1. Data Communication and Networks Lecture 6 Networks: (Packet Switching: X.25. ATM, Frame Relay) October 10, 2002 Joseph Conron Computer Science Department New York University conron@cs.nyu.edu

2. X.25 z1976 zInterface between host and packet switched network zAlmost universal on packet switched networks and packet switching in ISDN zDefines three layers yPhysical yLink yPacket

3. X.25 - Physical zInterface between attached station and link to node zData terminal equipment DTE (user equipment) zData circuit terminating equipment DCE (node) zUses physical layer specification X.21

4. X.25 - Link zLink Access Protocol Balanced (LAPB) ySubset of HDLC ysee chapter 7

5. X.25 - Packet zExternal virtual circuits zLogical connections (virtual circuits) between subscribers

6. X.25 Use of Virtual Circuits

7. Virtual Circuit Service zVirtual Call yDynamically established zPermanent virtual circuit yFixed network assigned virtual circuit

8. Virtual Call

9. Packet Format

10. Multiplexing zDTE can establish 4095 simultaneous virtual circuits with other DTEs over a single DTC-DCE link zPackets contain 12 bit virtual circuit number

11. Virtual Circuit Numbering

12. Flow and Error Control zHDLC at the link layer(Chapter 7) zSliding window at the VC layer

13. Packet Sequences zComplete packet sequences zAllows longer blocks of data across network with smaller packet size without loss of block integrity zA packets yM bit 1, D bit 0 zB packets yThe rest zZero or more A followed by B

14. Reset and Restart zReset yReinitialize virtual circuit ySequence numbers set to zero yPackets in transit lost yUp to higher level protocol to recover lost packets yTriggered by loss of packet, sequence number error, congestion, loss of network internal virtual circuit zRestart yEquivalent to a clear request on all virtual circuits yE.g. temporary loss of network access

15. Asynchronous Transfer Mode (ATM)

16. Protocol Architecture zSimilarities between ATM and packet switching yTransfer of data in discrete chunks yMultiple logical connections over single physical interface zIn ATM flow on each logical connection is in fixed sized packets called cells zMinimal error and flow control yReduced overhead zData rates (physical layer) 25.6Mbps to 622.08Mbps

17. Protocol Architecture (diag)

18. ATM Logical Connections zVirtual channel connections (VCC) zAnalogous to virtual circuit in X.25 zBasic unit of switching zBetween two end users zFull duplex zFixed size cells zData, user-network exchange (control) and network- network exchange (network management and routing) zVirtual path connection (VPC) yBundle of VCC with same end points

19. ATM Connection Relationships

20. Call Establishment Using VPs

21. VP/VC Characteristics zQuality of service zSwitched and semi-permanent channel connections zCall sequence integrity zTraffic parameter negotiation and usage monitoring zVPC only yVirtual channel identifier restriction within VPC

22. ATM Cells zFixed size z5 octet header z48 octet information field zSmall cells reduce queuing delay for high priority cells zSmall cells can be switched more efficiently zEasier to implement switching of small cells in hardware

23. ATM Cell Format

24. Header Format zGeneric flow control yOnly at user to network interface yControls flow only at this point zVirtual path identifier zVirtual channel identifier zPayload type ye.g. user info or network management zCell loss priority zHeader error control

25. Generic Flow Control (GFC) zControl traffic flow at user to network interface (UNI) to alleviate short term overload zTwo sets of procedures yUncontrolled transmission yControlled transmission zEvery connection either subject to flow control or not zSubject to flow control yMay be one group (A) default yMay be two groups (A and B) zFlow control is from subscriber to network yControlled by network side

26. Single Group of Connections (1) zTerminal equipment (TE) initializes two variables yTRANSMIT flag to 1 yGO_CNTR (credit counter) to 0 zIf TRANSMIT=1 cells on uncontrolled connection may be sent any time zIf TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections) zIf HALT received, TRANSMIT set to 0 and remains until NO_HALT

27. Single Group of Connections (2) zIf TRANSMIT=1 and no cell to transmit on any uncontrolled connection: yIf GO_CNTR>0, TE may send cell on controlled connection xCell marked as being on controlled connection xGO_CNTR decremented yIf GO_CNTR=0, TE may not send on controlled connection zTE sets GO_CNTR to GO_VALUE upon receiving SET signal yNull signal has no effect

28. Header Error Control z8 bit error control field zCalculated on remaining 32 bits of header zAllows some error correction

29. HEC Operation at Receiver

30. Cell Based Physical Layer zNo framing imposed zContinuous stream of 53 octet cells zCell delineation based on header error control field

31. Cell Delineation State Diagram

32. ATM Service Categories zReal time yConstant bit rate (CBR) yReal time variable bit rate (rt-VBR) zNon-real time yNon-real time variable bit rate (nrt-VBR) yAvailable bit rate (ABR) yUnspecified bit rate (UBR)

33. Real Time Services zAmount of delay zVariation of delay (jitter)

34. ATM Adaptation Layer zSupport for information transfer protocol not based on ATM zPCM (voice) yAssemble bits into cells yRe-assemble into constant flow zIP yMap IP packets onto ATM cells yFragment IP packets yUse LAPF over ATM to retain all IP infrastructure

35. Adaptation Layer Services zHandle transmission errors zSegmentation and re-assembly zHandle lost and mis-inserted cells zFlow control and timing

36. Frame Relay zDesigned to be more efficient than X.25 zDeveloped before ATM zLarger installed base than ATM zATM now of more interest on high speed networks

37. Frame Relay Background - X.25 zCall control packets, in band signaling zMultiplexing of virtual circuits at layer 3 zLayer 2 and 3 include flow and error control zConsiderable overhead zNot appropriate for modern digital systems with high reliability

38. Frame Relay - Differences zCall control carried in separate logical connection zMultiplexing and switching at layer 2 yEliminates one layer of processing zNo hop by hop error or flow control zEnd to end flow and error control (if used) are done by higher layer zSingle user data frame sent from source to destination and ACK (from higher layer) sent back

39. Advantages and Disadvantages zLost link by link error and flow control yIncreased reliability makes this less of a problem zStreamlined communications process yLower delay yHigher throughput zITU-T recommend frame relay above 2Mbps

40. User Data Transfer zOne frame type yUser data yNo control frame zNo inband signaling zNo sequence numbers yNo flow nor error control