updated notes computer networks

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


EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 30.05.2006 Session VIII Topic:ATM Faculty: Anita Kanavalli MSRIT  Source clock frequency recovery at receiver.  Source data structure recovery at receiver.  Monitoring and handling of AAL-PCI bit errors.  Monitoring and (possibly) correcting the bit errors in the user information field.  For circuit emulation, monitoring and maintenance of end-to-end QoS. AAL Type 2. This AAL type would be used with Class B (VBR). This type is not well defined and it seems possible that the it may be merged with AAL Type 1 in the future. Some of its functions are similar to ALL Type 1:  Segmentation and reassembly of user information.  Handling of cell delay variation (jitter).  Handling of lost and misinserted cells.  Source clock frequency recovery at receiver.  Monitoring and handling of AAL-PCI bit errors.  Monitoring and (possibly) correcting the bit errors in the user information field. It also has the additional functions:  Handle SDUs from a variable bit rate source.  Transfer timing information between source and destination.  Notify the higher layers of uncorrectable errors in AAL. AAL Type 3/4. There was once separate Type 3 and Type 4 AALs, but they have now been merged. This AAL is now intended to support both Class C (ABR) and Class D (UBR) services. In this AAL, the convergence sublayer is split into two (Figure ), the service specific convergence subslayer (SSCS) and the common part convergence sublayer (CPCS). The SSCS is application dependent, i.e. it could be for a VBR video application. The CPCS is responsible for constructing PDUs that can be sent to the other end user. There are two modes of operation of ALL Type 3/4; message mode and streaming mode. EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 30.05.2006 Session VIII Topic:ATM Faculty: Anita Kanavalli MSRIT Figure: AAL Type 3/4 sub-layering The message mode is intended for use framed data where the AAL-SDU is a logical unit of data with respect to the B-ISDN user (Figure ). It allows the the transport of a single AAL-SDU in one or (optionally) more than one CS-PDU. The CS-PDU may be then further spilt into several SAR-PDUs. The AAL-SDU can be of an arbitrary size. Figure: AAL Type 3/4 message mode service In streaming mode, the AAL-SDUs are of fixed size and one or more of them may be transported in a single CS-PDU (Figure ). Each AAL-SDU is delivered in a separate

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