1 Copyright © Monash University ECS5365 Lecture 2 ISDN Protocols and Standards Philip Branch Centre for Telecommunications and Information Engineering (CTIE) Monash University

2 Copyright © Monash University Outline ISDN protocol stack ISDN Physical layer –Basic Rate Interface –Primary Rate Interface ISDN Layer 2 Protocols –LAPD and LAP-B

3 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University Basic Rate Interface Frame Structure Attachment

11 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University Example of contention resolution Attachment

15 Copyright © Monash University Threshold in contention resolution Signalling –normal 8, low 9 Data –normal 10, low 11 After successful transmission threshold set low

16 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University Primary rate - Frame format Frame divided into 32 slots 1st slot used for frame alignment slots are used for the 30 B channels 1 slot for the D channel

19 Copyright © Monash University Primary Rate Interface Frame Structure Attachment

20 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University 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 Copyright © Monash University LAPD Frame Structure flag –bit stuffing Address field Control field Information field Frame Check Sequence (FCS) Flag

27 Copyright © Monash University 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 Copyright © Monash University Control Field Defines frame type –Information (0) –Supervisory (10) –Unnumbered (11)

29 Copyright © Monash University Information Field Carries data for layer 3 entities Packet data if X.25 Q.931 data if signalling

30 Copyright © Monash University FCS Field Frame check sequence field Cyclic redundancy check Error results in retransmission request

31 Copyright © Monash University Frame Types Information frames –layer 3 call setup information –flow and error control piggybacked Supervisory –flow and error control Unnumbered –link control functions

32 Copyright © Monash University Control field structure Attachment

33 Copyright © Monash University LAPD commands and responses Information Supervisory –RR, RNR, REJ Unnumbered –SABME, DM, UI, DISCUA FRMR, XID

34 Copyright © Monash University Examples of LAPD operation Attachment

35 Copyright © Monash University Summary Physical layer of the BRI and PRI Contention algorithm in BRI LAPD format LAPD messages and operation

36 Copyright © Monash University Preliminary Reading Signalling in ISDN Chapter 8 and 10 of Stallings

37 Copyright © Monash University 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 Copyright © Monash University 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 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?