Computer Networks and Internets with Internet Applications 4

Computer Networks and Internets with Internet Applications, 4 e By Douglas E. Comer Lecture Power. Points By Lami Kaya, LKaya@ieee. org © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Chapter 14 Connection-Oriented Networking And Asynchronous Transfer Mode (ATM) © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Topics Covered • • • 14. 1 Introduction 14. 2 A Single, Global Network 14. 3 ISDN And ATM 14. 4 ATM Design And Cells 14. 5 Connection-Oriented Service 14. 6 VPI / VCI 14. 7 Labels And Label Switching 14. 8 An Example Trip Through An ATM Network 14. 9 Permanent Virtual Circuits 14. 10 Switched Virtual Circuits © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Topics Covered (cont) • 14. 11 Quality Of Service • 14. 12 The Motivation For Cells And Label Switching – 14. 12. 1 Cells vs. Packets. – 14. 12. 2 Label Switching Vs. Routing • 14. 13 ATM Data Transmission And AAL 5 • 14. 14 Critique Of ATM • 14. 15 Multi Protocol Label Switching (MPLS) © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 1 Introduction This chapter • explores a single technology that was designed to fill both roles • presents a technical overview • explains the underlying motivation • introduces the connection-oriented PS paradigm © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 2 A Single, Global Network A new NW infrastructure with extremely ambitious goals: • Universal Service • Support For All Uses • Single, Unified Infrastructure – The new NW should not be formed from multiple technologies – NW should be capable of serving as a LAN or as a WAN • Service Guarantees – perform with the same reliability and efficiency as existing NWs – offer the same delivery guarantees that voice and video NWs • Support For Low-Cost Devices – allow users to connect small, low cost devices such as ATM telephones © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 3 ISDN And ATM • One of the attemp to satisfy the NW goals was – Integrated Services Digital NW (ISDN) • As the planning, standardization, development, and deployment of ISDN proceeded – NW changed rapidly and dialup modem technology advanced • When ISDN finally emerged – it was expensive for the moderate data speeds it offered – ISDN has not gained universal acceptance • After ISDN, the telecom industry developed a more comprehensive technology to satisfy its goals – Asynchronous Transfer Mode (ATM) © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 4 ATM Design And Cells (1) • ATM designers faced a difficult challenge – because three intended uses (voice, video, and data) have different sets of requirements • For example – both voice/video require low delay/ jitter (ie, low variance in delay) • make it possible to deliver smoothly without gaps or delays – video requires a substantially higher data rate than audio – most data NWs introduce jitter as they handle packets • Packet size is a key issue in designing a NW technology – Data achieves maximal throughput when packet sizes are large • Because the overhead from headers is minimized by carrying a maximum payload with each header © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 4 ATM Design And Cells (2) • Voice transmission cannot use such large packets for two reasons • First, PCM, produces an 8 -bit audio sample every 125 seconds – If packet size greater than 4000 octets, a sender must delay for more than half a second while accumulating enough samples to fill the packet • Second, users are also intolerant of echo – a problem that occurs when signals traveling in one direction are inadvertently amplified and transmitted back to the sender – telephone systems employ echo cancellation – Unfortunately, echo cancellation only works when delays are low • If large packets used, the delay introduced by waiting to fill a packet makes echo cancellation difficult • ATM technology divides data into small, fixed-size “cells” • Each ATM cell contains exactly 53 -octets – 5 octets of header information and 48 octets of data • Figure 141 illustrates the format of an ATM cell © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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14. 4 ATM Design And Cells (3) As the figure shows, • Most header bits are devoted to two VPI and VCI – These fields together identify the cell's destination • Other fields specify a payload type and give an 8 -bit CRC • PRIO is a Cell Loss Priority bit that identifies – if a packet can be discarded when the NW becomes congested © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 4 ATM Design And Cells (4) • Proponents of ATM argue that – A single technology must handle all possible communication • Critics suggest that – ATM is neither optimal for voice nor for data – the relative size of the header, is not suitable for data transmission • When ATM was designed, the designers decided – to limit the header to ten percent of the payload area – payload 48 bytes was chosen, the header was fixed at 5 bytes • For data NW 10% percent header overhead is high © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 5 Connection-Oriented Service • ATM uses a connection-oriented service paradigm – Before two computers can communicate, they must establish a ``connection'‘ – HW establishes a data path called a connection and returns a “connection identifier” to each of the two computers – An ATM switch extracts the connection identifier and consults a table to determine how to forward the cell © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 6 VPI / VCI (1) • Formally, an ATM connection is known as a – Virtual Channel (VC) • Term “virtual” is appropriate because – ATM connections are formed by storing values in memory rather than by attaching physical wires • The term “channel” is less descriptive • Many professionals expand the acronym VC to the more descriptive term – Virtual Circuit © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 6 VPI / VCI (2) • ATM assigns each VC a 24 -bit identifier that is divided into two parts • First part, a Virtual Path Identifier (VPI) – specifies the path the VC follows through the NW – As Figure 14. 1 shows, a VPI is 8 bits long • The second part, a Virtual Channel Identifier (VCI) – specifies a single VC within the path – A VCI is 16 bits long • The computer views the two parts as a single – 24 -bit binary value that gives the connection identifier • Because a computer refers to both parts together – the identifier is known as a VPI / VCI © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 7 Labels And Label Switching • Each ATM switch has multiple physical attachment points – connect to a user's computer or to another switch – the attachment points are known as “ports” • An ATM switch changes the VPI / VCI in each cell • Inside each switch is a “forwarding table” – Each entry in the table corresponds to a possible VPI / VCI • The table entry contains a replacement VPI / VCI • Switch rewrites the VPI / VCI in the cell header with the replacement, and forwards the cell – Known as “label rewriting” or “label switching” • A VPI / VCI in a cell does not remain the same as the cell traverses the NW, see Figure 14. 2 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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14. 8 An Example Trip Through An ATM Network • How label switching extends to a large ATM NW? • The forwarding tables in a series of switches must be coordinated to create a ``path'' through the NW • Figure 143 illustrates a set of three ATM switches – shows the values in the forwarding tables that correspond to a single VC between two computers © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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14. 9 Permanent Virtual Circuits • How and when are entries in an ATM forwarding table filled in? • How are entries coordinated across a large number of switches to ensure that they are correct? • This section answers each of these questions • ATM offers a facility that is analogous to a leased digital circuit – called permanent virtual channel (PVC) • To establish a PVC, the NW manager configures entries in the • forwarding tables manually • The technical term used by telephone companies (and ATM) for such configuration is “provisioning” © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 10 Switched Virtual Circuits • To handle dynamic interaction, ATM includes a facility that allows VCs to be established and terminated as needed • A VC can be created dynamically – called Switched Virtual Channel (SVC) • Each pair of ATM switches along the path communicate to choose a VPI / VCI to use for the VC and initialize their tables – If any switch along the path does not agree to provide the VC, an error message is sent back and the request is denied • How does a computer interact with an ATM to request an SVC? – The process, which is known as “signaling” requires control messages – ATM reserves a small set of VPI / VCI values for connection requests and NW control traffic – The reserved values can only be used across one connection © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 11 Quality Of Service (1) • To satisfy the needs of audio and video users, ATM has facilities that allow a subscriber to specify – quality of service (Qo. S) • ATM uses fine-grain quality of service because it allows a subscriber to specify quantitative values • Qo. S specifications are given when the connection is established, – and stay in effect until the connection is terminated – once a request has been approved • the switches must reserve capacity as promised – an ATM NW cannot behave like a shared channel © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 11 Quality Of Service (2) • ATM has several basic ways to specify Qo. S requirements • Constant Bit Rate (CBR) – For applications like compressed audio or video • the transmission rate depends on the amount of change in the input during each sample • Variable Bit Rate (VBR) – For applications that know in advance they will vary the rate of traffic • Available Bit Rate (ABR) – The transmission may be bursty, with short periods of intense communication, separated by long periods of silence – use whatever BW is available at a given time © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 12 The Motivation For Cells And Label Switching Several questions arise • Why did ATM designers choose fixed-size cells instead of variable-size packets? • What is the chief advantage of label switching, and why did ATM adopt it? • The answers are the same – to achieve maximum data rates and satisfy Qo. S requirements © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 12. 1 Cells vs. Packets NW HW could be optimized if all packets were exactly the same size and had exactly the same header format: • 1 st, variable-size packets can cause memory fragmentation; fixedsize cells do not • 2 nd, variable-size packets require the HW to accommodate the largest possible incoming packet and to detect the end of the packet • 3 rd, interface HW that transmits variable-size packets must interact with the switch • 4 th, variable-size packets make it difficult to guarantee Qo. S, especially low jitter • Thus, the ATM designers chose fixed-size cells rather than variablesize packets for efficiency © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 12. 2 Label Switching Vs. Routing • Label switching is another technology that increases HW speed and capacity • The HW has little to do when a cell arrives – extraction of the VPI / VCI and table lookup can both be implemented directly in HW; no CPU is required • As a result, the raw capacity of an ATM switch is usually quite high – For example, a typical ATM switch, designed as a LAN replacement, has an aggregate throughput capacity of 24 Gbps © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 13 ATM Data Transmission And AAL 5 • ATM defines a set of adaptation protocols – which can be thought of as a set of API • Although five adaptation protocols have been defined – Only ATM Adaptation Layer 5 (AAL 5) is used to send data – On the sending side, • AAL 5 accepts a block of data (up to 64 K octets), • divides the block into cells • and transfers the cells across the ATM NW as usual – On the receiving side, • AAL 5 accepts incoming cells • extracts the data • and delivers the original block – The process is known as “segmentation” and “reassembly” © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 14 Critique Of ATM The ATM technology has several drawbacks: • Expense • Connection Setup Latency • Cell Tax • Specification of Service Requirements • Lack Of Efficient Broadcast • Complexity of Qo. S • Assumption Of Homogeneity © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

14. 15 Multi Protocol Label Switching (MPLS) • Although ATM did not become popular – proponents continue to devise and promote variations of connection-oriented NW • One version of connection-oriented NW goes by the name Multi Protocol Label Switching (MPLS) • Instead of attempting to completely replace the Internet – MPLS is designed to serve in the Internet core • The basic idea is to use ATM-like label switches to provide virtual circuits among IP routers – Chapter 17 describes IP routers © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.