McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Network Operations & administration CS 4592 Lecture 11 Instructor: Ibrahim Tariq

9.2 DIGITAL SUBSCRIBER LINE DIGITAL SUBSCRIBER LINE After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops. ADSL ADSL Lite HDSL SDSL VDSL Topics discussed in this section:

9.3 ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. Note

9.4 ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line. Note

9.5 Discrete multitone technique(combination of QAM and FDM)

9.6 Bandwidth division in ADSL

9.7 ADSL modem

9.8 DSLAM Digital subscriber line access multiplexer

9.9 Summary of DSL technologies

9.10 CABLE TV NETWORKS The cable TV network started as a video service provider, but it has moved to the business of Internet access. In this section, we discuss cable TV networks per se; Traditional Cable Networks Hybrid Fiber-Coaxial (HFC) Network Topics discussed in this section:

9.11 Traditional cable TV network

9.12 Communication in the traditional cable TV network is unidirectional. Note

9.13 Hybrid fiber-coaxial (HFC) network

9.14 Communication in an HFC cable TV network can be bidirectional. Note

9.15 CABLE TV FOR DATA TRANSFER Cable companies are now competing with telephone companies for the residential customer who wants high-speed data transfer. In this section, we briefly discuss this technology.

9.16 Division of coaxial cable band by CATV

9.17 Downstream data are modulated using the 64-QAM modulation technique. Note

9.18 The theoretical downstream data rate is 30 Mbps. Note

9.19 Upstream data are modulated using the QPSK modulation technique. Note

9.20 The theoretical upstream data rate is 12 Mbps. Note

9.21 Cable modem (CM)

9.22 Cable modem transmission system (CMTS)

Packet Switching Difference between circuit switching and packet switching: Packet Switching Message is broken up into segments (packets). Each packet carries the identification of the intended recipient, data used to assist in data correction and the position of the packet in the sequence. Each packet is treated individually by the switching centre and may be sent to the destination by a totally different route to all the others.

Packet Switching Advantages: Security Bandwidth used to full potential Devices of different speeds can communicate Not affected by line failure (rediverts signal) Availability – do not have to wait for a direct connection to become available During a crisis or disaster, when the public telephone network might stop working, s and texts can still be sent via packet switching

Packet Switching Disadvantages Under heavy use there can be a delay Data packets can get lost or become corrupted Protocols are needed for a reliable transfer Not so good for some types data streams e.g real-time video streams can lose frames due to the way packets arrive out of sequence.

Circuit Switching There are three phases in circuit switching: Establish Transfer Disconnect The telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.

Circuit Switching Advantages: Circuit is dedicated to the call – no interference, no sharing Guaranteed the full bandwidth for the duration of the call Guaranteed Quality of Service

Circuit Switching Disadvantages: Inefficient – the equipment may be unused for a lot of the call, if no data is being sent, the dedicated line still remains open Takes a relatively long time to set up the circuit During a crisis or disaster, the network may become unstable or unavailable. It was primarily developed for voice traffic rather than data traffic.

Error Detection and Correction 10.29

10.30 Data can be corrupted during transmission. Some applications require that errors be detected and corrected. Note

INTRODUCTION Let us first discuss some issues related, directly or indirectly, to error detection and correction. Types of Errors Redundancy Detection Versus Correction Forward Error Correction Versus Retransmission Coding Modular Arithmetic Topics discussed in this section:

10.32 In a single-bit error, only 1 bit in the data unit has changed. Note

10.33 Figure 10.1 Single-bit error

10.34 A burst error means that 2 or more bits in the data unit have changed. Note

10.35 Figure 10.2 Burst error of length 8

10.36 To detect or correct errors, we need to send extra (redundant) bits with data. Note

10.37 The structure of encoder and decoder

10.38 In this book, we concentrate on block codes; we leave convolution codes to advanced texts. Note