Outline Computer Communication and Networks 1 Department of Computer Science & Information Technology University of Balochistan Lecture 03 1/26 2/26 Two major components The mesh of packet switches and links that interconnects the Internet s end systems The Network Edge 3/26 5/26
In a network application, end systems exchange messages with each other that may contain An email message, a JPEG image, or an MP3 audio file Source breaks long messages into smaller chunks of data called Packets Each packet travels through communication links and packet switches at a rate equal to the full transmission rate of the link A packet of L bits over a link with transmission rate R bits/sec, then the time to transmit the packet is L/R seconds 6/26 7/26 Transfer of information as payload in data packets Packets undergo random delays & possible loss Different applications impose differing requirements on the transfer of information Messages broken into smaller units (packets) Source & destination addresses in packet header Lower delay than message switching, suitable for interactive traffic Connectionless, packets routed independently Packet may arrive out of order Pipelining of packets across network can reduce delay, increase throughput 8/26 9/26
Store-and-Forward Transmission Store-and-Forward Transmission Types of Delay Packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link End-to-End Delay 10/26 11/26 Types of Delay As a packet travels from one node (host or router) to another node, it suffers from several types of delays at each Nodal Processing Delay (δproc ) Queuing Delay (δqueue ) Transmission Delay (δtrans ) Propagation Delay (δprop ) Greatly affects the performance of many Internet applications like search, browsing, email, maps, IM, & VoIP 12/26 13/26
Nodal Processing Delay (δproc ) Time that a packet spends in a queue at a node while waiting for other packets to be transmitted Queuing Delay (δqueue ) Time required to examine the packet s header and determine where to direct the packet Also include other factors like Length of δqueue of a packet depends on the number of earlier-arriving packets that are queued and waiting for transmission onto the link If the queue is empty then δqueue = Zero if the traffic is heavy & many packets are waiting to be transmitted then δqueue = Long Time needed to check for bit-level errors in the packet Nodal processing delay is usually negligible Related to the transmission delay (δtrans ) δqueue = δtrans lqueue 14/26 15/26 Transmission Delay (δtrans ) Packets are transmitted in a first-come-first-served manner Packet can be transmitted only after all the packets that have arrived before it have been transmitted δtrans = L R Propagation Delay (δprop ) Time required to propagate from the beginning of the link to a subsequent router Bit propagates at the propagation speed of the link For fiber optics, twisted-pair copper wire,... it is in the range of 2 108 meters/sec 3 108 meters/sec δprop = where, L = Length of a packet in bits and R = Transmission rate in bps D s where, D = Distance between two routers (nodes) and s = Propagation speed of the media 16/26 17/26
End-to-End Delay End-to-End Delay (δnodal ) Total (δnodal ) The total delay from source to destination δnodal = δproc + δqueue + δtrans + δprop Hands-on feel for end-to-end delay in a computer network, Traceroute program can be used δendtoend = N [δproc + δtrans + δprop ] Here, δqueue is negligible and N = Number of nodes in a network 18/26 19/26 Packet Loss Throughput A queue preceding a link has finite capacity that greatly depends on: Router design Router cost A packet can arrive to find a full queue With no place to store such a packet, a router will drop that packet; i.e., the packet will be lost Another reason for loss can be noise in any electronic circuit Instantaneous Throughput at any instant of time is the rate (in bits/sec) at which any host is receiving the file Average Throughput = F T bits/sec Fraction of lost packets increases as the traffic intensity increases A lost packet may be retransmitted on an end-to-end basis 20/26 21/26
Resources needed (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the session Traditional telephone networks Implies the need to first set up a dedicated, end-to-end path for the connection before the information transfer takes place 22/26 23/26 Establish: source creates circuit to destination Node along the path store connection info Nodes may reserve resources for the connection Transfer: source sends data over the circuit No destination address, since nodes know path Teardown: source tears down circuit when done 24/26 25/26
Advantages of Guaranteed bandwidth Predictable communication performance Not best-effort delivery with no real guarantees Simple abstraction Reliable communication channel between hosts No worries about lost or out-of-order packets Simple forwarding Forwarding based on time slot or frequency No need to inspect a packet header Low per-packet overhead Forwarding based on time slot or frequency No IP (and TCP/UDP) header on each packet 26/26