Chapter 18. Virtual Circuit Switching: Frame Relay and ATM
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Transcript of Chapter 18. Virtual Circuit Switching: Frame Relay and ATM
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Fall 2006 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
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Fall 2006 Computer Networks 18-2
Wide area network and switching methods
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Fall 2006 Computer Networks 18-3
Circuit switching
• Create a real circuit (dedicated line) between source and destination
• Physical layer technology
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Fall 2006 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
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Fall 2006 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
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Fall 2006 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
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Fall 2006 Computer Networks 18-7
VCI Phases
• Two approaches for the VC setup– Permanent virtual circuit (PVC)– Switched virtual circuit (SVC): setup, data transfer, teardown
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Fall 2006 Computer Networks 18-8
Data Transfer Phase
• All switches need to have a table entry for the virtual circuit
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Fall 2006 Computer Networks 18-9
Data Transfer using VCI
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Fall 2006 Computer Networks 18-10
SVC Setup: Request and Acknowledgment
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Fall 2006 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
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Fall 2006 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
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Fall 2006 Computer Networks 18-13
Frame Relay vs. T-line Network
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Fall 2006 Computer Networks 18-14
Frame Relay vs. X.25 Network
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Fall 2006 Computer Networks 18-15
Frame Relay Layers
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Fall 2006 Computer Networks 18-16
Comparing Layers: X.25 & Frame Relay
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Fall 2006 Computer Networks 18-17
Frame Relay Frame
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Fall 2006 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
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Fall 2006 Computer Networks 18-19
BECN
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Fall 2006 Computer Networks 18-20
FECN
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Fall 2006 Computer Networks 18-21
Four Cases of Congestion
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Fall 2006 Computer Networks 18-22
ATM
• Asynchronous Transfer Mode
• ATM is the cell relay protocol
• ATM uses asynchronous TDM
• Cells are transmitted along virtual circuits
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Fall 2006 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
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Fall 2006 Computer Networks 18-24
Synchronous vs. Asynchronous TDM
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Fall 2006 Computer Networks 18-25
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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Fall 2006 Computer Networks 18-26
VPI and VCI: Hierarchical Switching
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Fall 2006 Computer Networks 18-27
Identifiers and Cells
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Fall 2006 Computer Networks 18-28
VP Switch and VPC Switch
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Fall 2006 Computer Networks 18-29
ATM Layers
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Fall 2006 Computer Networks 18-30
ATM Layer and Headers
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Fall 2006 Computer Networks 18-31
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)
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Fall 2006 Computer Networks 18-32
AAL1
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Fall 2006 Computer Networks 18-33
AAL2
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Fall 2006 Computer Networks 18-34
AAL3/4
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Fall 2006 Computer Networks 18-35
AAL5
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Fall 2006 Computer Networks 18-36
ATM LAN
• Connectionless versus connection-oriented
• Physical addresses versus virtual connection identifiers
• Multicasting and broadcasting delivery