1. Lecture 12 X.25

2. X.25 is a packet switching wide area network developed by ITU-T in 1976. Originally it was designed for private use Definition : X.25 is an interface between data terminal equipment(DTE) & data circuit- terminating equipment(DCE) for terminal operation in the packet mode on public data networks.

3. X.25

4. Its a virtual-circuit switching network It uses a data transfer speed of 64 kbps It uses extensive error & flow controls at both data link & network layer X.25 defines how a packet-mode terminal can be connected to a packet network for exchange of data. It describes procedure necessary for establishing, maintaining & terminating connections. It also describes set of services, called facilities, to provide functions such as reverse charge, call direct & delay control

5. X.25 is what is known as a subscriber network interface(SNI) protocol. It defines how user’s DTE communicates with the network & how packets are sent over that networks using DCEs. It uses a virtual circuit approach to packet witching (SVC & PVC) & use asynchronous(statistical) TDM to multiplex packets.

6. X.25 Layers X.25 protocol specifies three layers: 1.Physical layer 2.Frame layer 3.Packet layer These layers defines functions of physical layer, data link layer & network layer of OSI model. Given figure explains the situation

7. X.25 Layers in Relation to the OSI Layers

9. Physical Layer X.25 specifies a protocol named as X.21 at physical layer. It is also called as X.21 bis. X.21 has been designed by ITU-T X.21 is quite similar to other physical layer protocols such as EIA-232 which is also supported by X.25

10. Frame Layer X.25 uses a bit oriented protocol at frame layer. This protocol is called as Link Access Procedure, Balanced (LAPB) LAPB is subset of High-level Data Control (HDLC), (which is bit oriented protocol used in point to point & multi point links) Figure shows general format of the LAPB packet.

11. Format of a Frame

12. Addressing at the Frame Layer

13. Frames are of three categories : I-Frames : I-frames are used to encapsulate PLP packets from the network layer. S-Frames : S-frames are for flow and error control in the frame layer. U-Frames : U-frames are used to set up and disconnect the links between a DTE and a DCE. The three packets most frequently used by LAPB in this category are SABM (or ESABM if the extended address mode is used) UA & DISC

14. Frame Layer Phases In the frame layer, communication between a DTE and a DCE involves three phases : link setup, packet transfer, and link disconnect Link set up phase The link between DTE and DCE must be set up before packets from the packet layer can be transferred. Either the DTE or the DCE can set up the link by sending an SABM (set asynchronous balanced mode) frame; the responding party sends a UA (unnumbered acknowledgement) frame to show that the link is actually set.

15. Transferring Data After the link has been established, the two parties can be send and receive network layer packets (data and control) using I-frames and S-frames. Link Disconnect When the network layer no longer needs the link, one of the parties can issue a disconnect (DISC) frame to request disconnection. The other party can answer with a UA frame.

16. Three Phases of the Frame Layer

17. Packet Layer The network layer in X.25 is called the packet layer protocol (PLP). This layer is responsible for establishing the connection, transferring the data, and terminating the connection. In addition, it is responsible for creating the virtual circuits and negotiating network services between two DTEs. While the frame layer is responsible for making a connection between a DTE and a DCE, the packet layer is responsible for making a connection between two DTEs (end-to-end connection). Note that X.25 uses flow and error control at two levels (frame layer and packet layer).

18. Virtual circuits in X.25 The X.25 protocol is a packet-switched virtual circuit network. The virtual circuits in X.25 are created at the network layer (not the data link layer as in some other wide area networks such as Frame Relay and ATM). This means that a physical connection established between a DTE and DCE can carry several virtual circuits at the network layer with each circuit responsible for carrying either data or control information, a concept called in-band signaling. Figure shows an X.25 network in which three virtual circuits have been created between DTE A and three other DTEs.

19. Virtual Circuits are further categorized into two types : Switched Virtual Circuit (SVC) – A switched virtual circuit is established at the request of a DTE and is terminated at the end of the call. The subnetwork resources are assigned for an SVC for the duration of the call. Permanent Virtual Circuit (PVC) – A permanent virtual circuit is a constant logical connection between two DTEs. It is similar to a leased circuit connection between two terminals. It is not to be established or terminated and is always in the data transfer phase.

20. Virtual Circuits in X.25 For Four Terminal Equipments

21. Advantages of X.25 The main advantages of the switching and routing technique are as follows – - Fast response times - No blocking except when the network storage is completely flooded. - High availability because of distributed routing. - High-speed data bursts can be handled as well as low-speed requirements.

22. Disadvantages of X.25 Disadvantages of X.25 packet switching network are as follows : (i)X.25 has a low data rate i.e. 64-kbps. (ii)X.25 has extensive flow and error control at both the data link layer and the network layer. Flow and error control at both layers create a large overhead and slow down transmissions.

23. (iii) X.25 was designed for private use, not for the Internet. X.25 has its own network layer which means that the user data are encapsulated in the network layer packets of X.25. Although, the Internet has its own network layer, which means if the Internet wants to use X.25, it has to deliver its network layer packet, known as datagram, to X.25 for encapsulation in the X.25 packet. Thus, the overhead doubles. (iv) Limitations of X.25 has given way to another technologies for WAN like Frame Relay & then ATM (Asynchronous Transfer Mode).

24. Other Protocols Related to X.25 X.121 Triple X ( collection of following three protocols) 1.X.3 2.X.28 3.X.29

25. Frame Relay Frame relay is a wide area network with the following features: 1.Frame Relay operates at a higher speed (1.544 Mbps and recently 44.376 Mbps). This means that it can easily be used instead of a mesh of T-1 or T-3 lines. 2.Frame Relay operates in just the physical and data link layers. This means it can easily be used as a backbone network to provide services to protocols that already have a network layer protocol, such as the internet. 3.Frame Relay allows bursty data. 4.Frame Relay allows a frame size of 9000 bytes, which can accommodate all local area network frame sizes.

26. 5.Frame Relay is less expansive than other traditional WANs. 6.Frame Relay has error detection at data link layer only. There is no flow control or error control. 7.There is not even a retransmission policy if a frame is damaged; it is silently dropped. Frame Relay was designed in this way to provide fast transmission capability for more reliable media and for those protocol that have flow and error control at the higher layers.

27. ATM Short for Asynchronous Transfer Mode, a network technology based on transferring data in cells or packets of a fixed size. The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line.networkpacketsvideo ATM (asynchronous transfer mode) is a dedicated- connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.bytecell

28. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible.switch The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. MbpsGbps Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).