NT1210 Introduction to Networking

Unit 7: Chapter 7, Wide Area Networks

Objectives ‰ Identify y the major j needs and stakeholders for computer networks and network applications. ‰ Identify the classifications of networks and how they are applied to various types of enterprises. ‰ Explain the functionality and use of typical network protocols. protocols ‰ Analyze network components and their primary functions in a typical data network from both logical and physical perspectives.

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Objectives ‰ Differentiate among g major j types yp of LAN and WAN technologies and specifications and determine how each is used in a data network. ‰ Explain basic security requirements for networks. ‰ Use network tools to monitor protocols and traffic characteristics. characteristics ‰ Use preferred techniques and necessary tools to troubleshoot common network problems. problems ‰ Differentiate among WAN technologies available from service providers p

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Objectives ‰ Evaluate how WAN devices function ‰ Define and describe WAN protocols ‰ Evaluate troubleshooting techniques for WAN connections

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Introducing Wide Area Networks: Basic Telco Services ‰ T Telephone, l h T Telcos, l and d companies i th thatt grew ffrom original Bell System impact how today’s WANs work ‰ Telcos built huge networks to support voice traffic traffic, long before computers could create and send bits

Timeline Comparison of Inventions Compared to Telephone 5

Figure 7-1

Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ S Switched it h d A Analog l Ci Circuits it ffor V Voice: i IIn early l d days off telephones, voice call required one analog electrical circuit between two phones p ‰ Telco installed 2-wire cable into each home: Local loop ‰ Other end connected equipment sitting in nearby Telco office: Central Office (CO) ‰ When user called number, Telco created electrical circuit from one telephone to other (source to destination)

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Introducing Wide Area Networks: Basic Telco Services – Circuit Switching

Early Voice: Telco Creates One Analog Electrical Circuit Between Phones 7

Figure 7-2

Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ S Switched it h d A Analog l Ci Circuits it ffor D Data: t T To create t fifirstt WAN connections, early computing devices had to act like telephones p ‰ One computer device would “make phone call” to other computer, encoding its bits using analog electrical signals

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Introducing Wide Area Networks: Basic Telco Services – Circuit Switching

Connecting from a PC to an ISP, Using Modems and an Analog Telco Circuit 9

Figure 7-4

Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ Beginning B i i mid-20 id 20th century t Telcos T l transformed t f d ‰ Invention and commercialization of computers: Started with few computers being rare and unusual to world where most companies owned computers ‰ Migration from Telcos as government monopolies to freemarket competition: Governments started removing monopoly status from different parts of Telcos’ business so allowed competition ‰ C Computerization off Telco’s own network: Revolutionized how Telco built its internal network to create better services at lower cost

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Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ Di Digital it l Ci Circuits it and d Leased Lines: Telcos started offering g service that used digital circuit between customer devices ‰ Endpoints still had circuit between them but could encode signal as bits with different electrical signals that followed encoding rules More Modern Routers Using a Digital Leased Line 11

Figure 7-5

Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ S Switched it h d Ci Circuits it and Circuit Switching: When user calls phone number, various circuit switches connect circuit on both sides of switch ( (see arrowed d lilines)) ‰ Circuit switches create effect of end-to-end circuit by switching/connecting circuits on various links Circuit Switching

Figure 7-6 12

Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ T To create t switched it h d circuits: i it S Switches it h and d customer t devices (telephones and modems) use signaling to setup p and tear down circuit ‰ Signaling messages allow switches to choose which switch-to-switch links (trunks) to use for particular call

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Introducing Wide Area Networks: Basic Telco Services – Circuit Switching ‰ Circuit: Ci it Communication C i ti path th b between t ttwo endpoints d i t ‰ Circuit Switching: Logic used by Telco network and devices called “circuit circuit switches” switches that allows them to switch circuits in and out of different physical trunks to create end-to-end circuit through network ‰ Switched Circuit: End-to-end circuit through Telco that changes over time because user calls number, hangs up calls another number up, number, and so on ‰ Dedicated Circuit (leased line): Circuit between two specific devices Telco never takes down 14

Introducing Wide Area Networks: Basic Telco Services – Packet Switching ‰ P Packet k t Switching: S it hi T Telcos l nextt started t t d offering ff i WAN services using packet switching services

General Timeline: Circuit Switching, Digital Circuits, and Packet Switching 15

Figure 7-7

Introducing Wide Area Networks: Basic Telco Services – Packet Switching ‰ All customer t d devices i need d di directt connection ti tto WAN via i circuit to packet switching service ‰ Customers: All devices can send data to every other device connected to packet switched service ‰ Telco (service provider): Must look at meaning of bits in customer’s headers and make forwarding decision per packet

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Introducing Wide Area Networks: Basic Telco Services – Packet Switching Packet Switching Example

Example of Packet Switching Service

Figure 7-8 17

Introducing Wide Area Networks: Routers ‰ Connect C t LANs LAN tto WAN WANs

Layer 3 IP Forwarding Logic

Figure 7-9 18

Introducing Wide Area Networks: Routers ‰ LAN might i ht b be simple i l Eth Ethernet-only t l LAN ‰ LAN might be simple 802.11 WLAN ‰ LAN might i ht b be more complex l campus LAN with ith b both th wired i d and wireless LANs

Example Enterprise Network, With LAN and WAN Details Revealed 19

Figure 7-10

Introducing Wide Area Networks: Routers ‰ Encapsulation E l ti and dD De-encapsulation l ti

Encapsulation that Happens During the IP Packet Forwarding Process 20

Figure 7-11

Introducing Wide Area Networks: Topologies ‰ Point-to-Point P i t t P i t Topology: T l Basic B i WAN service i ‰ LAN with10BASE-T or 100BASE-T cable has 2-pair: 1 pair for sending data in each direction ‰ Both LAN and WAN topologies allow full duplex operation and can share 1 link

Point-to-Point Topologies in WAN and LAN 21

Figure 7-12

Introducing Wide Area Networks: Topologies ‰ Hub and Spoke Topologies ‰ Reduces number of leased lines ‰ Provides way for packets to reach all sites ‰ Connects one router (hub router) to all other routers using leased lines

WAN Hub and Spoke Topology Vs. LAN Star Topology 22

Figure 7-13

Introducing Wide Area Networks: Topologies ‰ Hub H b and dS Spoke k T Topology l example l ‰ Routers in Enterprise network and how many leased lines required to connect every router to every other router

Number of Routers 20 40 60 80 100

Number of Leased Lines 190 780 1770 3160 4950

Formula: N(N-1)/2 N(N 1)/2 (20 x 19) / 2 (40 x 39) / 2 (60 x 59) / 2 (80 x 79) / 2 ((100 x 99)) / 2

Number of Leased Lines to Connect Every Pair of Routers 23

Table 7-1

Introducing Wide Area Networks: Topologies ‰ M Multipoint lti i t ttopologies: l i H Hub-and-spoke b d k topology t l h has some disadvantages ‰ Uses leased lines that might have to run hundreds or thousands of miles at large expense ‰ Packets that go from one spoke site to another th spoke k site it have h tto cross multiple WAN links

WAN Multipoint Topology

Figure 7-14 24

Understanding Leased Line WAN Links ‰ C Customer t expects t to t gett basic b i service i off sending di bit bits between two devices ‰ Customer buys right to send X number of bits per second constantly between two sites

Leased Line in Concept

Figure 7-15 25

Understanding Leased Line WAN Links ‰ Di Distance t lilimitations: it ti N No single i l circuit i it extends t d entire ti distance between two routers ‰ “Point Point to point” point circuits really series of circuits

Leased Line: Shorter Electrical Circuits, Knitted Together 26

Figure 7-16

Understanding Leased Line WAN Links ‰ T Telcos l i USA offer in ff leased l d liline speeds d iin multiples lti l off 64 Kbps: 64, 128, 192, 256, etc., up to 24 times 64 ((1.536 Mbps) p ) ‰ Speeds above 1.5 Mbps offered in multiples of 1.544 Mbps up to 28 times (around 43 Mbps) Incremental Speed Value Based on… DS0 DS1 ((T1))

Speed Increment

Up to this Many Increments

Speed Range

64 Kbps 1.536 Mbps p

24 28

64 - 1536 Kbps 1.536 - 43.008 Mbps p

Leased Line Speed Options, USA

Table 7-2 27

Understanding Leased Line WAN Links ‰ Telcos built networks to support digital transmission of voice in mid-20th century ‰ Earlier standards used 64-Kbps b ildi bl building block k called ll d digital di it l signal i l level 0 (DS0): Combined bits from slower speed p links into single g higher g speed physical link using multiplexing ‰ Bits from 24 DS0s (plus some overhead) combined onto single physical link called digital signal level 1 (DS1, T1) y links combined bits from 28 DS1s (p (plus ‰ Physical overhead) to create DS3 (T3) lines Visual Comparison of Speeds: DS0, DS1, DS3 28

Figure 7-17

Understanding Leased Line WAN Links ‰ T Telco l iinstalls t ll physical h i l cable bl b between t equipment i t iin CO to customer site ‰ 2 2-pair pair cable typically runs underground into customer buildings terminating near customer’s router

Cables in a Relatively Short Leased Line

Figure 7-18 29

Understanding Leased Line WAN Links ‰ C Customer t needs d tto plan l ffor cabling bli att end d off T Telco’s l ’ leased line cable ‰ Example: Customer’s Customer s router connects to cable installed by Telco

Components and Responsibilities on One Side of a Leased Line 30

Figure 7-19

Understanding Leased Line WAN Links ‰ L Leased d liline h has Ch Channell S Services i U Unit/Data it/D t S Services i Unit (CSU/DSU) function on each side of line at customer site ‰ Each site uses either internal or external CSU/DSU ‰ Internal CSU/DSU sits inside router as p part of serial interface card

Customer Equipment and Cabling with External CSU/DSU 31

Figure 7-20

Understanding Leased Line WAN Links ‰ E External t l CSU/DSU ttakes k more planning: l i N Need d tto kknow which connectors used by router and CSU/DSU device and then use correct serial cable to connect them

Drawings of Some of the Short Serial Cables (Router to CSU/DSU) 32

Figure 7-21

Understanding Leased Line WAN Links ‰ E Example: l Ci Cisco router t with ith ttwo slots l t ffor removable bl router interface cards (WICs) where serial cards are install ‰ Serial card on left has built-in CSU/DSU and uses RJ-48 connector ‰ Serial card on right does not have CSU/DSU so relies e es o on e external te a CSU/DSU 1921 router… http://www.cisco.com/en/US/prod/collateral/voicesw/ps6789/ps7290/ps10589/data_sheet_c78-598389.html WIC-1CSU: http://www.cisco.com/en/US/prod/collateral/routers/ps221/product_data_sheet09186a00801a9184.html http://www.cisco.com/en/US/prod/collateral/routers/ps5853/data_sheeet_serial_high_speed_waniInt_cards_for_1861.html

Photos of Router and Removable WAN Cards 33

Figure 7-22

Understanding Leased Line WAN Links ‰ Key K steps t for f installing i t lli lleased d lilines 1. Order leased line from Telco; include specs on line speed, cable connectors required, and exact location where cable should be installed (address, floor, identifying information for exact room) 2. Install router and serial interface cards in router as needed by leased line 3. If interface card does not have internal CSU/DSU, choose CSU/DSU and matching cable 4. Physically y y connect all cables 5. Configure devices (beyond scope of this chapter)

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Break Take 10

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Understanding Leased Line WAN Links: Multiplexing ‰ T T-Carrier C i system t and d multiplexing lti l i challenge: h ll T Too many trunks between sites ‰ Example: 3 customers (A, (A B, B C) who each have offices in same two towns about 15 miles apart and each has DS1 or T1 leased line between each site

Conceptual View, Three T1 Leased Lines, Three Customers (A, B, C) 36

Figure 7-23

Understanding Leased Line WAN Links: Multiplexing ‰ P Possible ibl solution: l ti T Telco l could ld iinstall t ll th three T1 ttrunk k lilines between CO switches

Telco Switching Connecting Incoming Customer T1s to T1 Trunks 37

Figure 7-24

Understanding Leased Line WAN Links: Multiplexing ‰ M More efficient ffi i t solution: l ti Time Ti Division Di i i M Multiplexing lti l i (TDM) uses TDM switches and one T3 trunk ‰ Telco connects cable using T3 card in each TDM switch to use T3 link (43.736 Mbps—28 times T1 speed)

CO Switches Multiplexing T1 Bits onto Faster T3 Circuit 38

Figure 7-25

Understanding Leased Line WAN Links: Multiplexing ‰ Top: 3 routers continually send bits to switch CO1 via 3 separate T1 lines ‰ Middle: Switch CO1 combines (“plexes”) bits onto single T3 for transmission ‰ Bottom: Switch CO2 demultiplexes incoming signal back into original bit streams, sending bits for customer A’s T1 to router A2, and B’s and C’s C s likewise A Three Part Drawing of the TDM Example 39

Figure 7-26

Understanding Leased Line WAN Links: Multiplexing ‰ Link types in T-carrier system (DS0 (DS0, DS1 DS1, DS3) define timing of when TDM switch can send bits over link ‰ Example: p On customer A’s T1 connection,, CO1 and CO2 TDM switches give A’s traffic “turn” on T3 on regular basis ‰ As long as number of bits allocated to A’s traffic totals 1.544 million every second, TDM switches can send all traffic for A’s T1

A View into a Longer Time Period over a T3 Link 40

Figure 7-27

Understanding Leased Line WAN Links: Multiplexing ‰ TDM switch it h considers id T3 ttrunk k as physical h i l serial i l lilink k with 28 logical T1 channels ‰ Physically Physically, T3 line uses 2-pair 2 pair cable that sends and receives bits serially (one at a time)) at 43.736 Mbps ‰ Logically, TDM switch views T3 as 28 T1 channels numbered 1 through 28

Matching TDM T3 Port and T1 Logical Channel on Both Ends of the T3 Trunk 41

Figure 7-28

Understanding Leased Line WAN Links ‰ Telco T l switches it h and d CSU/DSU CSU/DSUs view i bit bits as frames f

A T1 Frame, Shown as Sent Serially Over a T1 42

Figure 7-31

Understanding Leased Line WAN Links ‰ T1 frames f are sett off 193 bits bit that th t flow fl over link li k ‰ T1 equipment uses data in extra 193rd bit for several functions including functions, framing (can’t be used for customer data bits)

193-Bit T1 Frame

Figure 7-30 43

Understanding Leased Line WAN Links ‰ C Customer t b buys T1 liline att each h site it with ith ffullll T1 speed d (1.536 Mbps) ‰ What happens if customer router can only transmit at 768 Kbps?

Speed Differences on a 768-Kbps Leased Line WAN 44

Figure 7-29

Understanding Leased Line WAN Links ‰ Example: Serial cable connecting CSU/DSU and router includes wires that set clocking (speed of link) ‰ CSU/DSU signals router when to send or receive 1 bit ‰ If CSU/DSU configured to make router run at 768 Kbps, CSU/DSU sends clocking signal 768,000 times per second

CSU/DSU View of Serial Cable and Router, with Clocking 45

Figure 7-33

Understanding Leased Line WAN Links ‰ H How tto use entire ti T1 speed: d Adj Adjustt d data t speed d using i CSU/DSU ‰ CSU/DSU looks at T1 line as 24 separate DS0 channels plus overhead bits (framing) ‰ Every 1/8000th of second, each frame gets to send and receive one byte of data (i e each DS0 (i.e., channel gets 64 Kbps of the capacity of the T1) CSU/DSU Channel View of the T1 Line from Customer to Local CO 46

Figure 7-32

Understanding Leased Line WAN Links ‰ C Customer t b bought ht 1 1.536 536 Mb Mbps off T1 b between t routers t (full capacity of 24 DS0 channels) ‰ So CSU/DSU must clock router at slightly slower rate of 1.536 Mbps

Full T1 with CSU/DSU: 1.536 Mbps to the Router, 1.544 Mbps to the CSU/DSU 47

Figure 7-34

Understanding Leased Line WAN Links ‰ Wh When ttaking ki bit bits ffrom router, t CSU/DSU mustt fit them th into 24 DS0 channels in T1 frame

Role of the CSU/DSU in a Full T1 Leased Line 48

Figure 7-35

Understanding Leased Line WAN Links ‰ F Fractional ti l T1: T1 One O T1 line li di divided id d iinto t numerous DS0 channels ‰ Example: 256 Kbps (4 times 64-Kbps speed of DS0 channel) uses 4 DS0 channels on T1 line

Role of the CSU/DSU in a Fractional T1 Leased Line (256) 49

Figure 7-36

Understanding Leased Line WAN Links Type of Line Geography DS0 USA

Speed 64 Kbps

Number of Channels N/A

DS1 (T1)

USA

1.544 Mbps

24 DS0

DS3 (T3) E0

USA Europe

43.736 Mbps 64 Kbps

28 DS1 N/A

E1

E Europe

2 048 Mb 2.048 Mbps

32* E0

E3 J0

Europe Japan p

34.368 Mbps 64 Kbps p

16 E1 N/A

J1

Japan

1.544 Mbps

24 J0

J3

Japan

32.064 Mbps

20 J1

* 30 E0 channels are available for customer data; 2 E0 channels are for other functions.

Summary of Carrier TDM Line Standards

Table 7-3 50

Understanding Leased Line WAN Links ‰ Data D t Li Link kP Protocols t l 1. PC1 sends IP packet encapsulated in Ethernet frame to router R1 2. R1 receives frame, removes header/trailer, makes decision to send packet to R2; R1 adds header/trailer based on WAN Data Link protocol 3. R2 receives WAN frame, removes header /trailer, makes decision to send packet to host PC2; R2 adds Ethernet header/trailer so frame arrives at PC2

Encapsulation in a Serial WAN Data Link Protocol between Two Routers 51

Figure 7-37

Understanding Leased Line WAN Links ‰ Hi High-level hl lD Data t Li Linkk Control C t l (HDLC) protocol t lh has 2 main purposes: ‰ Deliver encapsulated data from sender to correct receiver ‰ Perform error detection

HDLC Frame Format

Figure 7-38 52

Understanding Leased Line WAN Links Field

Description

Shorthand Sh th d Reminder

Similar Si il to t Ethernet…

Flag

1-byte (7E) that signals beginning of frame

Here comes the frame!

Preamble + SFD

Address

Identifies destination device

To there

Destination MAC

Control

Defines many subfields used by older devices

Old; ignore

N/A

Data

Data pplus all headers from upper pp layers y

The actual data payload pa load

Data

FCS

Frame Check Sequence used for error checking (to see if any bits changed during transmission)

Check for errors

FCS

HDLC Header and Trailer Fields

Table 7-4 53

Understanding Leased Line WAN Links ‰ P Point-to-Point i t t P i t Protocol P t l (PPP): (PPP) D Designed i d ffor TCP/IP networks and multiprotocol routers

PPP Frame Format

Figure 7-39 54

Understanding Packet Switching and MultiMulti Access WANs ‰ P Packet k t switching it hi vs. TDM example: l T Telco l h has iinstalled t ll d 28 T1 leased lines for customers near two COs ‰ Each T1 mapped to one of 28 T1 channels in T3 trunk ‰ All 28 channels of T3 used

Single T3 Trunk Consumed by Supporting 28 Customer T1s 55

Figure 7-40

Understanding Packet Switching and MultiMulti Access WANs ‰ 28 T1 leased l d lines li stretch t t h ffrom ttwo CO COs ‰ Leased lines run through total of four TDM switches and total of three T3 trunks ‰ T1s take up entire capacity on all three TDM trunks

Multiple T3 Trunks Completely Used by Supporting 28 Customer T1s 56

Figure 7-41

Understanding Packet Switching and MultiMulti Access WANs ‰ H How packet k t switching it hi works: k D Devices i use same TDM physical link types as T-carrier system ‰ Use Data Link protocol that identifies destination address using hex code ‰ Telco packet switches must know location of each destination, typically using table

Packet Switching Concept: Telco Node Receives, Queues, Forwards Packets 57

Figure 7-42

Understanding Packet Switching and MultiMulti Access WANs ‰ With packet k t switching, it hi lilink k capacity it b between t switches it h used to forward packets as needed or available

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Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ F Frame Relay: R l All Allows any d device i connected t d tto network t k to communicate with any other network and details of Frame Relayy design g do not matter

Typical Drawing of a Frame Relay Design, One Customer, Ignoring Details 59

Figure 7-44

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ F Frame Relay R l physical h i l lilinks: k Ed Edge b between t customer t site and Frame Relay network ‰ Point of Presence (PoP): Where Telco devices/cables interface with customer premises ‰ DTE (Data Terminal Equipment): Customer device (e.g., router) ‰ Frame Relay switch: Telco device that forwards customer frames (also called DCE [Data Communications Equipment]) ‰ Access link: Physical link between DTE and DCE ‰ DLCI: Data Link Control Identifier, used instead of IP address

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Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ Frame F Relay R l tterms

One Possible Telco Implementation of the Frame Relay Network 61

Figure 7-45

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ Vi Virtual t l Ci Circuit it (VC) (VC): Vi Virtual t l lleased d liline b between t CO and d customer ‰ Telco has to permanently configure VCs so also called Permanent Virtual Circuits (PVCs) ‰ Example: p R1 is HQ router,, Branch are routers at other locations, leased lines connect R1 to each Branch router

Four Physical Circuits Between Routers

Figure 7-46 62

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ Same S d i using design i PVC PVCs iin F Frame R Relay l network t k

Frame Relay Virtual Circuit (VC) Concept; Partial Mesh 63

Figure 7-47

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ P Partial ti l and d ffullll mesh: h F Frame R Relay l full f ll mesh h design d i makes sense when most sites send lots of IP packets to everyy other site ‰ Full mesh: PVC between every pair of Frame Relay routers

Full Mesh of Frame Relay PVCs

Figure 7-48 64

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ E Every packet k t switching it hi ttechnology h l d defines fi protocols t l used to deliver data ‰ Link Access Procedure Frame (LAPF): Frame Relay protocol; often just called Frame Relay ‰ LAPF defines header/trailer ‰ Header includes LAPF address called Data Link Connection Identifier (DLCI)

Frame Relay Header/Trailer: Link Access Procedure Frame (LAPF) 65

Figure 7-49

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ E Example: l 4 routers t where h R1 iis att HQ with ith partial ti l mesh h of PVCs connected to 3 remote routers (R2, R3, R4) ‰ Uses 3 DLCIs that uniquely identifying Frame Relay PVCs to each connection

Example: Three DLCIs, Three PVCs, All Using a Single Access Link 66

Figure 7-50

Understanding Packet Switching and MultiMulti Access WANs: Frame Relay ‰ E Example l off DLCI use: R1 receives i IP packet k t ffrom LAN (not shown) and decides to forward packet to router R2

Example: Frame Relay Frame with DLCI 102 (R2) 67

Figure 7-51

Understanding Packet Switching and MultiMulti Access WANs ‰ Packet P k t Switching S it hi Services: S i SONET speeds d

Name OC-1 OC-3 OC-12 OC-24 OC 24 OC-48 OC-96 OC 192 OC-192

(Rounded) Line Speed (in Mbps) 52 155 622 1244 2488 4976 9952

SONET Optical Carrier (OC) Names and (Rounded) Line Speeds 68

Table 7-5

Understanding Packet Switching and MultiMulti Access WANs ‰ Ti Timeline li ffor WAN ttechnologies h l i (d (dates t are generall and d only meant for comparing some of technologies)

Comparison Timeline of WAN Technologies Mentioned in this Chapter 69

Figure 7-52

Summary, This Chapter… ‰ Compared switched circuits as used for a typical home telephone call with two computers sending data over a similar switched circuit using modems. ‰ Explained the basic differences between a circuit switching WAN service and a packet switching WAN service from the customer’s customer s perspective perspective. ‰ Illustrated the reasons why IP routers work well at forwarding g data between different types yp of LANs and WANs. ‰ Drew common WAN topologies.

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Summary, This Chapter… ‰ Drew and contrasted the different customer-site customer site cabling for a leased line WAN installed between two routers. ‰ Listed the types yp of p physical y links in the US T-carrier hierarchy, their approximate speeds, and the specific number of slowed-speed channels that fit in the next higher-speed line line. ‰ Explained how Telcos use CSU/DSUs to match a leased line speed p to a p physical y DS1 line,, using g an example of a 768 Kbps fractional T1 leased line between two routers. ‰ Compared and contrast the HDLC and PPP standards. 71

Summary, This Chapter… ‰ Explained the differences between packet switching and circuit switching from the Telco perspective. ‰ Used an example p network,, explain p how with Frame Relay, a router can have one physical link connected to the WAN, but send data to many other destination routers. routers ‰ Listed the other WAN packet switching services, and show whether theyy were introduced before or after Frame Relay.

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Questions? Comments?

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