Talk:X.25

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--86.132.179.112 15:51, 31 July 2007 (UTC) Can anyone confirm the source of this statement?

"For much of its history X.25 was used for permanent virtual circuits (PVCs) to connect two host computers in a dedicated link. This was common for applications such as banking, where distant branch offices could be connected to central hosts for a cost that was considerably lower than a permanent long distance telephone call. X.25 was typically billed as a flat monthly service fee depending on link speed, and then a price-per-packet on top of this. Link speeds varied, typically from 2400bit/s up to 2 Mbit/s, although speeds above 64 kbit/s were uncommon in the public networks."

I've heard something similar from the lips of a CCIE as well so this seems to be an urban myth that's going unchallenged.

Someone more cynical than I might suggest that the telecoms/network industry would like this to become a well established "truth" so people don't question why switched virtual circuits aren't offered on more modern services like MPLS, ATM etc. As this would allow reselling of capacity, usage based costs and a cheaper alternative to fixed circuits. Even today with much reduced national and international telecoms prices that would be a revenue impact.

SVC services are also more difficult to implement in engineering switch design and are less reliable in operation i.e. network capacity planning.

And avoid the nightmare of usage based tariffs and billing that go with switched virual services.

So it's understandable why "PVCs have always been used in networks" would be a message everyone would want to gain acceptance.

X.25 switches in the early eighties could barely cope with the traffic carried and with the buffer allocation demands of PVCs the operators soon learned that PVCs needed to be priced very highly as large numbers damaged revenues and impacted overall network service by using up switch capacity. Some operators even stopped taking orders for PVCs as the impact was so significant.

Even the example cited above of branch network would best be implemented as a SVC solution as more resilience would be available via call re-direction. Most network traffic was dumb termainal X.3/X.28/X.29 based and that was definetely SVC based traffic.

PVCs I think were not in the X.75 interface specification and none of the PTTs/operators offered them internationally as far as I am aware.

I think the more truthful and historically accurate statement would be that X.25 networks were overwhelmingly SVC based but later network technologies like MPLS, ATM, SMDS and Frame Relay learnt the lessons of X.25 and went for a simpler PVC offering (i.e. what was offered as a commercially available service at reasonable cost to the majority). For the reasons of engineering, network operation and billing complexity as described above.

If you care about being accurate then the statement in the article about PVCs needs to be changed.

Shouldn't this line:

"Speeds increased during the years, typically up to 48 or 96 kbit/s" read 4.8 or 9.6Kbps instead of 48 and 96?

I think the history of X.25 is too oriented on UK history. France for example made a lot of work to establish X.25 as a standard. TRANSPAC was also the first X.25 packet-switched commercial network in the world.134.214.165.97 18:14, 24 January 2006 (UTC)

Shouldn't "data terminating equipment" be "data terminal equipment"?

JTE, UK

Correct - in CCITT (now ITU-T) literature DTE is Data Terminal Equipment (and DCE is always Data Circuit Terminating Equipment, not the more colloguial Data Communications Equipment).

"X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware".

I disagree fundamentally with that opening statement. X.25 is the __recommendation__ (ie definition) for an __interface__ between a DTE and DCE "for terminals operating in the packet mode and connected to public data networks by dedicated circuit". Specifically it was not / is not a protocol suite for "wide area networks". Such networks were frequently constructed using X.25 with proprietary extensions to cover the missing parts of the protocol. Such networks were complex to maintain and manage because routing, recovery from route failure and private numbering schemes were specifically not covered by the X.25 recommendations ITU-T did not concern itself with the details of network internals. It specifies interfaces into and between network providers including exchange of (some) management, operation and charging/billing details. ITU-T work was delibterately called Recommendations not Standards because they were often a minimum set of rules to obtain a degree of international operation.

Speeds: For the UK, in the early 80's access to X.25 was provided using a V-series private-wire modem link at speeds of 2400, 4800 and 9600bps. High speed "group band" modems provided 48kbps access (at great expense, in a 48kHz analogue channel). When BT first introduced Public Packet Switching, the core of the network ran at 48kbps using V.35 modems. Public PADs provided dial-in async terminal access at 200 or 300bps full duplex (V.21) and 75/1200 (V.23) assymetric full duplex. I believe Germany and France had digital access services using X.21 circuit switching.

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[edit] In popular culture

In the movie Father of the Bride, the groom's character is a consultant who helps European banks link their x.25 networks with US-based banks' IP networks. Does this merit inclusion in the article? Jaysbro 20:26, 21 September 2006 (UTC)

Probably not, unless that fact is significant other than something mentioned in passing; I suspect most viewers of the movie would have little idea what an X.25 network is, and would only know what an IP network is because they might have seen references to "TCP/IP" in articles or in the parts of the UI of their computer that they don't like to use. :-) Guy Harris 00:18, 14 November 2006 (UTC)

[edit] X.25 protocols vs. OSI protocols

How many of the protocols listed there were expected to run primarily or solely over X.25? Most stuff above the transport layer was presumably supposed to run over the OSI transport layer protocols, not over X.25; were the transport protocols expected to run directly above X.25, or above OSI network-layer protocols? Guy Harris 00:21, 14 November 2006 (UTC)

---yaaaaa...gr8 work

[edit] Mobitex

Is it worth mentioning that the over the air interface for Mobitex is based around X.25? http://www.leapforum.org/published/internetworkMobility/split/node118.html Jonnyct 15:22, 9 June 2007 (UTC)

[edit] X.25 Today: NGN threat?

Does NGN (All IP) threaten to force the replacement of X.25 and similar non-IP protocols? My experience which such technologies is minimal so this may be an incorrect assumption of mine.

While there are assorted definitions of NGN, most assume highly reliable optical transmission media. X.25 remains a viable technique in certain media with high error rate, long delay, and low speed, including poor-quality landlines, amateur radio, some satellite links, etc. Howard C. Berkowitz 16:55, 8 August 2007 (UTC)