mickey@altos86.Altos.COM (Michael Thompson) (07/21/89)
The 1984 CCITT recommendation for X.25 (et al) mentions multilink protocol as a link level protocol for X.25. It does not go into specifics (of course) about the protocol. What are the major differences between multilink and HDLC? When is multilink used instead of HDLC? Is it reasonable to expect it to be used over the same medium (RS232) as HDLC? Where can I get documentation on this protocol? Thanks very much for your help.. -Michael Thompson ..sun!altos86!illogica!mickey
hanlon@APOLLO.COM (OSI Steamroller is a comin') (07/21/89)
New for 1984 CCITT X.25, Multilink Protocol performs the function of
distributing packets across available data links
and resequencing packets for delivery to the packet level.
This procedure has been available for sometime in X.75 and it allows
the grouping of multiple physical lines at Level 2 (data link) X.25
to appear as a single logical line (benefits: throughput,
resiliency among others) to Level 3 (packet) X.25.
This protocol is carried in a two-octet field within an I-frame
of LAP-B (LAP doesn't support Multilink). The protocol's main
responsibilities are to maintain sequencing, prevent duplication,
and protect against packet loss.
Details of the protocol should be in the CCITT X.25 1984 spec
(in my copy it's in section 2.5). CCITT Multilink used ISO DIS 7478
"Multilink Control Procedure" as the reference.
I'm not aware of any PDNs/VANs that support mutlilink at X.25 but
I've been out of the X.25 arena for a couple of years....
Bart Hanlon UUCP: ...{mit-eddie,yale,uw-beaver}!apollo!hanlon
Apollo Division/Hewlett Packard INTERNET: hanlon@apollo.com
Chelmsford, MA 01824 Phone: (508) 256-6600
-------levin@pong11.uucp (Larry Levin) (07/21/89)
In article <3552@altos86.Altos.COM> mickey@altos86.Altos.COM (Michael Thompson) writes: > >The 1984 CCITT recommendation for X.25 (et al) mentions multilink protocol >as a link level protocol for X.25. It does not go into specifics (of course) >about the protocol. What are the major differences between multilink and >HDLC? When is multilink used instead of HDLC? Is it reasonable to expect >it to be used over the same medium (RS232) as HDLC? Where can I get >documentation on this protocol? > The Multilink Procedure (MLP) is defined in the CCITT X.25 standard. Look at Section 2.5 of the RED Book X.25 Standard. MLP allows the use of multiple parallel physical circuits between a single DTE/DCE pair. I have never used it (or seen it implemented) but I imagine that is use would be in cases where only low speed lines are available. I group 4 2400 bps lines and get an aggregate approaching 9.6 kpbs. Hope this helps. Larry Levin ISED, SRI levin@spam.istc.sri.com
Christian.Huitema@MIRSA.INRIA.FR (Christian Huitema) (07/25/89)
A multilink protocol has been used betwen the Transpac nodes from the beginning. At that time (75), the max speed one could get was 72kbps on a 48khz "primary group" (analog). The multilink protocol allowed them to go for higher throughputs by grouping several such links on the heavily charged arteries, and to obtain better security of operation: if one link broke, the packets would be resent on the other links of the group, without loosing the virtual circuits. Indeed, this kind of protocol is much less useful today, with the generalisation of digital telephony: the equivalent of an analog "primary group" is a T1 link, and the network is more reliable. However, there still are cases where one has only the choice between 64 kbps and full T1, and would like to use 256 kbps only; and have networks ever been reliable enough? Christian Huitema PS. The multilink procedure referred to in X.25 1984 should not be confused with a multilink procedure analysed by Guy Pujolle and Otto Spaniol 10 years ago (numerous publications). They had the idea of splitting a high speed and long delay link into several subchannels in order to control it more efficiently.