ECS5365 Lecture 2 ISDN Protocols and Standards

1. ECS5365 Lecture 2ISDN Protocols and Standards Philip Branch Centre for Telecommunications and Information Engineering (CTIE) Monash University http://www.anspag.monash.edu.au/~pbranch/lect02.ppt

2. Outline • ISDN protocol stack • ISDN Physical layer • Basic Rate Interface • Primary Rate Interface • ISDN Layer 2 Protocols • LAPD and LAP-B

3. Layered Protocols in ISDN • Each layer provides a service to the above layer • Layer 1 physical layer • transmission issues • Layer 2 data link layer • connection from point to point in a network • Layer 3 network layer • connection between any two points in a network • Layer 4 transport layer • reliable connection between any points in a network

4. ISDN Standards • Physical layer (layer 1) • I.430 (BRI) and I.432 (PRI) • Data link layer (layer 2) • LAPD (Q.291) and LAP-B • Network layer (layer 3) • Q.931 and X.25

5. D Channel • Main function is signalling • Setting up, controling and releasing calls • Low priority packet data • using LAPD and X.25 packet layer • Telemetry (undefined) • eg meter reading

6. B Channel • Main function is user data • Packet switching interface defined • X.25 / LAP-B • Non ISDN terminal interface defined • I.465/V.120 • User is free to define protocols over B channel • eg. Point to Point protocol for IP

7. Basic Rate Physical • Connection between NT1 or NT2 and customer equipment TE1 or TA • Uses two pairs (four wires) • one pair receive, one pair transmit • 2B + D channels are transmitted each frame • Each frame 48 bits long • bit rate 192 kbps • time length of frame 250 microseconds

8. Basic Rate Coding • Pseudoternary coding • negative and positive voltage levels • binary 1 represented by no line signal • binary 0 alternates between positive and negative voltages for synchronisation • extra bits are added to frame to remove dc bias

9. Frame Format • 16 bits per B channel and 4 bits per D channel sent per frame • Full duplex between TE and NT • different frame format for TE to NT and NT to TE • The NT to TE frame echoes the values of the D bits received from the TE

10. Basic Rate Interface Frame Structure • Attachment

11. Multidrop configuration • Possible to use a passive bus to connect up to 8 terminals • Distance limit • 200 metres if more than one terminal per BRI • 1 km if one terminal per BRI • D Channel shared by all terminals • Contention resolution mechanism

12. Contention resolution • Each user terminal transmits 1s on D channel when no information to send • The NT reflects back the the D channel bits • If a terminal wishes to send it waits for a string of 1 bits greater than a threshold • Terminal checks echo bits after sending D channel bits • If they don’t correspond, a collision has occurred and the terminal must retransmit

13. Contention resolution (continued) • Threshold depends on terminal state • String of ‘1’s corresponds to signal absence • Terminal who writes a ‘0’ overrides terminal who writes a ‘1’ • will transmit first • other clients must resume waiting

14. Example of contention resolution • Attachment

15. Threshold in contention resolution • Signalling • normal 8, low 9 • Data • normal 10, low 11 • After successful transmission threshold set low

16. Primary rate - Physical • Uses two pairs (4 wires) • one pair transmit, one pair receive • 30 B+D channels transmitted each frame • Each frame 256 bits long • Bit rate = 2048 kbps • Time length of frame = 125 microseconds • Based on G.703, G.704 transmission standards

17. Primary rate - Coding • Pseudoternary coding - similar to basic rate but opposite assignment of codes • binary 0 - no line signal • binary 1 - alternates between positive and negative voltages for synchronisation • strings of 4 zeros replaced by sequence that violates the rules of alternate priorities • High density bipolar - 3 zeros (HDB3) code

18. Primary rate - Frame format • Frame divided into 32 slots • 1st slot used for frame alignment 0011011 • 30 slots are used for the 30 B channels • 1 slot for the D channel

19. Primary Rate Interface Frame Structure • Attachment

20. Data link layer - D channel • LAPD (Link Access Procedure) for the D channel • Based on LAP-B • developed for X.25 and HDLC • Link Access Protocol - Balanced • Supports multiple user terminals across UNI • Support multiple layer 3 entities • signalling and X.25

21. LAPD services • Unacknowledged information transfer • no flow control or error control • point to point and broadcast • Acknowledged information transfer • similar to HDLC • guarantees all frames delivered in order • sliding window flow control • error control via retransmission

22. LAPD Frame • Messages sent in frames • Flags identify location of frames in bit stream • Address field must cope with multiple user devices per physical interface and multiple layer 3 entities per device

23. TEI • TEI can be set manually by the user or automatically by network • LAPD supports multiple logical connections via Data Link Connection Identifier = combination of TEI and SAPI

24. Protocol Nature • peer-to-peer protocol • terminal equipment and network termination have equal status • balanced operation • once connection is established both sides can send data either side can initiate disconnect

25. Management functions • TEI management • request a TEI number from network • check value of a TEI • remove a TEI assignment • Parameter negotiation • each parameter has a default value • XID command used to change parameter

26. LAPD Frame Structure • 01111110 flag • bit stuffing • Address field • Control field • Information field • Frame Check Sequence (FCS) • 01111110 Flag

27. Address Field • TEI terminal endpoint identifier • SAPI Service Access Point Identifier • C/R command response bit • user side commands 0, responses 1 • network side commands 1, responses 0

28. Control Field • Defines frame type • Information (0) • Supervisory (10) • Unnumbered (11)

29. Information Field • Carries data for layer 3 entities • Packet data if X.25 • Q.931 data if signalling

30. FCS Field • Frame check sequence field • Cyclic redundancy check • Error results in retransmission request

31. Frame Types • Information frames • layer 3 call setup information • flow and error control piggybacked • Supervisory • flow and error control • Unnumbered • link control functions

32. Control field structure • Attachment

33. LAPD commands and responses • Information • Supervisory • RR, RNR , REJ • Unnumbered • SABME, DM, UI, DISCUA FRMR, XID

34. Examples of LAPD operation • Attachment

35. Summary • Physical layer of the BRI and PRI • Contention algorithm in BRI • LAPD format • LAPD messages and operation

36. Preliminary Reading • Signalling in ISDN • Chapter 8 and 10 of Stallings

37. Review Questions from last week • Why don’t all TE1 devices need to connect to NT2 equipment? • The BRI provides 2 B channels and 1 D channel, total 144 kbps. However, a BRI interface is defined at 192 kbps. Why? • In what way might a carrier treat a 64 kbps 8kHz structured speech bearer service differently to a 64kbps, unrestricted, 8kHz structured bearer service? • Which bearer services might be used for G4 fax?

38. Review Questions (not for assessment) • The BRI D channel contention algorithm would fail if any TE1 sent more than 8 consecutive ‘1’s as data over the D channel. Why does this never happen? • Why is the overhead for the BRI so much greater than for the PRI? • Why is there no contention mechanism for PRI? • How would data consisting of the bit sequence 0111110 be coded within a LAPD frame? • Why does LAPD define Supervisory frames for flow and error control when Information frames can piggyback the same information?