I sent this to Priscilla on the topic and she suggested that the group might benefit from my response, so here it is.
Priscilla, I think that you may find it helpful to separate end - to - end data transfer from signalling. Very few L2 protocols offer error correction. The modern approach is to require the L1 transmission to provide intrinsically reliable communication and hence it is a waste of bandwidth to implement error correction both on hop by hop and end to end basis as per X.25. Modern WAN digital transmission systems are designed to offer transmission error rates of fewer than 1 bit error in 10^9 bits. On Telco Wan links it is common on this side of the pond to require transmission media to offer error rates better than 1 in 10^9 and often 1 in 10^11. Indeed the commissioning tests call for fewer than 1 error in a 20 minute period on a basic E3 (34 Mb) link and fewer than 1 error in 24 hours on International links prior to acceptance from Transmission into Networks for operational trunks. That is not to say that links may not degrade but if the error rates became worse than 1 in 10^9 it would be time for Network operations to call 'holes & poles' (Transmission) to fix it. The fundamental assumptions in both Frame Relay and ATM is that they are running over intrinsically reliable transmission media. The low error rates being achieved either by correctly engineered transmission paths or by the use of significant forward error correction built in to the transmission equipment. ATM, and Frame Relay, implement error correction, or more precisely re- transmission in the interface to the signalling protocols. ISDN relies on the hop by hop error correction offered by LAPD. However, they tend to leave the issue of payload error correction to any high level end-to- end protocols being run on top of these L2 Datalinks. ATM offers no direct protection of payload content, the HEC only protects the ATM header. However, some AALs do offer protection if not correction of the payload. Even AAL5 - most common for IP has a check polynomial (CRC32) to protect the CS PDU. It performs error detection but not correction. In the case of Q.2931, SAAL (version of AAL5 to carry signalling) will detect faulty PDUs. If you want to look at ATM signalling take a look at Q.2931 essentially an enhanced and extended version of narrow band ISDN Q.931 signalling. Take a look at the ATM forum website. www.atmforum.org Frame Relay has Frame Check Sequence that again will detect faulty frames. (Incidentally Carrier Switches tend to drop frames with a faulty FCS). Incidentally Frame Relay is sometimes known as LAPF. Take a look at the frame relay forum web site. www.frforum.org there are some good white papers and the frf's recommendations that you can download. ISDN B channel - is a 64 Kbit clear channel and the network makes no assumptions about the contents. It could be any number of data formats or indeed it could be 64 K G.711 PCM voice. The most ubiquitous use of data over ISDN is to encapsulate it in PPP which is intrinsically multi- protocol. However, it is also possible to use HDLC, X.25, Frame Relay, or any number of specialist protocols. D channel usage is somewhat different. L2 on D channel is Q.921 (as you say also known as LAPD). It is perhaps worth pointing out the ISDN signalling is NOT an end to end protocol! ISDN signalling only traverses the single hop to the signalling processor on the nearest switch. This signalling processor then signals to the signalling processor of the next switch and finally the signalling processor on the last switch communicates with the far end CPE. In Public Carrier Networks the signalling between switches is normally SS7 or C7 as it is sometimes known. The D channel is normally used for signalling but in the case of Basic Rate may also be used for permanently on low speed data services such as X.31 (9k6 X.25 in D channel, which uses LAPD for L2 and normal X.25 L3) Q.931 is used on public networks to communicate with the Carrier's CO switch and is fairly primitive in its feature set. QSIG is essentially a superset of Q.931 used on private telephony networks to signal between PABXs and offers an enhanced set of features such as 'camp on extension', 'ring back when free', redirect calls etc. X.25 has hop by hop error detection and correction in L2 - LAPB and also end to end in the L3. Sometimes known as 'belt and braces' or 'The Pony Express' of data communications. 'We get the data through, eventually, no matter how crummy the analogue link is!' Not being of IBM extraction I am not in a position to comment on SDLC or Bisync. I hope that this helps Peter -- Peter Whittle Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=27659&t=27568 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
