>"VoIP Guy" wrote, I was wrong. > >I looked it up last night and there is a seq. number in the control field of >LAPB, HDLC, and LABD.
No problem with LAPB and LAPD, which are proper subsets of full HDLC. Again, full HDLC is more of an architecture -- I've never known ANYONE to implement ever feature of it. >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. SS7 doesn't use Q.931, but SSCOP (don't ask me what the ITU number is). SSCOP is meant as the ultimately reliable protocol -- it allows multilink operation, so retransmission requests are made only if no frame with a correct CRC arrives on any link. > >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. See a few comments below > > >""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. You can visualize it as: Q.931\ ----Switch------SS7 QSIG / | | H.323---Gateway--------+ (SIGTRAN, MEGACO) > > >> >> X.25 has hop by hop error detection and correction in L2 - LAPB and also > > end to end in the L3. Recommendations for the use of X.25, however, deprecate packet level retransmission other than for buffering problems. There is no packet level error checking. >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=27757&t=27568 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
