Communication Engineering: Protocol X.25

Wow! X.25 is really old. I believe it was one of the first attempts at packet switching. I think sooner or later it has to ride on top of T1/E1. We're talking about 1.54 Mbits or fractions of 64 Kbits.

Check out this link: http://en.wikipedia.org/wiki/X.25

hello!

thanks for helping me

ok would u please tell me completly about T1/E1 i saw this in my studing GSM books but exactly i dont know what is it and its function would u please help me thank

T1 and E1 are WAN protocols. T1 is a US standard, while E1 is the European standard.

Specifically, T1 was originally a protocol to allow time division multiplexing (TDM) over private lines - This allowed users to purchase as much or as little bandwidth as they needed. You can think of a T1 line as a pipe with 24 (?) smaller pipes inside. Collectively, the T1 line provides 1.54 Mbits of bandwidth. However, this bandwidth consists of 24 (?) 64 Kbit channels. A customer can purchase a single 64Kbit channel or several 64 Kbit channels - up to a max of 24 or the full 1.54 Mbits T1 pipe. T1 lines still exist, and they go for thousands of dollars a month. No wonder internet protocols are so popular.

Also, a newer standard T3/E3 provides about 54 Mbits (if you rent the whole pipe) and uses basically the same TDM scheme. Further, T3 lines can be split up into multiple T1 components, and in turn, those "inner" T1s can be split up into 64 Kbit channels. All by the use of the Time Division Multiplexing process.

Again, I direct you to Wikipedia for the complete and official definition:

http://en.wikipedia.org/wiki/T1/E1

As Vermin said this is really old stuff.

Yes it is actually a protocol for packet switching data networks that can use a multitude of physical link technologies.

Basically what you have is a series of X25 exchanges/nodes connected in a network. If one computer that is connected to the network wishes to talk with another machine on the network a virtual circuit is set up through the nodes on the network that all the packets being transferred between the to machines will follow.

As each packet is transmitted from one node to the next the receipt is acknowledged by the next node but the system is set to allow for burst of up to seven packets before receiving the acknowledgement for the first.

This limit of a maximum of 7 packets being in transit between nodes proved to be a problem with satellites so the protocol was modified to what is called Extended X25 which allowed for up to 255 packets being in transit between nodes before an acknowledgement was required.

The term X25 refers to the protocol that was used to create these virtual circuits and make sure that all the packets get to where they need to go and that none are lost.

hello!

thanks for all or you that help me .

when u are talking about E1/T1 are they use as wired or wirless becuse u told me that think of these 2 as a pip and secondly

would u please tell me about the (digital signal level 0) DS0 ?

and also here i stadied that most of the time E1 connection with 31 DSOs is not enough to connect two switching centers with each others in this case E3 connection can be use my question is what is E3 which can support up to 512 DOSs.

I am not familiar with the T1/E1 standards but with X25 the physical link can use just about any sort of link you can think of the X25 is the actual protocol that the two nodes use to transfer the data and the actual physical way the link operates is irrelevant.

Modern inter computer communications are based on the OSI seven layer system of communications. The way this system works is that at the top or level seven is you and what you see on the screen of your computer. The information that needs to be transferred from the computer you are using to the destination system is passed from layer 7 down through layers till it reaches level 1 which is the actual physical communications layer. At the other end the data is passed up through the seven layers back up to wherever it is needed.

The concept behind the system can be made transparent to the end user and is was intended to be operating system independent. The concept was never really fully implemented but even so modern communications is fundamentally based on the concept so have a read of the link above and if you have any questions post them here.

If I remember correctly, the 64 Kbit channels of a T1 line are usually numbered ds0, ds1, ds2, and so on. Physical lines (I believe originally deployed by telcos) usually carry the TDM signals between switching and multiplexing/de-multiplexing locations. Here, you may want to Wikipedia the terms DSU/CSU.

Also, being for the most part at the physical layer, you can carry just about any protocol across a T1 connection. For example, Ethernet, ATM, etc.. These days a T1 line would really be a bottleneck for sending other data-link traffic. T3 or DS3 is better, but still very expensive and not the absolute solution for bandwidth. That's why for more and faster transport, telcos have deployed large (sometimes proprietary) ATM switches. Also, SONET is another protocol typically used for passing lots of data through a MAN. We're talking hundreds or thousands of megabits. And finally, terabit Ethernet is not far off.

More recently, Multi Protocol Label Switching (MPLS) is starting to make an appearance on the Internet. Actually, MPLS is basically good old IP that's been reverse engineered to act like ATM-type switch connections across the internet. A dedicated virtual link (like a wire, but made of silicon) is created by sending traffic through a set of specified (labeled) routers. The path taken across the internet changes only if a part of the link or one of its corresponding routers goes down. When working properly, this keeps the IP packets and Ethernet frames from being sent in a hundred different directions as router decisions are made, which is the usual IP internet circumstance.

Hope some of this helps. Perhaps I've wandered off topic. If so, feel free to haul me back.

One last point that needs to be made is that these newer forms of switching provide for very accurate and granular deployment of Quality of Service (QoS) and Class of Service (CoS). This allows telcos and ISPs to offer all kinds of extra bells and whistles regarding how your info is carried across their networks. Of course, the better the service, the higher the price.

In North America and Japan, the basic building block to carry data is the DS0.

A DS0 is a chunk of bandwidth, namely 64 kbps. This rate was choosen because if you wish to carry PCM (pulse coded modulated) voice, and the sampling rate is 8 kbps at 8 bits, then you have 64 kbps.

Physically, circuits come in higher bit rates, carrying multiple DS0s. The first example is the T1. The T1 carries 24 DSOs, or 24 voice calls. The data rate in a T1 is 1.544 Mbps. Although I make reference to voice calls, you could establish a Point-to-Point internet link with a T1 so you can surf the web, and this would give you the full 1.544 Mbps. So I'll continue to make reference to voice calls, but you can make use of this bandwidth for whatever you need requires.

So you made reference to switching centers: If you want to carry voice traffic between switcing centers, you need voice circuits. In the example above, each T1 will carry 24 voice calls (i.e. 24 channels of 64 kbps.) To link two cities together, there will certainly be more than 24 concurrent voice calls between two switching centers ... so we need mutliple T1s.

So, 7 T1's = 1 T2. (by the way, I've never seen a T2, so don't write that down).

The next physical circuit that we can purchase is the T3. One T3 carries 28 T1's which carries 24 channels of 64 kbps voice calls, for a total of 28X24 = 672 calls.

In practice, phone networks have optical fiver rings through switching centers, so that there are multiple paths between the same switching centers, and the data between switching centers are of the order of OC-3, OC-12, OC-48 and OC-192 rates.

To use an example, in a wireless network, there are T1s to the cellsites, and these are aggregated regionally into T3s and added with other T3s to be backhauled on OC-3 links into larger OC-48 circuits until they reach the Mobile switching center.

Going back to basics, if you needed 448 kbps, you can do this by use several DS0s of a T1, and this is accomplished by multiplexing timeslots.

And going back to the original X.25 question, it's a technology that uses addresses to route packets of data between two points.

If you have three computers on a X.25 cloud, you can communicate between them by addressing your packet. As the packet enters the X.25 cloud (network), each router will send the packet to the router to get the packet to the end computer.

If you need more info about X.25, search the internet for X.25 and HDLC.

In all my examples, I talked about T1s, but E1s are like T1s except a E1 carries 32 DS0s.

One last critical piece of information ... T1s and E1s are synchronous data protocols, meaning that telco equipment has to have timing equipment to synchronize clocks so that data is sent and received properly.

I hope this helps you.

That was a very good explanation, ve9gfi!