Fall 2005 Computer Networks 18-1

Chapter 18. Virtual Circuit Switching:Frame Relay and ATM

18.1 Virtual Circuit Switching

18.2 Frame Relay

18.3 ATM

Fall 2005 Computer Networks 18-2

Wide area network and switching methods

Fall 2005 Computer Networks 18-3

Circuit switching

• Create a real circuit (dedicated line) between source and destination

• Physical layer technology

Fall 2005 Computer Networks 18-4

Packet Switching: Datagram Approach

• Mostly used in the network layer

• Routing (selecting the best route for a packet) is performed at each router

Fall 2005 Computer Networks 18-5

Packet Switching: Virtual Circuit Approach

• Packets (frames) are switched along a pre-determined path from source to destination

• Data link layer technology

Fall 2005 Computer Networks 18-6

Virtual Circuit Identifier

• Virtual circuit network has two addresses– Global address which is unique in the WAN– Virtual circuit identifier which is actually used for data transfer

• VCI has switch scope; it is used between two switches

• Each switch can use its own unique set of VCIs

Fall 2005 Computer Networks 18-7

VCI Phases

• Two approaches for the VC setup– Permanent virtual circuit (PVC)– Switched virtual circuit (SVC): setup, data transfer, teardown

Fall 2005 Computer Networks 18-8

Data Transfer Phase

• All switches need to have a table entry for the virtual circuit

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Data Transfer using VCI

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SVC Setup: Request and Acknowledgment

Fall 2005 Computer Networks 18-11

Frame Relay

• Frame Relay is a virtual circuit wide area network

• VCIs in Frame Relay are called DLCI(Data Link Connection Identifier)s

Fall 2005 Computer Networks 18-12

Frame Relay Features

• Frame relay operates at a higher speed. It can easily be used instead of a mesh of T-1 or T-3 lines

• Frame relay operates just the physical and data link layers. It is good as a backbone to provide services to protocols that already have a network layer protocol, such as Internet

• It allows bursty data

• It allows a frame size of 9000 bytes accommodating all LAN frame sizes

• It is less expensive than other traditional WANs

• It has error detection at the data link layer only. There is no flow control pr error control

Fall 2005 Computer Networks 18-13

Frame Relay vs. T-line Network

Fall 2005 Computer Networks 18-14

Frame Relay vs. X.25 Network

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Frame Relay Layers

Fall 2005 Computer Networks 18-16

Comparing Layers: X.25 & Frame Relay

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Frame Relay Frame

Fall 2005 Computer Networks 18-18

Congestion Control

• Frame relay requires congestion control, because– Frame Relay does not have a network layer– No flow control at the data link layer– Frame Relay allows the user to transmit bursty data

• Congestion avoidance– Two bits in the frame are used– BECN(Backward Explicit Congestion Notification)– FECN(Forward Explicit Congestion Notification)

• Discarding: Traffic control

Fall 2005 Computer Networks 18-19

BECN

Fall 2005 Computer Networks 18-20

FECN

Fall 2005 Computer Networks 18-21

Four Cases of Congestion

Fall 2005 Computer Networks 18-22

ATM

• Asynchronous Transfer Mode

• ATM is the cell relay protocol

• ATM uses asynchronous TDM

• Cells are transmitted along virtual circuits

Fall 2005 Computer Networks 18-23

Multiplexing using Cells

• The variety of packet sizes makes traffic unpredictable

• A cell network uses the cell as the basic unit of data exchange– A cell is defined as a small, fixed-sized block of information– Cells are interleaved so that non suffers a long delay– A cell network can handle real-time transmissions– Network operation is more efficient and cheaper

Fall 2005 Computer Networks 18-24

Synchronous vs. Asynchronous TDM

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Virtual Connection

• Connection between two endpoints is accomplished through– Transmission path (TP)– Virtual path (VP)– Virtual circuit (VC)

• A virtual connection is defined by a pair of numbers: VPI and VCI

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VPI and VCI: Hierarchical Switching

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Identifiers and Cells

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VP Switch and VPC Switch

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ATM Layers

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ATM Layer and Headers

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Application Adaptation Layer (AAL)• Convert data from upper-layer into 48-byte data units for the ATM cells• AAL1 – constant bit rate (CBR) video and voice• AAL2 – variable bit rate (VBR) video and voice• AAL3/4 – connection-oriented/connectionless data• AAL5

– Sequencing and error control mechanisms are included in the upper layers

– SEAL (Simple and Efficient Adaptation Layer)

Fall 2005 Computer Networks 18-32

AAL1

Fall 2005 Computer Networks 18-33

AAL2

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AAL3/4

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AAL5

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ATM LAN

• Connectionless versus connection-oriented

• Physical addresses versus virtual connection identifiers

• Multicasting and broadcasting delivery