Ch. 4 - WAN, Wide Area Networks

Transcription

1 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 1

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. 2

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. 3

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. 4

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. 5

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. 6

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. 7

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. 8

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. 9

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. 10

11 Frame Relay is offered at access speeds of 56 kbps up to 2 Mbps. Compare this to typical 56 kbps or 64 kbps normally used in X.25. The Frame Relay Forum is working on plans to push Frame Relay's maximum speed to 45 Mbps. Frame Relay services offer flexible bandwidth over the public network. This flexibility is provided through two parameters: CIR, which stands for Committed Information Rate AR, which stands Access Rate The CIR establishes the minimum amount of bandwidth that will be available. If the public network gets overloaded, various mechanisms come into play to ensure the CIR is available. If, on the other hand, the network has free capacity you are able to use higher bandwidths than CIR. The actual bandwidth that is used in the physical connection is called access rate. 11

12 Frame Relay services emerged in Since then almost every major long distance and local Telephone Company, as well as resellers and others, have started to offer frame Relay. Revenues have doubled every year according to some estimates. End users who would otherwise chose private networks are now buying frame Relay. One of the questions often raised about Frame Relay is the competition with the ATM technology. Frame Relay is more mature than ATM and much cheaper. It will still take some time before ATM is an alternative for the majority of users. Within the next years ATM will become more available, and it may ultimately replace Frame Relay. However, the death of frame Relay will not come about rapidly. The advantages of Frame Relay are 1. High bandwidth 2. High capacity 3. Handles much more transferred data than X.25 due to less overhead 4. One physical connection but many logical connections The disadvantages of Frame Relay are 1. An error is detected later than in X Not as fast as ATM. 12

13 ATM which stands for Asynchronous Transfer Mode is a De facto standard developed by the ATM Forum. ATM is a method of communication, which can be used as the basis for both LAN and WAN technologies. ATM specifications are being written to ensure that ATM smoothly integrates numerous existing network technologies, at several levels, for instance, Frame Relay, Ethernet and TCP/IP. ATM is a switched-based technology. By providing connectivity through a switch (instead of a shared bus) several benefits are provided: 1. Dedicated bandwidth per connection. 2. User defined quality of service. 3. Flexible access speeds. Equipment, Services and Applications for ATM are available today and are being used in live networks. In the picture two LANs are connected through an ATM network. This ATM network could be either a WAN or a LAN. It is more common to use ATM as a WAN service because of high costs. The reason for using ATM in LAN is not high speed, because you could get higher speeds with fast or Gigabit Ethernet. The real reason is when you want to use your LAN for different kinds of services, like voice, video and data. 13

14 ATM is not only used for data communication but also for telecommunication. ATM is available at various speeds. The most commonly used are 25, 155 and 622 Mbps. 14

15 The physical layer provides for transmission of cells over a physical medium. This physical layer consists of two sublayers: the PMD, Physical Medium Dependent sublayer, and the TC, Transmission Convergence sublayer. The PMD sublayer handles timing and synchronization and TC handles error control and packs ATM cells into appropriate frames suitable for the physical medium. ATM layer is responsible for handling connections and also for cell redistribution. AAL, which stands for ATM Adaptation Layer, adapts the different classes of applications to the ATM layer. This is necessary in order for ATM to support many kinds of services with different traffic characteristics, for instance continuous or bursty, and different system requirements like real time voice and video applications or normal data traffic. There are four types of AAL: AAL-1: ATM Adaptation Layer Type 1 offers support for constant bit rate and timedependent traffic such as voice and video. AAL-2: ATM Adaptation Layer Type 2 is a placeholder for variable bit rate video transmission, which means that it can be used by applications which do not have high requirements for real time handling. AAL-3/4: ATM Adaptation Layer Type 3/4 is used for variable bit rate, delay-tolerant data traffic requiring some sequencing and/or error detection support. AAL-3/4 was originally two AAL types, connection-oriented and connectionless, which have been combined. AAL-5: ATM Adaptation Layer Type 5 is used for ordinary LAN traffic having variable bit rate, delay-tolerant connection-oriented data traffic requiring minimal sequencing or error detection support. 15

16 Above ATM Adaptation Layer, there are higher layers in form of different network protocols such as IP and IPX. 16

17 Information to be sent using ATM, is segmented into fixed length cells. These cells are then transported to and re-assembled at the destination. The ATM cell has a fixed length of 53 bytes. The length being fixed allows the information to be transported in a predictable manner. This predictability accommodates different traffic types on the same network. Since the switches know that the cells are always the same lengths they don t need to count the number of bytes or look at the length information field in the header data. This makes ATM very fast. The cell is broken into two main sections, the Header and the Payload. The Payload, which is 48 bytes, is the portion, which carries the actual information. The Header, which is 5 bytes, carries the addressing information. 17

18 There are three commonly used ATM services. 1. LAN Emulation, used for emulation of standard LANs like Ethernet and Token Ring. 2. Classical IP and ARP over ATM, used for handling TCP/IP traffic over ATM networks. 3. Multiprotocol over ATM, is a mechanism for handling ISO layer 3 protocols transparanet over ATM. 18

19 When information needs to be sent, the sender negotiates a "requested path" with the network for a connection to the destination. When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service. An analogy for this negotiation of qualities would be similar to determining a method of delivery using US mail. One can choose to send 1st class, overnight or budget delivery. 19

20 Classical IP and ARP over ATM, is a way of making the TCP/IP traffic run over ATM in the most efficient manner possible. ATM IP and ARP over ATM service is used by the host and the router in the picture. Both the host and the router must usually be equiped with an ATM network interface card. 20

21 The ATM Forum's Multiprotocol over ATM (MPOA) specification is the industry's first standard based solution that allows transport of Layer 3 protocols transparent over ATM networks. At the same time MPOA takes the advantage of the specific benefits that ATM offers, like lower latency, high performance and quality of service. 21

22 ATM protocols are organized by the ATM Forum. After agreement by members of the ATM Forum, standards are presented to the International Telecommunications Union (ITU). The ATM Forum is an international non-profit organization formed with the objective of accelerating the use of ATM products and services. In addition, the Forum promotes industry cooperation and awareness. The ATM Forum consists of a worldwide Technical Committee, three Marketing Committees for North America, Europe and Asia-Pacific as well as the User Committee, through which ATM end-users participate. There are also RFC standards from the Internet Architecture Board, IAB which describe how to use ATM together with TCP/IP. Examples of ATM standard RFC s are shown in the picture: 22

23 If you want to connect two LANs with each other through an ATM network there are two possibilities. The first one is to adapt the communication software on LANs for ATM. The second is to adapt the ATM network so you don t have to change anything on the LANs. This is called LANE, which stands for LAN Emulation. The LANE protocol creates an emulated LAN by making an ATM network look and behave like an Ethernet or Token Ring. LANE works as a bridging protocol at layer 2 of the OSI model. It doesn t use CSMA/CD or token passing for media access control. Instead it uses its own solution, but seen from the LAN side the LANE solution is totally transparent. 23

24 What will the future hold for ATM? 1. ATM will provide a single network for all types of traffic like voice, data, video and so on. This will give improved efficiency and manageability since you only have to manage one network instead of several. 2. Because ATM is not based on a specific type of physical transport, it is compatible with currently deployed physical networks. ATM can be carried by twisted pair, coax and fiber optics. 3. The bandwidths used by ATM are not so important to LANs because there are much cheaper alternatives. But ATM will have a big impact on WANs, because it is still very expensive to get high bandwidths with alternative WAN technologies. 4. One important part of ATM is that the user can define and pay for quality of service, which means that he pays more for traffic with higher priority. 5. ATM has been designed from the onset to be scalable and flexible in: geographic distance, number of users and bandwidths. As of today, the speeds range from megabits to gigabits. This flexibility and scalability assures that ATM will be around for a long time. 24

25 Everybody is talking about ATM today. The most unique aspect of ATM is the possibility for users to define and buy priority for their traffic. ATM treats every kind of service as 53- byte data to be delivered. This means that you have one network for everything. ATM will dramatically increase the bandwidths for WANs. Since every cell has the same length the switches can handle them with high speed. Another advantage of ATM is that it works both on LANs and WANs. What are the disadvantages of ATM? Primarily the price, ATM products are very expensive and the market is waiting for more standards. While the vendors wait for standards they develop their own solutions, which gives poor interoperability between different products. 25