1 Fall 2005 Virtual Circuit Switching and ATM: Asynchronous Transfer Mode Qutaibah Malluhi CSE Department Qatar University

2 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)

3 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: Mbps, T3: Mbps –E Services (Europe & Qatar) »E1: Mbps, E2: Mbps, E2: Mbps –Other high-speed Services »E.g., Optical Carrier (OC) Standard: Mbps (OC-1) to 2, Mbps (OC-48)

4 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

5 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

Datagram

Virtual Circuit

8 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

9 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)

10 Source-to-Destination Data Transfer

11 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

12 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)

13 SVC Setup Request

14 SVC Request Acknowledgement

15 ATM: Asynchronous Transfer Mode

16 Asynchronous Transfer Mode (ATM)  Designed by phone companies  Single technology meant to handle –Voice –Video –Data  Intended as LAN or WAN

17 ATM Characteristics  End-to-end (application to application)  Connection-oriented interface: –Establish “connection” –Send data –Teardown connection  Performance guarantees (statistical)  Uses cell switching

18 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

19 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)

20 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

21 Variable Frame-Size vs. Cell Multiplexing

22 ATM Cell Switching

23 ATM Cell Formats NNI Cell UNI Cell

24 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

25 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

26 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

27 VPI/VCI: Identify a Virtual Connection Virtual connection is defined by a pair: VPI/VCI

28 ATM Switch Forwarding/Rewriting Forwarding Table

29 Example II Of VPI/VCI Rewriting  Sender uses VPI/VCI 3  Receiver uses VPI/VCI 6  Intermediate VPI/VCIs are established within each switch

30 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

31 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)

32 ATM Quality Of Service  Specified when connection established  Endpoint specifies –Type of data transfer –Throughput desired –Maximum packet burst size –Maximum delay tolerated

33 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