CMPE 150 Introduction to Computer Networks FINAL REVIEW Venkatesh Rajendran Spring 2003 UCSC CMPE150 1 Class Final Exam Final exam Three hours 40 50 questions comprehensive Multiple choice as the midterm Scantron bring your sheet and pencils Wednesday June 11th 8 00 11 00am Six short days Tick tick tick Spring 2003 UCSC CMPE150 2 Principles of Computer Communication Protocol specification The description of the protocol is complete and accurate Safety A protocol does what it is supposed to do all the time Liveness A protocol does not leave any deadlocks Efficiency A protocol makes efficient use of available resources Fairness Fair or contractual use of resources Simplicity is desirable but not necessary Spring 2003 UCSC CMPE150 3 Layering Model Purpose is to divide and conquer complex software and hardware needed to implement services Partition services and functions needed in system into layers Each layer of service is provided by peer protocol entities Communication can be point to point or multipoint Layer N packets NODE A Layer N Protocol Entity virtual communication interface NODE B protocol Layer N 1 Protocol Entity Spring 2003 Layer N Protocol Entity Layer N 1 Protocol Entity UCSC CMPE150 4 Protocol Correctness A protocol must be safe and live Safety Liveness Protocol provides the desired service all the time Protocol has no deadlocks no process waits forever for an event to occur Proving one may depend on the other Spring 2003 UCSC CMPE150 5 Protocol Performance Average delay Throughput or capacity Time between transmission of an information bit and reception of the bit at the receiver Number of information bits sent divided by the time between transmission of first bit and delivery of the last bit Computations will make strong assumptions in most cases results of analytical model provide only a rough approximation Most effective for comparative analysis Spring 2003 UCSC CMPE150 6 Basic Network Services S 1 2 2 2 2 1 1 2 1 1 D Data may take different paths to destination 1 Shared network resources Connection oriented service Reliable data transfer In order delivery no duplicates or missing data Flow control Do not congest the receiver s Congestion control Do not congest the network s Spring 2003 UCSC CMPE150 7 Basic Network Services S 1 2 3 2 1 3 D Shared network resources Connectionless service No delivery guarantees needed from the network Any connection oriented service to application is provided by end to end protocol Spring 2003 UCSC CMPE150 8 S Circuit Switching D call request call accept DATA call termination termination ack Portion of physical resource is assigned to a single connection Delay and signaling overhead in establishing and ending connections Spring 2003 UCSC CMPE150 9 Message Switching S D message Message from sender is sent on a store and forward basis Message has a header used for forwarding Resources shared among different calls Spring 2003 UCSC CMPE150 10 Statistical Multiplexing S1 Share the same communication channel among multiple connections without fixed allocations of the resource to those connections m2 m1 2 m m1 m1 Link is shared based on the statistics of each connection or flow m2 D1 m1 m1 S2 m2 m2 1 m m2 D2 Limitation Entire message must be received at a switch before it can be forwarded Spring 2003 UCSC CMPE150 11 S Packet Switching D packet 1 packet 2 packet 3 packet 4 Resources are shared among connections Packets from the same connection can be processed concurrently Connection setup delay can be avoided using datagrams Spring 2003 UCSC CMPE150 12 Packet Switching Information is organized into packets A packet consists of a header and a payload Header specifies the control information needed to transport the packet from origin to destination Packets are forwarded from source to destination using routing tables There are two basic approaches to packet switching datagrams virtual circuits Spring 2003 UCSC CMPE150 13 Datagrams a b a a e c d a e 5 3 go to 2 go to 3 go to 2 go to 3 7 b 4 c d To b To d To e To 4 e a b e 1 6 a c a b a e 2 d next next next next Routing table specifies next hop to each destination Packets are forwarded based on the routing table Each packet is routed independently Spring 2003 UCSC CMPE150 14 Virtual Circuits VC1 2 a c VC3 1 3 e 6 7 5 VC2 b 4 d Virtual circuits are established and terminated much like circuits in circuit switching Statistical multiplexing using packets rather than FDM or TDM is used to share links among connections Spring 2003 UCSC CMPE150 15 Transmission Media We consider the physical layer as a black box We are interested in the characteristics and services provided by the transmission media that impact the link layer and higher layers Parameters Bandwidth Delay or latency average and variance aka jitter Storage capacity bandwidth delay product Reliability and security Order of delivery Type of sharing or access Spring 2003 UCSC CMPE150 16 Bandwidth We think of the bandwidth of a network or link as the number of information bits that can be transmitted over it in a certain period of time e g bits per second The bandwidth of a link is really the frequency range tolerated by the channel without major attenuation Telephone line is 3000 Hz 300Hz to 3300 Hz Available bandwidth depends on the rate at which channel can change stored energy We can model waveforms as sums of sine waves of different frequencies Channel attenuates and delays each frequency component differently causing distortion Spring 2003 UCSC CMPE150 17 Sources of Packet Delay transmission time of packet over each link A B queueing nodal processing delay nodal queueing processing delay propagation delay of each link Spring 2003 UCSC CMPE150 18 Sources of Packet Delay Nodal processing Queueing delay Time waiting at output link for transmission Depends on congestion level of router Transmission delay Checking for bit errors Determining output link Time to send bits into link L R where R link bandwidth bps and L packet length bits Propagation delay Time for each bit to traverse a link d s where d length of physical link and s propagation speed in medium 2x108 m sec Spring 2003 UCSC CMPE150 19 Bandwidth Delay Product The amount of data stored in the link Think of a link as a pipe the latency is the length of the pipe and the bandwidth is its diameter The BD product gives the volume of the pipe Example A channel of 50 ms latency and just 45 Mbps bandwidth can hold 2 25 million bits the same as the memory of a PC of early 80s We are
View Full Document
Unlocking...