I was wrong. I looked it up last night and there is a seq. number in the control field of LAPB, HDLC, and LABD. Both, the sending and receiving stations must keep the same seq. numbers when transmitting, but I cannot find anything on retransmission at that layer. But I asked an old IBM guy I used to work with and he said that SDLC and all the related layer two protocols do require retrans when bad packets are found or missing. So I would assume that LAPD layer two is reliable. And as everyone else said, the SS7 signalling (Q.931) is just control and status messages over D channel.
And B channel is a different retrans technique, based upon the higher layer protocols it carries. If an ISDN frame gets corrupt, both channels will be retransmitted, but by differnt methods. So ISDN D channel is inherently reliable at layer two and B channel is reliable only if that higher layer protocol is. ""Peter Whittle"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > 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=27694&t=27568 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
