computer networks

EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 29.05.2006 Session VII Topic:ATM Faculty: Anita Kanavalli MSRIT Why ATM? •All digital telephone network •Need for data applications like computer communications and facsimile •Nonvoice applications like videoconferencing need to be included in the future networks •Circuit switching not suitable but packet switching would accommodate this •ISDN were the first effort to address this ISDN Overview •There are many types of WAN technologies that can be used to solve the problems of users who need network access from remote locations. •ISDN has been specifically designed to solve the low bandwidth problems that small offices or dial-in users have with traditional telephone dial-in services. •Telephone companies developed ISDN with the intention of creating a totally digital network whilst making use of the existing telephone wiring system. •ISDN works very much like a telephone - When you make a data call with ISDN, the WAN link is brought up for the duration of the call and is taken down when the call is completed •ISDN allows digital signals to be transmitted over existing telephone wiring. ••This became possible when the telephone company switches were upgraded to handle digital signals. ••ISDN is generally viewed as an alternative to leased lines, which can be used for telecommuting and networking small and remote offices into LANs. •ISDN's ability to bring digital connectivity to local sites has many benefits, including the following: ••ISDN can carry a variety of user traffic signals including: –digital video, packet-switched data, and telephone network services. •ISDN offers much faster call setup than modem connections because it uses out-of- band (D, or delta, channel) signaling. –For example, some ISDN calls can be setup in less than one second. •ISDN provides a faster data transfer rate than modems by using the bearer channel (B channel of 64kbps). –With multiple B channels, ISDN offers users more bandwidth on WANs than some leased lines. –For example, if you were to use two B channels, the bandwidth capability is 128Kbps because each B channel handles 64Kbps. –•ISDN can provide a clear data path over which to negotiate PPP links. BRI (Basic Rate Interface) Connection from the ISDN office to the user location provides for access to three channels. The channels are two 64Kb B-channels and one 16Kb D-cha




EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 17.05.2006 Session III Topic: TCP Faculty: Anita Kanavalli MSRIT TCP connection management • Server waits by executing LISTEN and ACCEPT primtives • Client executes a CONNECT primitive specifying IP and PORT no, max TCP segment size and user data • CONNECT sends TCP segment with SYN bit ON and ACK off • Server can either accept or reject connection • In call collision only one connection is established • Connection released using FIN bit • One FIN and one ACK in each direction • possible to combine first ACK and second FIN in the same segment Finite state machine • Management on client side • When client issue CONNECT, TCP entity sends SYN segment • Separate state for each connection • When ACK arrives the final ACK sent and switches to new state • Data transfer • when no data issue CLOSE primitive sends FIN segment • One side goes to WAIT and waits for the FIN from other side • Packet life time taken care too • Management on server side • When server issue LISTEN and waits for incoming request • When SYN comes the server responds with ACK • When three way handshake complete then server goes to new state • FIN is sent when server want to close TCP transmission policy • Sender & receiver negotiate on window size • In the figure below the sender sends 2K data and the initial seq no • The receiver sends the ack for the next seq no it is expecting and also advertises the widow size • Suppose the window is zero then sender waits and then sends a probe and then sends the next set of data • The diagram shows how the two sides communicate • Suppose there is only one byte to be sent the 41 byte packet to be sent instead Nagle algorithm suggest that when the first byte comes it is sent and the other buffered till the ack received and then send the rest and wait for the ack and like this a sufficient no of bytes go in one segment EDUSAT SESSION FOR COMPUTER NETWORKS-I (C


EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 22.05.2006 Session IV Topic: TCP Faculty: Anita Kanavalli MSRIT TCP Congestion control • How TCP prevents congestion • when connection established, window size chosen • Receiver specifies seeing its buffer size • Still congestion occurs • The two problems are Network Capacity and Receiver Capacity • Solution? • Solution • Sender maintains two windows: one the receiver granted • the other Congestion Window • at the connection establishment- the congestion window is set to the size of the maximum segment in use on the connection • Each burst acknowledged doubles the congestion window • Congestion window grow exponentially • This is called the Slow Start algorithm • Another Solution? initialize: Congwin = 1 for (each segment ACKed) Congwin++ until (loss event OR CongWin > threshold)




EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 23.05.2006 Session V Topic: Performance Issues Faculty: Anita Kanavalli MSRIT Different performance issues in network • Performance Problems in Computer Networks • Network Performance Measurement • System Design for Better Performance • Fast TPDU Processing • Protocols for Gigabit Networks Performance problems in computer networks • Overloads Example 1: TPDU containing the bad parameter when broadcast may clog the n/w results in broadcast storm due to error message • synchronous overload due to power failure-DHCP contacted for booting • Apart from this problems due to insufficient memory TPDUs lost • Not setting the timeout correctly the TPDUs lost • Gigabit n/w pose new problems • The next figure explains this here the transmission line used only for .5msec greatly reducing the efficiency • The useful quantity is the Bandwidth-Delay product • The product is the capacity of the pipe from sender to receiver and back to sender in bits EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 23.05.2006 Session V Topic: Performance Issues Faculty: Anita Kanavalli MSRIT • In the above example it is 40 million bits but the actual utilisation is only 1.25 percent of the pipe capacity • therefore for good performance the receiver window must be at least as large as the Bandwidth-Delay product • Another performance problem could be jitter to avoid a small standard deviation is used The basic loop for improving network performance. • Measure relevant network parameters, performance. • Try to understand what is going on. • Change one parameter Precautions taken while measuring • Sample size should be large enough • Samples should be representative • To be careful while using coarse grained clock • Nothing unexpected going on while tests are conducted • Caching problem • Understanding the measurements • Extrapolation of the result System Design for Better Performance Rules: • CPU speed is more important than network speed. • Reduce packet count to reduce software overhead. • Minimize context switches. • Minimize copying. • You can buy more bandwidth but not lower delay. • Avoiding congestion is better than recovering from it. • Avoid timeouts. Fast TPDU Processing • TPDU processing overhead has two components • one –overhead per TPDU • other – overhead per byte • Example take the sending side