159.334 Computer Networks

Introduction to the OSI Model and the Physical Layer Professor Richard Harris School of Engineering and Advanced Technology (SEAT)

Learning Objectives 159.334 Computer Networks

You will be able to:

Describe the concept of Layering as it applies to communication protocols Identify and describe the basic components in the OSI Reference Model such as: the N+1, N and N-1 Layers, Layer Entities, the Service Access Point (SAP), the role of primitives, the difference between Inter-Layer and peer communication, the basic functions of the various Layers of the OSIRM

Identify and describe the layered components of a TCP/IP network Computer Networks - 1/2 1/2

References 159.334 Computer Networks

Forouzan, “Data Communications and Networking”, 4th Edition Tanenbaum, “Computer Networks”, 4th Edition Cisco CCNA1 Module 10 - part 1 Stallings, William 2000 ‘Data and Computer Communications’, Prentice Hall, Sixth Edition Russell, Travis 1997 ‘Telecommunications Protocols’, McGraw Hill

Computer Networks - 1/3 1/3

Introducing Protocols (Protocols in a Data Network) 159.334 Computer Networks

D D

X.25 A A

Network Access

Internal Protocols

Network Access

High Layer (Level) User Protocols (End to End)

Network Access Protocols Highly standardise d Describe how to use a network Specified as if each user is the only network user. Internal Computer Networks - 1/4 1/4 Protocols

Introducing Protocols (A Protocol Model for a Data Network) 159.334 Computer Networks

The Packet Switching Network of the Previous Slide can be viewed in terms of a Layered Model

A

D Network

Higher Layer (Level) Protocols

Higher Layers X.25 (L3)

X.25 (L3)

LAPB X.21

LAPB X.21

x y z

x y z

X.25 (L3)

X.25 (L3)

LAPB X.21

LAPB X.21

Node D

Node A X.25 Network Access Layers 1,2 & 3

Higher Layers

Internal Protocols

X.25 Network Access Layers 1,2 & 3 Computer Networks - 1/5 1/5

Introducing Protocols (A Definition) 159.334 Computer Networks

According to the “Webster's New World Dictionary ” A protocol is .. "a set of rules governing the communications and the transfer of data between machines, as in computer systems". Other dictionaries will talk about the “rules of interaction” between organisations, countries, people or machines. The point here is that for two entities to interact / communicate / co-operate there must be a set of rules that describe how the communication is to take place (including definitions of the semantics and syntaxes involved) Computer Networks - 1/6 1/6

Introducing Protocols (A Definition - 1) 159.334 Computer Networks

To “handle” complex protocols designers “compartmentalised” communication protocols into Layers Communications and data transfer between machines takes place at many "layers". Each layer provides a Service (via InterLayer Communication) to it’s higher Layer through the SAP And communicates with its peer in the remote system

Layer N+1 Layer N Layer N-1 L1 SAP

Layer 1

Physical Medium Computer Networks - 1/7 1/7

Introducing Protocols (A Definition - 2) 159.334 Computer Networks

These layers can be implemented as software, hardware or a combination of both InterLayer Communications can be anything from software messages to the raising or lowering of voltage levels Peer Communication is always in terms of abstract Protocol Data Units (PDUs). The only actual communication is on the Physical Medium

Layer N+1 Layer N Layer N-1 L1 SAP

Layer 1

Physical Medium Computer Networks - 1/8 1/8

Introducing Protocols (Entities and Primitives) 159.334 Computer Networks

Layer N+1 Layer N

Entities in each Layer

(N+1) Entity

Response

Indication

Confirm

Request

Primitives at the Layer Boundary

(N+1) Entity

Layer N+1 Protocol

(N) Entity

Layer N Protocol

(N) Entity

Computer Networks - 1/9 1/9

Introducing Protocols (Goals of the OSIRM) 159.334 Computer Networks

The Open Systems Interconnection Reference Model (OSIRM) effort strove to define the functions and protocols necessary for any computer system to connect to any other computer system, regardless of the manufacturer. In 1977, the International Organisation for Standardisation (ISO) started work on the OSIRM. The Basic reference model was documented in 1983 as ISO standard 7498.

OSI has standardized many of these protocols, but only a few are in widespread use.

The layering concept, however, has been widely adopted by every major computer and communications standards body and most proprietary implementations as well. Computer Networks - 1/10 1/10

Introducing Protocols (OSIRM and the Future) 159.334 Computer Networks

Whilst ISO defined protocols for the OSIRM may not be in wide use. Communication product vendors relate their commercial offerings to the OSIRM. That is…… Protocol X is a Transport Layer Protocol, or Protocol Y is a Session Layer Protocol It is, and will continue to grow to be the framework for modelling of communication systems While “Internet” protocols (i.e. those developed by the IETF) will be commercially dominant. Computer Networks - 1/11 1/11

OSI Layered Model 159.334 Computer Networks

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Communication between layers 159.334 Computer Networks

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Overview of the Layers 159.334 Computer Networks

Application

To translate, encrypt and compress data

Presentation

Session To provide reliable process to process message delivery and recovery

To establish, manage and terminate sessions

Transport

Network To organise bits into frames; to provide hop-to-hop delivery

To allow access to network resources

To move packets from source to destination; to provide internetworking

Data Link

Physical

To transmit bits over a medium; to provide mechanical and electrical specifications Computer Networks - 1/14 1/14

Introducing Protocols (The Basic OSI Reference Model - 1) 159.334 Computer Networks

Relating certain network equipment classes to the OSIRM

7. Application 6. Presentation 5. Session S/W Layers

Comms Subnetwork

H/W Layers

Gateway

4. Transport 3. Network

Router

2. Data Link

Bridge

1. Physical

Repeater Computer Networks - 1/15 1/15

Introducing Protocols (The Basic Reference Model - 2) 159.334 Computer Networks

7. Application 6. Presentation 5. Session 4. Transport 3. Network

Defines network segmentation and network address scheme. Connectivity over multiple network segments Cornerstone on which all upper layers are based. Units is “Packets” Creates packet headers & checksum trailers. Packages datagrams into frames. Detects errors. Regulates data flow. Maps hardware addresses. (FDDI, Ethernet, T1) Units is “Frames”

2. Data Link 1. Physical

Defines physical and electrical specifications for transmission. Defines connector types and pin-outs, voltage and current. (e.g., RS-232, V.35) Units is “Bits” Computer Networks - 1/16 1/16

Introducing Protocols (The Basic Reference Model - 3) 159.334 Computer Networks

7. Application 6. Presentation 5. Session 4. Transport 3. Network

Connection negotiation, establishes and maintains connection, and synchronises dialog. (RPC, Nethios, service 'ports') Unit is “Message” Assures end-to-end reliability. Translates & manages message communication through subnetwork. Insure data integrity and deals with packet sequencing. (e.g. TCP) Unit is “Segment”

2. Data Link 1. Physical Computer Networks - 1/17 1/17

Introducing Protocols (The Basic Reference Model - 4) 159.334 Computer Networks

7. Application 6. Presentation

Manages program requests that require access to services provided by a remote system.(FFP, NFS, MHS, Netware requester) Unit is “Message”

5. Session 4. Transport 3. Network

Translates data format of sender to data format of receiver. Also performs encryption. Provides data compression, translation and encryption. Unit is “Message”

2. Data Link 1. Physical Computer Networks - 1/18 1/18

Introducing Protocols (End and Intermediate Systems - 1 ) 159.334 Computer Networks

Various Levels of Peer Communication 7. Application

7. Application

6. Presentation

6. Presentation

5. Session

5. Session

4. Transport

4. Transport

3. Network

3. Network

3. Network

2. Data Link

2. Data Link

2. Data Link

1. Physical

1. Physical

1. Physical

User A End System

Intermediate Node

User B End System Computer Networks - 1/19 1/19

Introducing Protocols (End and Intermediate Systems - 2 ) 159.334 Computer Networks

Each of the seven layers represents one or more protocols that define the functional operation of communications between user and network elements. All protocol communications between layers are "peer-to-peer" - depicted as horizontal arrows between the layers. Standards span all seven layers of the model.

Computer Networks - 1/20 1/20

Introducing Protocols (Following Interlayer & Peer Communication) 159.334 Computer Networks

Actual Communications Path

Various Levels of Peer Communication

7

7

6

6

5

5

4

4

3

3

3

2

2

2

1

1

1

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Connection-oriented and connectionless services 159.334 Computer Networks

The service each layer provides to the layer above can be classified into two different types, Connection oriented service requires a pair of processes in the two computers to establish a connection before sending data. In connectionless service, data packets are independent of each other and they may not arrive at the destination in the proper order.

Computer Networks - 1/22 1/22

Connection-oriented service over a single link 159.334 Computer Networks

In connection-oriented service, a logical connection is established by having both sides initialize variables and counters needed to keep track of which frames have been received and which ones have not In connectionless service, this connection does not need to be established, therefore data frames can get lost, corrupted, duplicated, and out of order.

Computer Networks - 1/23 1/23

Connection-oriented service over multiple links 159.334 Computer Networks

In connection-oriented service, a path from the source to the destination must be established before any data packets can be sent. This connection is called a VC (virtual circuit). In connectionless service, no advance setup is needed. In this context, the packets are called datagrams. The datagrams carries the full destination address, and each is routed through the system independent of all the others.

Computer Networks - 1/24 1/24

Application layer 159.334 Computer Networks

Provides the user interface, application programs, access to distributed information services ex. File Transfer Protocol (FTP), E-Mail

Other important tasks Identification and checking the availability of the called party Agreement on encryption mechanisms and responsibility for error recovery Procedures for initiation and release of a dialog

Computer Networks - 1/25 1/25

Presentation Layer 159.334 Computer Networks

Concerned with the representation of syntax of data during transfer of information between two application processes Performs the necessary conversion if two application processes use different syntaxes ex. Two people who speak different languages communicating with each other. The PL does the job of the interpreter. The PL at each end translate the message to English before handing it to SL

Performs encryption using a secured key if requested by the AL

Computer Networks - 1/26 1/26

Session Layer 159.334 Computer Networks

Sets up and clear the dialog between two PL protocol entities In the case of half duplex (two way alternate) mode of dialog, it controls the exchange of data (dialog units) Token management- in case both sides are not supposed to attempt the same operation at the same time, users are issued with tokens. Only the user who has the token at a given time can perform the critical operation Can establish synchronization points during lengthy transactions as requested by the user via SL Reports of non-recoverable data to higher layers. Computer Networks - 1/27 1/27

Transport Layer 159.334 Computer Networks

Provides higher layers with a message transfer facility which is efficient, reliable and cost-effective and independent of the network layer Can offer various classes of service because of the varied quality of service (QOS) provided by different networks - ranging from basic functions needed for connection establishment and data transfer to full error control and flow control

Computer Networks - 1/28 1/28

Network Layer 159.334 Computer Networks

Concerns with routing messages from the source to the destination through the network. This may require making multiple hops at various nodes in the network The main tasks of the NL are Providing services to the TL, such as connection oriented or connection less, explicit confirmation of delivery and priority schemes Routing of packets Congestion control compatibility issues associated with internetworking

Computer Networks - 1/29 1/29

Data Link Layer 159.334 Computer Networks

DLL converts a raw transmission media (coaxial cable, satellite channel) into a line that appears free of transmission errors to the network layer and it concerns about the link established between two ends of a single transmission facility, i.e. no nodes or switching centres in the middle The main tasks are Services provided to the NL,such as unacknowledged connectionless service, acknowledged connectionless service and connection oriented service framing error control flow control Computer Networks - 1/30 1/30

Physical Layer 159.334 Computer Networks

The physical layer concerns with issues related to the utilisation of a given physical medium (copper wire, optical fiber, free space) for digital data transmission The main tasks of the physical layer are defining voltage levels of various signals and bit rates analogue to digital and digital to analogue conversion error control coding multiplexing schemes modulation and demodulation

Computer Networks - 1/31 1/31

Some Revision on TCP/IP Networks 159.334 Computer Networks

Nodes in a TCP/IP network are modelled using a 5 layer model. They can also be modelled using the OSI 7 layer model.. It is just that the TCP/IP community use a 5 layer model Note that in the past they have used a three layered model…. So their model complexity is growing

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A TCP/IP Node and a Rough Equivalence to OSI Layering 159.334 Computer Networks

An element of the communications stack

Transport Control Protocol User Processes

User Datagram Protocol - Unreliable Datagram service but supports the ability to find multiple Applications on the target machine

5,6,7 Applications UDP

TCP

4 3 2 1

IP NDP

- Provision of a reliable connection oriented service InterNet Protocol Layer - Routing on IP Addresses - Provision of an unreliable Datagram service over multiple networks Network Dependent Protocols - Those protocols necessary to connect to a network Computer Networks - 1/33 1/33

A TCP/IP Network Model 159.334 Computer Networks

The Internetwork

User Processes

- A collection of networks that provide an unreliable Datagram service for the transport of UDP or TCP messages

User Processes

Applications

Applications

UDP

UDP

TCP IP

NDP

IP Network 1

NDP

TCP IP

Network 2

NDP

Computer Networks - 1/34 1/34

A TCP/IP Network Model 159.334 Computer Networks

User Processes

User Processes

Applications

Applications

UDP

UDP

TCP IP

NDP

IP FR or ATM

NDP

TCP IP

FR or ATM

NDP

The Subnetwork - A collection of networks that can carry an IP Datagram (e.g. Frame Relay or ATM) Computer Networks - 1/35 1/35

Addresses in TCP/IP 159.334 Computer Networks

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Relationship of layers and addresses in TCP/IP 159.334 Computer Networks

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Physical addresses 159.334 Computer Networks

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