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Computer Networks

by: Earlene Cremin III

Computer Networks CPSC 5157U

Earlene Cremin III

GPA 3.91

Edward Bosworth

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Edward Bosworth
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This 7 page Class Notes was uploaded by Earlene Cremin III on Sunday October 11, 2015. The Class Notes belongs to CPSC 5157U at Columbus State University taught by Edward Bosworth in Fall. Since its upload, it has received 24 views. For similar materials see /class/221201/cpsc-5157u-columbus-state-university in ComputerScienence at Columbus State University.


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Date Created: 10/11/15
CPSC 5157 Computer Networks Lecture 12 Lecture for Wednesday June 28 2006 Lecture Topics Chapter 14 Connection Oriented Networking and ATM Design goals for ISDN ATM Establishing Virtual Circuits PVC and SVC ATM Design and Cells Comments on ATM Chapter 15 Network Characteristics Ownership Service and Performance Networks Private Public and VPN Delay Throughput and Jitter NOTE Many of the issues discussed theoretically in chapter 15 will reappear later and more concretely in the discussion of IP We deal with many of them then Slide 1 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 Design goals for ISDN ATM Establish a single global network for all possible types of traf c including data audio and telephony video Broad goals included 1 Universal Service 2 Support for all Uses 3 A Single Uni ed Infrastructure 4 Service Guarantees see discussion of Delay and Jitter The goals may have been overly ambitious Who wants television delivered over telephone lines Such service is available but cable TV is already here and works Many customers your instructor included adopt the rst option to market and do not change without good reason As will be seen the desire for universal service lead to compromises in the design of ATM cells Slide 2 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 Real Circuits Virtual Circuits and Datagrams In a real circuit your instructor s terminology the connection between two users is established and remains until the end of the session A long distance telephone call is a good example of a real circuit A VC Virtual Circuit or Virtual Channel is a path through the network from one application to another that is used to send data For each intermediate switch node in the path there is an incoming link and an outgoing link No transmission occurs until a VC has been established As a part of this initialization each intermediate switch reserves capacity for that VC The rule for the switch is if a cell comes in on this link it goes out on that link Intermediate switches do not check a destination address as such address appears only in the messages that set up the VC In a Connectionless Datagram approach such as IP each intermediate switch inspects the destination address and consults a routing table to determine the proper outgoing link for the datagram There is no circuit setup and no capacity reserved This approach does not guarantee QoS Quality of Service but just best effort Slide 3 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 Establishing Virtual Circuits PVC and SVC As just mentioned the VC must be set up with a speci ed Quality of Service and capacity reserved at each intermediate switch before communication can begin The ATM Virtual Channels come in two varieties PVC A Permanent Virtual Circuit analogous to a leased telephone line SVC A Switched Virtual Circuit analogous to a dialed telephone call Each of the two types of channels is likely to go through a number of intermediate switches It is just that the SVC is set up dynamically used and then torn down The setup phase of the SVC begins with one application sending a connection request to the network This is also known as signaling and uses a dedicated VPIV CI value see later The network responds by creating a complete path between the two communicating applications reserving appropriate network capacity and updating the forwarding tables in each intermediate node In a PVC this setup is done once It is often called provisioning a channel Each channel is identi ed by a 24 bit identi er called VPIVCI the 8 bit Virtual Path Identi er identi es the path through the network the 16 bit Virtual Channel Identi er identi es a link in the circuit Slide 4 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 ATM Design and Cells The study of ATM design is useful because it focuses on the compromises that are usually made when a network is set up Recall the three goals for ATM video audio and data Data transmission calls for large packet sizes Audio transmission especially telephony calls for small packet sizes The ATM compromise yields a packet called a cell with a size of 53 octets ATM goals are high speed low delay and low jitter In a network jitter is de ned as variance in delay If there is a wide difference in the delay of two cells the cells will arrive out of sequence For a telephone call this is annoying The ATM specification calls for 53 octets per cell 5 octets for the header 48 octets for the data Note that 553 0094 a 94 overhead for the header This is called cell tax For comparison an IP packet has 26 octets of overhead and between 46 and 1500 octets of data This is a tax of anywhere between 2672 36 and 26 1526 17 Slide 5 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 Comments on ATM ATM has a few strengths and some problems The use of a xed cell size 53 octets allows optimization of the network hardware Memory fragmentation does not occur in ATM switches The reason is that memory can be allocated in a xed number of slots An ATM cell can be placed into any available cell A switch must place an IP packet into a continuous segment of memory that is large enough for that packet End of packet detection is not an issue for ATM All packets called cells have the same xed length Just count the bits But ATM is expensive as it is designed to provide such a wide variety of services ATM has a latency time delay associated with the channel setup that might be comparable to the time required to send the entire sequence of cells ATM cannot open a channel if it cannot provide the required Quality of Service IP will send at the best available rate Quality of Service might be overrated If the network has enough capacity the QoS is unnecessary If the network does not have the capacity nothing is sent Slide 6 of8 Slides CPSC 5157 Revised 06282006 CPSC 5157 Computer Networks Lecture for Wednesday June 28 Networks Private Public and VPN We now begin Chapter 15 material A private network uses components switches and digital lines that are owned by one person or corporation It is isolated from other networks and thus difficult to hack into A public network uses components provided by a public carrier such as ATampT Obviously messages on a public network are public There are a number of amazing regulatory constraints on private networks One common requirement is that any cable crossing a public road must be owned by a common carrier such as ATampT Verizon etc A VPN Virtual Private Network is a connection over a public network that can be treated as a private network Each site that participates in a VPN connects to the public network through a system hardware andor software that restricts both incoming and outgoing packets to a list of approved correspondents A Virtual Private Network will often use encryption to enhance privacy While no system can guarantee absolute privacy current crypto systems can give a very high assurance that communications are not read by third parties Slide 7 of8 Slides CPSC 5157 Revised 06282006


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