Ch 18. Virtual Circuit Networks

Old WAN Technology

• X.25– Virtual-circuit switching network (designed in 1970s)– Operating at the network layer and below additional encapsulation of IP packets

– Low data rate: 64 Kbps– Large overhead: flow & error controls at multiple layers

• Leasing T-1 or T-3 lines– Costly, since it requires n(n-1)/2 lines to connect n

branches (mesh networks)– Inefficient and cannot handle “bursty data”

18.1 Frame Relay• Virtual-circuit WAN – Higher speed: 1.544 Mbps or 44.376 Mbps– No network layer: operates in PHY and data link layers– Allows bursty data– Support large frame up to 9000 bytes– Error detection only (no flow control or error control)

Architecture• Virtual circuits– A VC is identified by data link connection id (DLCI)– Support both permanent and switched virtual circuits– Switch has a table with (incoming-port, DLCI) pairs as we have

discussed in Chapter 8• Data link layer– Addressing (DLCI)– Congestion control– Error detection

• Physical layer– No specific protocol is defined

Frame Format

• DLCI: address• Command/response (C/R):

not used• Extended address (EA)• Forward(destination) explicit

congestion notification (FECN)

■ Backward(sender) explicit congestion notification (BECN)

■ Discard eligibility (DE): priority level of frame when a frame needs to be discarded

Extended Address

• To increase the range of DLCI, the Frame Relay address have been extended from the original 2-byte address to 3- or 4-byte addresses.

• Three address formats by setting EA

Other Options• To handle frames arriving from other prototocls .Frame

relay uses assembler/disassembler (FRAD)– Mux/demux frames from other protocols

• Voice over frame relay (VOFR), voice is digitized (PCM) and the compressed– Quality is not good as a circuit-switched network

• Originally Frame relay was designed to provide PVC connections. Local management information (LMI) is added to provide more management features– Keep-alive (check if data is flowing), multicast, switch status check

18.2 ATM

• Goals of Asynchronous Transfer Mode (ATM)– High data rate (e.g., transmission over optical fiber)– Interface with existing systems providing WAN inter-

connectivity– Low-cost implementation– Inter-operability with legacy telephone systems– Connection-oriented for accurate and predictable

delivery– Move most functions to hardware

Cell Networks

• Problem of existing systems– Multiplexing frames of different size can lead to

“unfair delay” for small frames

• Cell (a small fixed-size data block) network that uses the cell as the basic unit of data exchange can solve the problem

ATM Architecture

• Uses asynchronous time-division multiplexing

• User-to-network interface (UNI) and network-to-network interface (NNI)

Virtual Connection• Virtual circuit (VC)– A single message flow

• Virtual path (VP)– A set of VCs

between two switches• Transmission path (TP)– Physical connection between two end points

ATM Cell

• A virtual circuit is uniquely identified by a pair of (VPI, VCI)

• Cell (the basic data unit) includes (VPI, VCI)

• Switching/routing

Layering Structure

• PHY can be any physical layer– Originally intended for SONET

• ATM layer– Routing, traffic management, switching, multiplexing

• Application adaptation layer (AAL)– Data segmentation to fit in a cell

ATM Header• Header format

Generic flow control (GFC): UNI level

VPI and VCI

Payload type (PT)

Cell loss priority (CLP): congestion control

Header error correction (HEC): CRC to correct errors

AAL Layer – AAL1

• Support any type of payload– Four versions: AAL1, AAL2, AAL3/4, AAL5

• AAL1– Support applications with constant bit rates

- Sequence number (SN): 4-bit

- Sequence number protection (SNP): 4-bit

AAL Layer – AAL2• AAL2– Intended to support variable-data-rate bit stream– Used for low-bit-rate traffic and short-frame traffic

AAL Layer – AAL3/4

• AAL3/4– For connection-

oriented and connectionless services

AAL Layer – AAL5

• AAL5– Simple and efficient adaptation layer (SEAL)

ATM Layers in Devices and Switches

• ATM has a congestion control and quality of service

18.3 ATM LANs

• Adopt ATM technology to LANs– High data rate– Support permanent and temporary connections– Support multimedia traffic with different bandwidths

• Pure ATM and Legacy ATM LANs

ATM LANs• Mixed architecture

• Issues in LAN Emulation (LANE)– Connectionless versus connection-oriented– Physical addresses versus virtual-circuit identifiers– Multicasting and broadcasting– Interoperability

LAN Emulation (LANE)

• Client/Server model– LAN emulation client (LEC): Let upper layers

unaware of the existence of the ATM technology■ LAN emulation configuration server (LECS):

initial connection between the client and LANE

■ LAN emulation server (LES): create virtual circuit for a request of frame delivery

■ Broadcast/unknown server (BUS): responsible for multicast and broadcast service

Example of Mixed Architecture

Homework

• Exercise– 17– 20– 21– 28