Communication Systems and Computer Networks PDF 2024

Communication Systems and Computer Networks (1404703) Prepared by: Prof. Shawkat K. Guirguis Professor of Computer Science & Informatics Part 5 2024-11-0...

Communication Systems and Computer Networks (1404703) Prepared by: Prof. Shawkat K. Guirguis Professor of Computer Science & Informatics Part 5 2024-11-04 (c) Prof.Shawkat K. Guirguis 1  EXAMPLE NETWORKS 1.Internet 2.X.25, Frame Relay and ATM 3.Ethernet 4.IEEE 802.11, the standard for wireless LANs. 2024-11-04 (c) Prof. Shawkat K. Guirguis 2  1 The Internet History: Evolved from the ARPANET, its subnet consisted of minicomputers called (IMP: Interface Message Processor) connected with 56-kbps. For reliability, each IMP should be connected to at least two other IMPs. It was a datagram network … etc. up to the WWW. (Long story self reading) 2024-11-04 (c) Prof. Shawkat K. Guirguis 3  The ARPANET (2) The original ARPANET design 2024-11-04 (c) Prof. Shawkat K. Guirguis 4  The ARPANET Growth of the ARPANET (a) December 1969. (b) July 1970. (c) March 1971. (d) April 1972. (e) September 1972. 2024-11-04 (c) Prof. Shawkat K. Guirguis 5  2. Connection-Oriented Networks: X.25, Frame Relay, and ATM Since the beginning of networking, a war has been going on between the people who support connectionless (i.e., datagram) subnets and the people who support connection-oriented subnets. 2024-11-04 (c) Prof. Shawkat K. Guirguis 6  View1: The main proponents of the connectionless subnets come from the ARPANET/Internet community. Remember that DoD’s original desire in funding and building the ARPANET was to have a network that would continue functioning even after multiple direct hits by nuclear weapons wiped out numerous routers and transmission lines. Thus, fault tolerance was high on their priority list; billing customers was not. This approach led to a connectionless design in which every packet is routed independently of every other packet. As a consequence, if some routers go down during a session, no harm is done as long as the system can reconfigure itself dynamically so that subsequent packets can find some route to the destination, even if it is different from that which previous packets used. 2024-11-04 (c) Prof. Shawkat K. Guirguis 7  View2: The connection-oriented camp comes from the world of telephone companies. In the telephone system, a caller must dial the called party’s number and wait for a connection before talking or sending data. This connection setup establishes a route through the telephone system that is maintained until the call is terminated. All words or packets follow the same route. If a line or switch on the path goes down, the call is aborted. This property is precisely what the DoD did not like about it. 2024-11-04 (c) Prof. Shawkat K. Guirguis 8  Why do the telephone companies like it then? There are two reasons: 1. Quality of service. 2. Billing. 2024-11-04 (c) Prof. Shawkat K. Guirguis 9  1. Quality of Service By setting up a connection in advance, the subnet can reserve resources such as buffer space and router CPU capacity. If an attempt is made to set up a call and insufficient resources are available, the call is rejected and the caller gets a kind of busy signal. In this way, once a connection has been set up, the connection will get good service. With a connectionless network, if too many packets arrive at the same router at the same moment, the router will choke and probably lose packets. The sender will eventually notice this and resend them, but the quality of service will be jerky and unsuitable for audio or video unless the network is very lightly loaded. Needless to say, providing adequate audio quality is something telephone companies care about very much, hence their preference for connections. 2024-11-04 (c) Prof. Shawkat K. Guirguis 10  2. Billing The second reason the telephone companies like connection-oriented service is that they are accustomed to charging for connect time. When you make a long distance call you are charged by the minute. When networks came around, they just automatically gravitated toward a model in which charging by the minute was easy to do. If you have to set up a connection before sending data, that is when the billing clock starts running. If there is no connection, they cannot charge for it. Ironically, maintaining billing records is very expensive. If a telephone company were to adopt a flat monthly rate with unlimited calling and no billing or record keeping, it would probably save a huge amount of money, despite the increased calling this policy would generate. Political, regulatory, and other factors weigh against doing this, however. 2024-11-04 (c) Prof. Shawkat K. Guirguis 11  X.25 and Frame relay X.25, was the first connection-oriented public data network. It was deployed in the 1970s at a time when telephone service was a monopoly everywhere and the telephone company in each country expected there to be one data network per country—theirs. To use X.25, a computer first established a connection to the remote computer, that is, placed a telephone call. This connection was given a connection number to be used in data transfer packets (because multiple connections could be open at the same time). 2024-11-04 (c) Prof. Shawkat K. Guirguis 12  X.25 and Frame relay (cont.) Data packets were very simple, consisting of a 3-byte header and up to 128 bytes of data. The header consisted of a 12-bit connection number, a packet sequence number, an acknowledgement number, and a few miscellaneous bits. X.25 networks operated for about a decade with mixed success. 2024-11-04 (c) Prof. Shawkat K. Guirguis 13  Frame Relay In the 1980s, the X.25 networks were largely replaced by a new kind of network called frame relay. The essence of frame relay is that it is a connection-oriented network with no error control and no flow control. Because it was connection-oriented, packets were delivered in order. The properties of in-order delivery, no error control, and no flow control make frame relay akin to a “Wide Area LAN”. Its most important application is interconnecting LANs at multiple company offices. Frame relay enjoyed a some success. 2024-11-04 (c) Prof. Shawkat K. Guirguis 14  Asynchronous Transfer Mode (ATM) ATM is far more important, connection-oriented network. The reason for the somewhat strange name is that in the telephone system, most transmission is synchronous and ATM is not. ATM was designed in the early 1990s ATM was to solve all the world’s networking and telecommunications problems by merging voice, data, cable television, telex, telegraph, carrier pigeon, tin cans connected by strings, smoke signals, and everything else into a single integrated system that could do everything for everyone. 2024-11-04 (c) Prof. Shawkat K. Guirguis 15  It did not happen!! In large part, the problems were similar to those we described earlier concerning OSI, that is, bad timing, technology, implementation, and politics. To make a long story short, ATM was much more successful than OSI, and it is widely used deep within the telephone system, often for moving IP packets. Because it is mostly used by carriers for internal transport, users are often unaware of its existence. 2024-11-04 (c) Prof. Shawkat K. Guirguis 16  ATM virtual circuits Since ATM networks are connection-oriented, sending data requires first sending a packet to set up the connection. As the setup packet wends its way through the subnet, all the routers on the path make an entry in their internal tables noting the existence of the connection and reserving whatever resources are needed for it. 2024-11-04 (c) Prof. Shawkat K. Guirguis 17  ATM Virtual Circuits 2024-11-04 (c) Prof. Shawkat K. Guirguis 18  ATM virtual circuits (cont.) Connections are often called virtual circuits, in analogy with the physical circuits used within the telephone system. Most ATM networks also support permanent virtual circuits, which are permanent connections between two (distant) hosts. They are similar to leased lines in the telephone world. Each connection, temporary or permanent, has a unique connection identifier. Once a connection has been established, either side can begin transmitting data. 2024-11-04 (c) Prof. Shawkat K. Guirguis 19  ATM virtual circuits (cont.) The basic idea behind ATM is to transmit all information in small, fixed-size packets called cells. The cells are 53 bytes long, of which 5 bytes are header and 48 bytes are payload, as shown in next figure. Part of the header is the connection identifier, so the sending and receiving hosts and all the intermediate routers can tell which cells belong to which connections. This information allows each router to know how to route each incoming cell. Cell routing is done in hardware, at high speed. 2024-11-04 (c) Prof. Shawkat K. Guirguis 20  ATM Virtual Circuits An ATM cell. 2024-11-04 (c) Prof. Shawkat K. Guirguis 21  ATM virtual circuits (cont.) The main argument for having fixed-size cells is that it is easy to build hardware routers to handle short, fixed-length cells. Variable- length IP packets have to be routed by software, which is a slower process. Another plus of ATM is that the hardware can be set up to copy one incoming cell to multiple output lines, a property that is required for handling a television program that is being broadcast to many receivers. Finally, small cells do not block any line for very long, which makes guaranteeing quality of service easier 2024-11-04 (c) Prof. Shawkat K. Guirguis 22  ATM virtual circuits (cont.) All cells follow the same route to the destination. Cell delivery is not guaranteed, but their order is. If cells 1 and 2 are sent in that order, then if both arrive, they will arrive in that order, never first 2 then 1. But either or both of them can be lost along the way. It is up to higher protocol levels to recover from lost cells. Note that although this guarantee is not perfect, it is better than what the Internet provides. There packets can not only be lost, but delivered out of order as well. ATM, in contrast, guarantees never to deliver cells out of order. 2024-11-04 (c) Prof. Shawkat K. Guirguis 23  ATM virtual circuits (cont.) ATM networks are organized like traditional WANs, with lines and switches (routers). The most common speeds for ATM networks are 155 Mbps and 622Mbps, although higher speeds are also supported. The 155-Mbps speed was chosen because this is about what is needed to transmit high definition television (HDTV). The exact choice of 155.52 Mbps was made for compatibility with AT&T’s SONET transmission system. The 622 Mbps speed was chosen so that four 155-Mbps channels could be sent over it. 2024-11-04 (c) Prof. Shawkat K. Guirguis 24