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January 09, 2014

X.25 Networks - An Introduction and Overview


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Friends, Computer Networks is one of the important sections for upcoming IBPS Specialist Officers and GATE 2014 Exams. We have already discussed in brief introduction and OSI models. Today we shall discuss about a very important topic of Computer Networks. That is, X.25 Networks. Ofcourse, this topic is kind of hard and boring. But its important. So please be focused.

The X.25 is an International Telecommunication Union-Tell-Communication standardization (ITU-T). The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. The X.25 protocols works at the Layers 1 to 3 of the OSI model. Basically these network devices fall into 3 general categories. Those are
  1. Data Terminal Equipment (DTE)
  2. Data Circuit Terminating Equipment (DCE) and
  3. Packet Switching Exchange (PSE)
DTE are usually terminals, PC or network hosts. DCE devices are communication devices, such as modem and packet switches and PSE are generally located in the carrier's facilities. 

X.25 defines the procedures for the exchange of data between user devices and a packet network node. It is interface between Data Terminal Equipment (DTE0 and Data Circuit Terminating Equipment (DCE) for terminals operating in the packet mode on Public Data Networks. The x.25 standard specifies an interface between a host system and a packet switching network. As mentioned above, the standard specifically calls for three layers for functionality. Those are,
  1. Physical Layer
  2. Data Link Layer
  3. Network Layer.
These three layers correspond to the lowest three layers of the OSI model.

The physical layer deals with the physical interface between computer and the link that attaches that station to the packet switching node. The X.25 makes use of the physical layer specification in a standard known as X.21 but in many cases, other standard such as EIA-23 are substituted. The physical level of X.25 dos not perform significant control functions. It is more of a passive conduit, with control provided by the data link and network layers.
The Data Link Layer provides for the relaible transfer of data across the physical link by transmitting the data as a sequence of frames. The data link layer standard is referred to as Link Access Protocol Balanced (LAPB). LAPB is the subset of HDLC. The X.25 packets are carried within the LAPB fram as the information field  (I). LAPB's job is to ensure that teh X.25 packets are transmitted error free across an error prone channel frmo or to the DTE and DCE.

X.25 Requires LAPB to use specific addresses at the data link layer. The subscriber DTE must be A (binary 11000000) and the DEC (network node) must be B (binary 10000000).

X.25 allows both set asynchronous resopnse mode (SARM) and Set Asynchronous Balanced Mode (SABM) to support LAP and LAPB respectively.

However, SABM is encouraged while LAP with SARM is rarely used now. Both X.25 and LAPB use send (S) and recieve (R) numbers to accounts for traffic in their respective layers. X.25 uses P (S) and P (R) to describe its sequence number.

Network Layer provides an external virtual circuit service. User data are passed down to X.25 level 3, which appends control information as a header, creating a packet. X.25 protocol uses this control information for its operation.

The enitre X.25 packet is then passed down to the LAPB entity, which appends control information at the front and back of the packet, forming an LAPB frame. X.25 operates on the premise of the virtual curcuit services. X.25 provides for two types of virtual circuit. Those are,
  1. Virtual Call (VC)
  2. Permanent Virtual Circuit (PVC)
 Raghunada Reddy

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  1. this x.25 network is also in ccna course...........good explenation.................

  2. Please provide all IT materials in single PDF for quick reference before the exam. Thank you!


    1. U r rite.. it will help us a lot

    2. Quick Reference guide may not work Friends. As the syllabus is vast. We will try to post subject wise pdf files.

      All the Best.

    3. Hi.Pls post multiple choice question exam

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    2. i hope today late night.......................

    3. i think result will come out tomorrow....

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