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Fall 2005

Virtual Circuit Switching andATM: Asynchronous Transfer

Mode

Qutaibah MalluhiCSE DepartmentQatar University

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Types of WANs

Dedicated-circuit Networks

Switched Networks– Circuit-switched and packet-switched Networks

» Virtual Circuit approach

Local Loop Technologies– E.g., DSL, Cable Modem

Wireless WAN (Cellular Network)

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Dedicated Circuit Networks

Lease circuits from common carriers All connections are point-to-point. A flat fee per month. Unlimited use of the circuit Several standards specified by the telephone

industry in each country– T-Series Carrier Services (US)

» T1: 1.5 Mpbs, T2: 6.312 Mbps, T3: 44.736 Mbps

– E Services (Europe & Qatar)» E1: 2.048 Mbps, E2: 8.448 Mbps, E2 : 34.368 Mbps

– Other high-speed Services » E.g., Optical Carrier (OC) Standard:

51.840 Mbps (OC-1) to 2,488.320 Mbps (OC-48)

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Switched Networks

Circuit-switched networks – Form a dedicated connection (circuit) between two points

– Guaranteed capacity but high-cost (cost is fixed and is independent of the traffic)

– Circuit-switched networks operates over the PSTN (public switched telephone network)

– No link sharing

– E.g. Plain Old Telephone Service (POTS) and Integrated Services Digital Network (ISDN)

Packet switched networks– Data divided into small packets. Each packet is sent

individually

– Link is shared by multiple transmissions

– E.g. IP datagram delivery

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Packet Switching

Datagram Switching– Each packet contains the destination address and

sequence number to each packet– A route is independently chosen for EACH packet.– The packets may arrive out of sequence.– E.g., Ethernet, IP

Virtual Circuit Switching– Similar to dedicated circuits unlike circuit switching,

which is a physical layer technology, it is a Data link layer technology.

– A preplanned route is established before data transmission.

– All packets for one transmission take the same route– Each packet contains a virtual circuit identifier.– E.g. Frame Relay and ATM

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Datagram

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Virtual Circuit

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VCI: Virtual Circuit Identifier

VCI is used to identify a virtual circuit between the sender and the receiver

A small number that has the switch scope Used by a frame between two switches VCI in a data frame changes from one switch to

another

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Switch Forwarding Table

Has a table entry for each VC 4 columns: (In Port, In VCI), (Out VCI, Out Port) Maps an incoming (port, VCI) into an outgoing

(port, VCI)

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Source-to-Destination Data Transfer

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Communication Phases

Sender and receiver go through three phases– Setup

» create virtual circuit forwarding table entries

– Data transfer– Teardown

» Delete VC virtual circuit forwarding table entries

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Types of Virtual Circuits

Permanent Virtual Circuit (PVC)– Forwarding table entries are setup manually by the

administrator– Like a point-to-point leased telephone circuit– No dialing is needed, circuit is always up and ready– Costly (pay even if you are not using it)

Switched Virtual Circuit (SVC)– Dynamic on-demand creation of connections– Exists when data is being transferred. Tear down when

session ends.– Connection (dialing) phase is required (Also called

Signaling)

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SVC Setup Request

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SVC Request Acknowledgement

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ATM: Asynchronous Transfer Mode

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Asynchronous Transfer Mode (ATM)

Designed by phone companies Single technology meant to handle

– Voice– Video– Data

Intended as LAN or WAN

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ATM Characteristics

End-to-end (application to application)

Connection-oriented interface:– Establish “connection”– Send data– Teardown connection

Performance guarantees (statistical) Uses cell switching

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ATM Switch

Building block of ATM network Connections to

– Computers—User-Network Interface (UNI)– Other ATM switches– Network-Network Interface

(NNI) Accepts and forwards cells

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ATM Design Issues

Different traffic has different demands Variable packet size introduces more

jitter (variance in delivery time) Even sending at a constant rate,

contention can result jitter Small packets incur less jitter and

delay, but less efficient Large packets more efficient, delay

and jitter is more serious (packet loss)

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ATM Cell

A cell network uses the cell as the basic unit of data exchange.

A cell is a small, fixed-sized block of information. Size chosen as compromise between voice

(small) and data (large)– 5 octet header– 48 octet payload

Note: size not optimal for any application

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Variable Frame-Size vs. Cell Multiplexing

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ATM Cell Switching

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ATM Cell Formats

NNICell

UNICell

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ATM Cell Format

GFC: Flow control – local flow control (user-to-network only). doesn't appear in

network-to-network interface VPI/VCI: virtual circuit identification

– together provide identification of the cell connection (see later)– Only difference between NNI and UNI cells is that NNI VPI is

a larger field PT - Payload type

– indicates the type of the cell (e.g. user data cell, resource management cell, operation and maintenance cell)

CLP: Cell Loss Priority– one bit specifying whether or not the cell can be dropped (e.g.

voice/video is loss tolerant) HEC - Header Error Control

– 8-bit CRC

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Cell Forwarding

Virtual Circuit Switching Also label switching: Uses label in cell

– Label is used to identify the virtual circuit– Label is specified by the pair: Virtual Path

Identifier/Virtual Channel Identifier (VPI/VCI)

Like standard virtual circuit switching, VPI/VCI is rewritten at each switch

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TP, VP and VC

TP: Transmission Path– Physical connection

VP: Virtual Path– specified by VPI– Set of connections

between two end devices– Path the VC follows

through the network VC: Virtual Circuit

– specified by VCI– All cells of the same

message follow the same VC

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VPI/VCI: Identify a Virtual Connection

Virtual connection is defined by a pair: VPI/VCI

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ATM Switch Forwarding/Rewriting

Forwarding Table

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Example II Of VPI/VCI Rewriting

Sender uses VPI/VCI 3 Receiver uses VPI/VCI 6 Intermediate VPI/VCIs are established within

each switch

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ATM Layers

Physical Layer– Can be one of several physical layer technologies (e.g.

SONET) ATM Layer

– Routing, traffic management, switching and multiplexing

– Receives 48 byte segment from AAL sublayers and transform it into a 53 byte cell

AAL: Application Adaptation Layer– Depend on type of application: data frames, stream of

bits, CBR

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Application Adaptation Layers AAL1:

– Constant bit rate (CBR) – Example: audio

AAL2: – Variable bit rate (VBR)

» Example: video with adaptive encoding– Low bit rate and short packet traffic

» Audio, video, and fax AAL3/4:

– conventional packet switching with sequencing and error control

AAL5: – Used for sending data (not audio/video)– Simple and efficient adaptation layer– No sequencing: assumes cells corresponding to a message

travel sequentially– No error control: left to upper layers (e.g. TCP/IP)

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ATM Quality Of Service

Specified when connection established Endpoint specifies

– Type of data transfer– Throughput desired– Maximum packet burst size– Maximum delay tolerated

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Issues

More expensive than traditional LAN technology More connection setup time Cell tax (header/data ~= 10%) Need to specify service requirement at the

connection, some may not know which to specify Lack of efficient broadcast Complexity of QoS (Quality of Service): one can

specify the request, but hard to enforce it Assumption of homogeneity