrounded by a glass cladding. Sometime cladding is surrounded by PVC to increase the cable strength and provide support for the cladding. This cladding is sometime surrounded by metal fibers to give strength to the cable. It is finally surrounded by an outer cover known as sheath. In this, the electrical signal are converted into light signal and transmitted through it. An emitter sends the signals from one end of cable and a light sensor (photo diode) senses this signal and then converts into a digital equivalent. There is transceiver located at both the ends of cable where the signal are converted from electrical to light signal and vice versa. The fiber optic cables are available in different sizes with varying core and cladding mechanism. But the most common used fiber optic cable is 62.5/125 micrometer. The diameter of core is 62.5 and cladding of 125 micrometer. Fiber optic technology makes use of light to carry digital signals via optical cables. By attaching a light source to one end of an optical fiber and a detector to the other, we get a unidirectional data transmission system. An optical transmission system has three components: the light source - a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit the transmission medium - ultra-thin fiber of glass the detector - generates an electrical pulse when light falls on it.

Features of fiber-optic cables: • Speed and throughput (Bandwidth)—More than 1 GBPS • Average cost per node—Expensive • Media and connector size—Small Connector – ST, SC, SMA and MIC • Maximum cable length—More than 10 km for single mode; up to 2 km for multimode Interference susceptibility – best among all types of cable Types of fiber optic It is classified into two types depending on the way the light is transmitted:  multi-mode fiber (MMF)  single-mode fiber (SMF) USE and NEED: Fiber optic cables are mainly used in environment that are highly susceptible to noise and other interference. These cables do not emit any external noise. Because of greater bandwidth (2Gbps), smaller diameter, lighter weight, low attenuation, immunity to electromagnetic interference and longer repeater spacing, optical fiber cables are finding widespread use in long-distance telecommunications. Advantages: 1. Completely immune to crosstalk, EMI and RFI 2. Much smaller and lighter in weight as compare to copper wire 3. Unaffected by chemical or atmospheric conditions 4. Able to carry much more information than copper cable 5. Support much higher data rates 6. Covers a long distance 7. Much more difficult to tap, making it is good choice for environment with security concerns. Disadvantages:    

Difficult to terminate, requires fiber hubs etc. Difficult to install, connect and repair Costly and dedicated as compare to copper wires More fragile than copper wire and difficult to split.

Advantages and Disadvantages of Optical Fiber Advantages Fiber-optic cable has several advantages over metallic cable (twisted pair or coaxial). Higher bandwidth: Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available. Less signal attenuation: Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable.

Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables. Resistance to corrosive materials: Glass is more resistant to corrosive materials than copper. Light weight: Fiber-optic cables are much lighter than copper cables. Greater immunity to tapping: Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antenna effects that can easily be tapped. Disadvantages There are some disadvantages in the use of optical fiber. Installation and maintenance: Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere. Unidirectional light propagation: Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed Cost: The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified.

CONNECTORS USED FOR FIBER OPTIC CABLES Following types of connectors are widely used for fiber communication are: SC (Straight Tip) ST (Subscribe/standard connector) SMA(Sub miniature Type) MIC(Medium Interference connector)

Comparison of wired media Data Transfer Rate Data Transfer Range Interference Susceptibility Cost of Cable

Twisted 10Mbps-10Gbps 100 m More Least Cost

Coaxial 100 Mbps 185m-500 m Less than Ethernet cable More than Ethernet

Fiber Optic More than100 Gbps -NIL Very Expensive

Comparing media types(Guided media) Comparing the different guided media as following: Media Type

Max. Segment Length

Speed

Cost

UTP

100 m

10 mbps to Least 100 mbps expensive

Easy to install; Susceptible to widely available interference; can cover and widely used only a limited distance

STP

100 m

10 mbps to More 100 mbps expensive than UTP

Reduced Difficult to work with; crosstalk; more can cover only a limited resistance to EMI distance than Thinnet or UTP

Coaxial

500 m

10 mbps to Relatively 100 mbps inexpensive, but more costly than UTP

Les susceptible to EMI interference than other types of copper media

Fiber Optic

10 km and 100 mbps to Expensive farther 100 gbps (single mode) 2 km and farther 100 mbps to 9.9 gbps (multi

Advantages

Disadvantages

Difficult to work with(Thicknet); limited bandwidth; damage to cable can bring down entire N/W

Can be used for Difficult to terminate longer distance; has a higher data rate than coaxial and twisted cable

mode )

UNGUIDED FORM OF TRANSMISSION/MEDIA (WIRELESS TRANSMISSION) Unguided transmission is used when running a physical cable (either fiber or copper) between two end points is not possible. Unguided media relates to data transmission through the air and is commonly referred to as wireless. The transmission and reception of data is carried out using antenna. There are two main ways that antenna work: Directional (in a beam) :

Focused bean Careful alignment required Omni directional (all around): Signals spread out in all directions Can be received by many antennas Transmission using unguided media is done though:    

RADIO WAVES TRANSMISSION MICROWAVES TRANSMISSION INFRARED TRANSMISSION SATELLITE TRANSMISSION

RADIO WAVES TRANSMISSION Wireless communication uses Radio Frequencies (RF) or Infrared (IR) waves to transmit data between devices on a LAN. Wireless networking involves the use of radio waves to send and receive signals. Wireless technology can be used in short as well as long geographical distance.  

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 

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To receive the signals from the access point, a PC or laptop must install a wireless adapter card (wireless NIC). Wireless signals are electromagnetic waves that can travel through the vacuum of outer space and through a medium such as air. Therefore, no physical medium is necessary for wireless signals, making them a very versatile way to build a network.(It can easily penetrate through building but susceptible to interference problems from other electrical equipment) A popular technology called Bluetooth uses radio waves to establish communication. Radio frequency do not maintain line of sight between the sender and receiver. It can communicate in Omni direction. Wireless signals use portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from 3 kilohertz (kHz) to 300 gigahertz (GHz). The frequency is divided into VLF(very low frequency), LF(low frequency), MF(middle Frequency), HF(high frequency), VHF(very High frequency), UHF(ultra High frequency), SHF(Super High Frequency) and EHF(Extremely High frequency) The data-transmission rates range from 9 kilobits per second (kbps) to as high as 54 Mbps. Some common applications of wireless data communication include the following: • Accessing the Internet using a cellular phone • Establishing a home or business Internet connection over satellite • Using a wireless keyboard and mouse for the PC

Characteristics of Radio Wave Transmission: These waves are Omni-directional, so the transmitting and receiving antennas need not be aligned. (Recall when you throw a stone in a pond, circular waves are generated and spread outwards. Similarly, radio waves are generated by the transmitter and spread in all the directions.) Relatively inexpensive than wired media. It offers ease of communication over difficult terrain The transmission can be interfered by motors or other electrical equipment Permission from concerned authorities is required for use of radio wave transmission Less secure mode of transmission

 MICROWAVE TRANSMISSION Micro waves have a frequency range of 300MHz (0.3 GHz) to 300 GHz. This range has some overlapping portion (0.3GHz - 3GHz) with radio waves as there is no clear cut demarcation between radio waves and micro waves. Microwaves travel in straight lines and cannot penetrate any solid object. Therefore for long distance microwave communication, high towers are built and microwave antennas are put on their tops. Distance between two microwave towers depends on many factors including frequency of the waves being used and heights of the towers. These waves travel in straight lines and therefore the sending and receiving antennas have to be aligned with each other. An example of usage of microwaves for communication is as follows: In the big cities where land is very costly and a lot of formalities have to be completed to get permission to dig land for cabling, microwave antennas can be put on top of high rise buildings and communication can be started in a short time. Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country. Characteristics of Micro Wave Transmission: Free from land acquisition rights Relatively inexpensive than wired media Offers ease of communication over difficult terrain The transmission is in straight lines so the transmitting and receiving antennas need to be properly aligned (line of sight transmission)

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth.

Advantages: a. b. c. d.

They require no right of way acquisition between towers. They can carry high quantities of information due to their high operating frequencies. Low cost land purchase: each tower occupies small area. High frequency/short wavelength signals require small antenna.

Disadvantages: a. b. c. d.

Attenuation by solid objects: birds, rain, snow and fog. Reflected from flat surfaces like water and metal. Diffracted (split) around solid objects Refracted by atmosphere, thus causing beam to be projected away from receiver.

INFRARED TRANSMISSION Infrared wireless transmission network uses infrared light to send and receive signals. It is invisible to human beings. These lights are of high frequency beams in order to avoid interference. Devices generally use high frequency beams of infrared light to communicate with each other. For communication to take place, line of sight must be maintained between the devices. It is mainly used in LAN environment. LED or ILE(Injection Laser Diode) is used to transmit information and photodiode are used to receive the signals. Infrared light waves can’t penetrate through obstacles and also become week as they travel away from the transmitter. Security is better than radio wave. No govt. license is required to operate an infrared system. The remote controls used in TV, VCR and stereos all are infrared communication.



SATELLITE TRANSMISSION

Satellite links are used for very long distance wireless communication which may range from intercity to intercontinental. Transmission from the earth to a satellite is known as uplink. Transmission from a satellite to the earth is known as downlink. There are multiple micro wave frequency bands which are used for satellites links. Frequency used for uplink varies from 1.6 GHz to 30.0 GHz and that for downlink varies from 1.5GHz to 20.0GHz. Downlink frequency is always lower than the uplink frequency. For example, the uplink frequency is 6.0GHz, and the corresponding downlink frequency is 4.0 GHz. A communications satellite is a relay station in orbit above the earth that receives, regenerates, and redirects signals carried on a specific frequency. The satellite system is very expensive but its area coverage and fringe benefits compensate for the expenses. Communication satellites are normally owned by governments or by government approved organizations of various countries. Satellite communication is based on ideas similar to those used for line-of-sight. A communication satellite is essentially a big microwave repeater or relay station in the sky. Microwave signals from a ground station is picked up by a transponder, amplifies the signal and rebroadcasts it in another frequency, which can be received by ground stations at long distances as shown in Fig(a).

A satellite can be used for point-to-point communication between two ground-based stations or it can be used to broadcast a signal received from one station to many ground-based stations as shown in Fig(b).

 Optimum frequency range for satellite communication is 1 to 10 GHz.  Another interesting property of satellite communication is its broadcast capability. All stations under the downward beam can receive the transmission. It may be necessary to send encrypted data to protect against piracy. Difficulties with wireless communication: 1. Weather interferes with signals. For instance, clouds, rain, lightning, etc. may adversely affect communication. 2. Radio transmissions easy to tap. A big concern for companies worried about competitors stealing plans. 3. Signals bouncing off of structures may lead to out-of-phase signals that the receiver must filter out. TRANSMISSION IMPAIRMENT Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal

at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.

Attenuation Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal.

Decibel To show that a signal has lost or gained strength, engineers use the unit of the decibel. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified.

Variables P1 and P2 are the powers of a signal at points 1 and 2, respectively. Distortion Distortion means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed (see the next section) through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if the delay is not exactly the same as the

period duration. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same.

Noise Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise is a spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on.

Signal-to-Noise Ratio (SNR) To find the theoretical bit rate limit, we need to know the ratio of the signal power to the noise power. The signal-to-noise ratio is defined as

SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise). A high SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. Because SNR is the ratio of two powers, it is often described in decibel units, SNRdB, defined as

DATA RATE LIMITS Data rate depends on three factors: 1. The bandwidth available 2. The level of the signals we use 3. The quality of the channel (the level of noise) Two theoretical formulas were developed to calculate the data rate: one by Nyquist for a noiseless channel, another by Shannon for a noisy channel. Noiseless Channel: Nyquist Bit Rate For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate BitRate = 2 x bandwidth x log2L In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and BitRate is the bit rate in bits per second. According to the formula, we might think that, given a specific bandwidth, we can have any bit rate we want by increasing the number of signal levels. Although the idea is theoretically correct, practically there is a limit. When we increase the number of signal levels, we impose a burden on the receiver. If the number of levels in a signal is just 2, the receiver can easily distinguish between a 0 and a 1. If the level of a signal is 64, the receiver must be very sophisticated to distinguish between 64 different levels. In other words, increasing the levels of a signal reduces the reliability of the system. Noisy Channel: Shannon Capacity In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a noisy channel: Capacity = bandwidth x log2(1+SNR) In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and capacity is the capacity of the channel in bits per second. Note that in the Shannon formula there is no indication of the signal level, which means that no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel. In other words, the formula defines a characteristic of the channel, not the method of transmission.

PERFORMANCE Bandwidth One characteristic that measures network performance is bandwidth. However, the term can be used in two different contexts with two different measuring values: bandwidth in hertz and bandwidth in bits per second. Bandwidth in Hertz Bandwidth in hertz is the range of frequencies contained in a composite signal or the range of frequencies a channel can pass. For example, we can say the bandwidth of a subscriber telephone line is 4 kHz. Bandwidth in Bits per Seconds The term bandwidth can also refer to the number of bits per second that a channel, a link, or even a network can transmit. For example, one can say the bandwidth of a Fast Ethernet network (or the links in this network) is a maximum of 100 Mbps. This means that this network can send 100 Mbps. Relationship There is an explicit relationship between the bandwidth in hertz and bandwidth in bits per seconds. Basically, an increase in bandwidth in hertz means an increase in bandwidth in bits per second. The relationship depends on whether we have baseband transmission or transmission with modulation. Throughput The throughput is a measure of how fast we can actually send data through a network. Although, at first glance, bandwidth in bits per second and throughput seem the same, they are different. A link may have a bandwidth of B bps, but we can only send T bps through this link with T always less than B. In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data. For example, we may have a link with a bandwidth of 1 Mbps, but the devices connected to the end of the link may handle only 200 kbps. This means that we cannot send more than 200 kbps through this link. Latency (Delay) The latency or delay defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source. We can say that latency is made of four components: propagation time, transmission time, queuing time and processing delay. Latency = propagation time + transmission time + queuing time + processing delay Propagation Time Propagation time measures the time required for a bit to travel from the source to the destination. The propagation time is calculated by dividing the distance by the propagation speed.

The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. Transmission Time In data communications we don't send just 1 bit, we send a message. The first bit may take a time equal to the propagation time to reach its destination; the last bit also may take the same amount of time. However, there is a time between the first bit leaving the sender and the last bit arriving at the receiver. The first bit leaves earlier and arrives earlier; the last bit leaves later and arrives later. The time required for transmission of a message depends on the size of the message and the bandwidth of the channel.

Queuing Time The third component in latency is the queuing time, the time needed for each intermediate or end device to hold the message before it can be processed. The queuing time is not a fixed factor; it changes with the load imposed on the network. When there is heavy traffic on the network, the queuing time increases. An intermediate device, such as a router, queues they arrived messages and processes them one by one. If there are many messages, each message will have to wait.

1 2 3 4 5 6 7

1 2 3 4 5

Peer to Peer Use less expensive computer network It is easy to set up It is built redundancy May hurts user performance Not very secured Difficult to backup No special NOS required

Analog Signal It is in form of continuous signal High error prone Accuracy of transmission is less Reliability is less It is highly noise signal

Client Server Needs intensive and expensive network Require professional administrator More reliable Better performance Highly secured Centralized backup Required NOS ad required number of client license

1 2 3 4 5 6 7

1 2 3 4 5

Digital Signal It is in the form of discrete signal Less error prone Accuracy of transmission is higher Reliability is higher It is completely noise free signal

1 2 3 4 5 6

MAC address IP address Function at the data link layer 1 Function at the Network layer Support the hardware implementation 2 Support the software implementation of OSI of OSI model model It is fixed 3 It changes with the relocation of device from one network to another It is physical address 4 It is logical address It changes when the packet moves from 5 It remains the same from the original source one network to another to final destination It is 48 bit address 6 It is 32 bit address IPv4

IPv6

Addresses are 32 bits (4 bytes) in length.

Addresses are 128 bits (16 bytes) in length

IPSec is optional and should be supported externally

IPSec support is not optional

Header does not include QoS handling of data

Header contains Flow Label field, which Identifies packet flow for QoS handling

Both routers and the sending host fragment packets. Routers do not support packet fragmentation. Sending host fragments packets Header includes a checksum.

Header does not include a checksum.

Header includes options.

Optional data is supported as extension headers.

Broadcast addresses are used to send traffic to all

IPv6 uses a link-local scope all-nodes multicast

nodes on a subnet.

address.

Configured either manually or through DHCP.

Does not require manual configuration or DHCP.

Must support a 576-byte packet size (possibly

Must support a 1280-byte packet size (without

fragmented).

fragmentation).

Differentiate between 802.3, 802.4 and 802.5 IEEE standard. Sr.no

Parameter of comparison

1

Physical topology

Linear

Linear

Ring

2

Logical topology

None

Ring

Ring

3

Contention

Random chance

By token

By token

4

A new station can be added almost Distributed algorithms are Adding stations anywhere on the needed to add new stations. cable at any time.

802.3 Ethernet

Stations often transmit immediately 5

Performance

under light loads,but heavy traffic can reduce the effective data to nearly 0.

802 4Token Bus

802.5Token Ring

Must be added between two specified stations.

Stations must wait for the Stations must wait for the

token even if no other

token even if no other station station is transmitting. is transmitting. Under heavy

Under heavy loads, token

load, token passing provides

passing provides fair access

fair access to all stations.

to all stations.

Bounded, depending on distance spanned and 6

Maximum delay before None transmitting

Bounded, depending on distance spanned and number of stations.

number of stations. However, if priorities are used, a low priority station may have no maximum delay.

7

8

Maintainence

Cable used

No central

Distributed algorithm provide

maintenance

maintenance

Twisted pair, co-axial co axial fiber optic

A designated monitor station performs maintenance. Twisted pair and fiber optic.

ETHERNET Networking allows one computer to send information to and receive information from another. We can classify network technologies as belonging to one of two basic groups. Local area network (LAN) technologies connect many devices that are relatively close to each other, usually in the same

building. The library terminals that display book information would connect over a local area network. Wide area network (WAN) technologies connect a smaller number of devices that can be many kilometers apart. In comparison to WANs, LANs are faster and more reliable, but improvements in technology continue to blur the line of demarcation. Fiber optic cables have allowed LAN technologies to connect devices tens of kilometers apart, while at the same time greatly improving the speed and reliability of WANs. Ethernet has been a relatively inexpensive, reasonably fast, and very popular LAN technology for several decades. Two individuals at Xerox PARC -- Bob Metcalfe and D.R. Boggs-- developed Ethernet beginning in 1972 and specifications based on this work appeared in IEEE 802.3 in 1980. Ethernet has since become the most popular and most widely deployed network technology in the world. Many of the issues involved with Ethernet are common to many network technologies, and understanding how Ethernet addressed these issues can provide a foundation that will improve your understanding of networking in general. The Ethernet standard has grown to encompass new technologies as computer networking has matured. Specified in a standard, IEEE 802.3, an Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless LANs. Ethernet uses the CSMA/CD access method to handle simultaneous demands. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds upto 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second). 10-Gigabit Ethernet provides up to 10 billion bits per second. IEEE standards IEEE developed a set of network standards. They include: • IEEE 802.1: Standards related to network management. • IEEE 802.2: General standard for the data link layer in the OSI Reference Model. The IEEE divides this layer into two sub-layers -- the logical link control (LLC) layer and the media access control (MAC) layer. The MAC layer varies for different network types and is defined by standards IEEE 802.3 through IEEE 802.5. • IEEE 802.3: Defines the MAC layer for bus networks that use CSMA/CD. This is the basis of the Ethernet standard.

• IEEE 802.4: Defines the MAC layer for bus networks that use a token-passing mechanism (token bus networks). • IEEE 802.5: Defines the MAC layer for token-ring networks. • IEEE 802.6: Standard for Metropolitan Area Networks (MANs). Ethernet and IEEE 802.3 Frame Formats Figure illustrates the frame fields associated with both Ethernet and IEEE 802.3 frames. Field lengths, in bytes 8 6 6 2 46-1500 4 Preamble Destination Source Type Data FCS Address Address Field lengths in bytes 7 1 Preamble SOF

6 6 Destination Source Address Address

2 Length

46-1500 802.3 header data

4 FCS

SOF = Start-of-Frame Delimiter FCS = Frame Check Sequence The Ethernet and IEEE 802.3 frame fields illustrated in Figure 7-2 are described in the following summaries: • Preamble — The alternating pattern of ones and zeros tells receiving stations that a frame is coming (Ethernet or IEEE 802.3). The Ethernet frame includes an additional byte that is the equivalent of the Start of Frame (SOF) field specified in the IEEE 802.3 frame. • Start-of-Frame (SOF)—The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame-reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet. • Destination and Source Addresses—The first 3 bytes of the addresses are specified by the IEEE on a vendor-dependent basis. The last 3 bytes are specified by the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single-node) address. The destination address can be unicast, multicast (group), or broadcast (all nodes). • Type (Ethernet)—The type specifies the upper-layer protocol to receive the data after Ethernet processing is completed. • Length (IEEE 802.3) — The length indicates the number of bytes of data that follows this field. • Data (Ethernet)—After physical-layer and link-layer processing is complete, the data contained in the frame is sent to an upper-layer protocol, which is identified in the Type field. Although Ethernet Version 2 does not specify any padding (in contrast to IEEE 802.3), Ethernet expects at least 46 bytes of data. Data (IEEE 802.3)—After physical-layer and link-layer processing is complete, the data is sent to an upper-layer protocol, which must be defined within the data portion of the frame, if at all. If data in the frame is insufficient to fill the frame to its minimum 64-byte size, padding bytes are inserted to ensure at least a 64-byte frame. • Frame Check Sequence (FCS) — This sequence contains a 4-byte cyclic redundancy check (CRC) value, which is created by the sending device and is recalculated by the receiving device to check for damaged frames.

Transmission Speed/Types of Ethernet system1. 10Base-T The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. 2. 100BASE-T Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. The 100BASE-T standard consists of five different component specifications. These include the Media Access Control (MAC) layer, the Media Independent Interface (MII), and the three physical layers, (100 BASE-TX, 100BASET4, and 100BASE-FX). 3. Gigabit Ethernet Gigabit Ethernet provides an even higher level of backbone support at 1000 MBPS [megabits per second] (1 gigabit or 1 billion bits per second). Used mostly for backbones, the first IEEE standard (802.3z) for Gigabit Ethernet (GigE) was defined in 1997 for use over multimode optical fiber. 802.3z provides full-duplex operation from switch to end station or to another switch and half-duplex using CSMA/CD in a shared environment. 4.10 Gigabit Ethernet 10 Gigabit Ethernet is an upcoming Ethernet technology that transmits at 10 Gbps. 10 Gigabit Ethernet enables a familiar network technology to be used in LAN, MAN and WAN architectures. However the CSMA/CD method for gaining access to the physical medium is not employed and half duplex operation is not supported. 10 Gigabit Ethernet uses multimode optical fiber up to 300 meters and single mode fiber up to 40 kilometers. Limitations of Ethernet There are practical limits to the size of our Ethernet network. A primary concern is the length of the shared cable. Electrical signals propagate along a cable very quickly, but they weaken as they travel, and electrical interference from neighboring devices (fluorescent lights, for example) can scramble the signal. A network cable must be short enough that devices at opposite ends can receive each other's signals clearly and with minimal delay. This places a distance limitation on the maximum separation between two devices on an Ethernet network. Additionally, since in CSMA/CD only a single device can transmit at a given time, there are practical limits to the number of devices that can coexist in a single network. Ethernet networks face congestion problems as they increased in size. If a large number of stations connected to the same segment and each generated a sizable amount of traffic, many stations may

attempt to transmit whenever there was an opportunity. Under these circumstances, collisions would become more frequent and could begin to choke out successful transmissions, which could take inordinately large amounts of time to complete. One way to reduce congestion would be to split a single segment into multiple segments, thus creating multiple collision domains. This solution creates a different problem, as now these now separate segments are not able to share information with each other. To overcome these problems, Ethernet networks implemented bridges. Bridges connect two or more network segments, increasing the network diameter as a repeater does, but bridges also help regulate traffic. They can send and receive transmissions just like any other node, but they do not function the same as a normal node. The bridge does not originate any traffic of its own; like a repeater, it only echoes what it hears from other stations. Define multiplexing and de-multiplexing? How are they useful? Multiplexing is when several connections are supported by a single connection. According to the formal definition, this applies to layers (so that three presentation service connections could be multiplexed into a single session connection). However, it is a term generally used for all kinds of connections, such as putting four modem calls down a single modem line. De-multiplexing is the reverse of multiplexing, in which one connection is split into several connections. Multiplexing is a key to supporting many connections at once with limited resources. A typical example is a remote office with twenty terminals, each of which is connected to the main office by a telephone line. Instead of requiring twenty lines, they can all be multiplexed into three or four. The amount of multiplexing possible depends on the maximum capacity of each physical line. Ques: What is Multiplexing? Ans: Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic.

There are three basic multiplexing techniques: frequency-division multiplexing, wavelength-division multiplexing, and time-division multiplexing. The first two are techniques designed for analog signals, the

third, for digital signals.