wan, wide area networks

1 Ch. 4 - WAN, Wide Area Networks 1 X.25 - access 2 X.25 - connection 3 X.25 - packet format 4 X.25 - pros and cons 5 Frame Relay 6 Frame Relay - access 7 Frame Relay - frame format 8 Frame Relay - addressing 9 Frame Relay - access rate and CIR 10 Frame Relay - pros and cons 11 ATM 12 ATM - Reference Model 13 ATM - Cell format 14 ATM - Services 15 ATM - Access 16 Classical IP and ARP over ATM 17 Multiprotocol over ATM (MPOA) 18 ATM - Standardization 19 ATM - LANE 20 ATM - Future 21 ATM - Pros and cons Ch. 4 - WAN, Wide Area Networks 2 One of the most common WAN services is X.25. It is a packet switched network based on the ITU standard X.25. The big difference between X.25 and leased lines is no dedicated leased connection between the source and the destination exists in X.25. In a packet switched network, the data is transmitted to the nearest packet switch in the network. Then the packets go from one switch to another until they reach the destination node. In X.25, hosts with different bandwidths can talk to each other. Conversion of the bandwidth is integrated in the network. Maximum bandwidth used in an X.25 network is 64 kbps per virtual channel. A virtual channel is logical connection which is established between source- and destination host before the data can be transferred. The X.25 network has many functions for secure data transfer, which guarantee that the packets arrive uncorrupted at the destination. Equipment can be connected to the X.25 network in many different ways. Equipment with synchronous leased connections can deliver packets to the closest packet switch in the network. There is also one standard for dial-up synchronous service to an X.25 network and it is called X.32. X.32 is often used when you want a temporary connection to a node in X.25 network. In the ISDN standard there is also a specification on how to use the D-channel to connect to an X.25 network. Ch. 4 - WAN, Wide Area Networks 3 Asynchronous terminals can not create the ordinary X.25 packets, so a PAD, which stands for Packet Assembler Disassembler, must be used to convert the asynchronous data traffic to synchronous and then retransmit the data to the nearest packet switch. This standard is called X.28 and it is a dial-up service used to reach X.25 networks. Ch. 4 - WAN, Wide Area Networks 4 When you establish a virtual channel through X.25 a call setup is sent to the network. This packet is called ”call request” and contains address information and other things. The network routes this packet to the destination address, which receives it as an ”incoming call”. The destination host will then answer this request and send a packet back with the information ”Call accepted” included. The network transports the packet back to the initial source which receives the ”Call Connected” information. The logical channel is now established and the data transmission can start. The normal packet length in X.25 network is 128 bytes. Ch. 4 - WAN, Wide Area Networks 5 X.25 is an interface between layer 3, the network layer, and layer 4, the transport layer, in the OSI model. X.25 offers a network service to higher layers in the OSI model. On layer 1, the physical layer, X.21 is used. On layer 2, the data link layer, LAPB, which stands for Link Access Procedure Balanced, is used. An X.25 packet is composed of a series of fields, as shown in the picture. The fields include data, addressing and control information. Layer 3, X.25 fields make up an X.25 packet and include a header and user data. Ch. 4 - WAN, Wide Area Networks 6 X.25 was pronounced dead when frame relay networks arrived in early 1990s. Despite such proclamations, reality has shown X.25 is still going strong. The X.25 market just keeps on growing, satisfying customers with low cost and reliable service. The U.S. market for X.25 services has continued to grow about 5 percent per year. There are three major factors responsible for this: 1. X.25 connection among private networks and LANs 2. Access to on-line services, like gambling and cash dispensing. 3. Dial-in LAN access. Advantages of X.25 are: 1. X.25 is a worldwide service, offered by many providers. 2. Equipment vendors support X.25 in their products. 3. Different access methods, both synchronous and asynchronous, with various bandwidths are supported. Disadvantages of X.25 are: 1. X.25 is expensive for large traffic volumes. 2. Due to large amount of control data the performance is low. Ch. 4 - WAN, Wide Area Networks 7 Frame Relay was originally conceived as a protocol for use over ISDN. Initial standards were done by ITU and ANSI. Frame Relay provides a fast packet switching capability that is used between user devices such as hosts and routers and network equipment such as switching nodes. User devices are often referred to as ”data terminal equipment”, while the network equipment is referred to as ” data circuit-terminating equipment”. The network providing the Frame Relay interface can be either a public network or a privately owned network. Seen from the user’s point of view, Frame Relay and X.25 are very similar. However, Frame Relay differs significantly from X.25 in its functionality and format. In particular, Frame Relay is a more streamlined protocol, facilitating higher performance and greater efficiency. Ch. 4 - WAN, Wide Area Networks 8 Normally Frame Relay is used together with leased lines. Frame Relay as a dial-up service is seldom used. The real advantage of using Frame Relay is that you have one physical connection but many logical connections. If you had leased lines to several connections, without using Frame Relay, you would be forced to have a unique line for every connection. Ch. 4 - WAN, Wide Area Networks 9 The frame used in Frame Relay is shown in this picture. The flag fields delimit the beginning and end of the frame. After the first flag there are two bytes of address information. Ten bits of these two bytes make up the actual circuit ID, called the DLCI, which stands for Data Link Connection Identifier. After the address info the data follows. The frame ends with check sum and the end flag. A frame can contain up to 1600 bytes. Ch. 4 - WAN, Wide Area Networks 10 The 10-bit DLCI value is the heart of the Frame Relay header. It identifies the logical connection that is multiplexed into the physical channel. In the basic mode of addressing, DLCIs have local significance; that is, the end devices at two different ends of a connection may use a different DLCI to refer to that same connection. If host A wants to communicate with host B it sends packets to the router at A with the IP address of host B. The router looks in its routing table and sees that this address belongs to a Frame Relay network and that the destination is to be reached through the Frame Relay address DLCI 204. The Frame Relay network is predefined by the network operator so that the DLCI 204 at the source end matches DLCI 50 at the destination end. The packets from A reach the router at B and are delivered to the host B. . support. Ch. 4 - WAN, Wide Area Networks 16 Above ATM Adaptation Layer , there are higher layers in form of different network protocols such as IP and IPX. Ch. 4 - WAN, Wide Area Networks 17 Information. up an X.25 packet and include a header and user data. Ch. 4 - WAN, Wide Area Networks 6 X.25 was pronounced dead when frame relay networks arrived in early 1990s. Despite such proclamations, reality. switch. This standard is called X.28 and it is a dial-up service used to reach X.25 networks. Ch. 4 - WAN, Wide Area Networks 4 When you establish a virtual channel through X.25 a call setup is sent