Voice Over Packet Networks (Wiley Series on Communications Technology) - Hardcover

Wright, David J.

Hardcover

ISBN 10: 0471495166 ISBN 13: 9780471495161

Verlag: Wiley, 2001

Inhaltsangabe

One of the hottest topics in the telecommuncations industry today is the migration of voice traffic from the circuit switched Telco networks to packet switched networks such as IP, ATM, Frame Relay and access technologies such as DSL (Digital Subscriber Line) and Packet Cable. The sheer volume of voice traffic, together with the efficiencies of packet transport and the opportunity to offer new features on voice calls, has made internetworking, between circuit and packet transport, vital while this migration is in progress.
 
Presents a concise treatment of a complex subject providing up-to-date coverage of all relevant packet voice technologies and an evaluation of how they compare
Explains how and why to transport voice over packet networks
Includes MEGACO: Media Gateway Control
Protocol, Voice over Packet Cable and Voice over DSL
Enables the reader to obtain information rapidly by featuring numerous bulleted lists and extensive diagrams
 
Voice over Packet Networks is comprehensive and compact and will be welcomed by both engineers and planners in the telecommunications industry, as well as graduate and senior undergraduate students on telecommunication courses.

Über diesen Titel

Über die Autorin bzw. den Autor

David J. Wright University of Ottawa, Canada

Auszug. © Genehmigter Nachdruck. Alle Rechte vorbehalten.

Introduction

This book provides information on how voice is transported over packet networks: a topic which bridges between the circuit networks from which the voice traffic is being migrated and the packet networks onto which it is being migrated. Interworking between circuit and packet transport is the name of the game while that migration is in progress. This book is therefore designed for readers with a background in the circuit world as well as for readers with a background in the packet world. The purpose of this introduction is to provide a primer to bring each reader up to speed on whichever technology they may not be familiar with. This material is very introductory, covering the basic concepts, terminology and acronyms and may well be skipped by many readers.

1. Introduction to the Circuit Network The PSTN, public switched telephone network, transports voice and switches it in minimum bandwidths of 64 Kbps. Customers use a range of technologies to access the PSTN:

* Analog twisted pair copper phone lines. These lines run typically from residential customers to the network and carry an electrical signal representing the analog audio waveform in the region 300 Hz to 3.4 Kl-lz.

* Integrated services digital network (ISDN). These digital lines provide two options: Basic rate interface (BRI) provides two user channels at 64 Kbps and one signaling channel at 16 Kbps Primary rate interface (PRI) provides 23 or 30 user channels at 64 Kbps and one signaling channel at 64 Kbps

* TI/E1.TlisaNorthAmericanstandardwith24channelsat64Kbps.ElisaEuropean standard with 30 channels at 64 Kbps, plus signalling and management channels.

We now describe a day in the life of a telephone call in order to introduce the reader to all the components of the PSTN and how they work together.

A customer (called the calling party) picks up the phone which results in an "off-hook" message being sent to the network equipment, which responds by generating a dial tone for the customer to hear. This confirms that the network is ready to accept a call. The customer dials a number (called dialed digits) in order to place a call to a called party. The digits are sent down the access line from the customer to the network in one of a variety of formats:

* Dual tone multifrequency (DTMF). This is used on analog access lines for the dialed digits themselves. * Channel associated signaling (CAS). This uses some of the least significant bits in the voice channel to transport codes corresponding to the dialed digits. * Common channel signaling (CCS). This uses a single channel to carry the codes corresponding to the dialed digits of a number of calls.

The dialed information is converted into signaling messages by the edge network equipment, also known as the ingress node or Class 5 switch. The signaling messages are interpreted by the switch in order to figure out the next hop on the path to the final destination, and the switch reserves one time slot (8 bits) on the outgoing transmission facility. The outgoing transmission facility may be, for instance, a TI (1.5 Mbps), T3 (45 Mbps) or STS-N (n X 5 1.8 Mbps) and it connects the ingress switch to the next switch. Whatever the transmission facility is, it is controlled by a master clock which ticks 8000 times/s. TI, T3 or STS-N frames containing time slots are generated at each tick of the clock. The 8 reserved bits for our phone call repeated 8000 times/s correspond to 64 Kbps.

The signaling message is then passed into the SS7 network which transports it to the next switch downstream, where the next hop of the route is determined and the next time slot reserved. This process is repeated until we reach the destination edge switch, which is another Class 5 switch also known as the egress switch. The egress switch causes the phone to ring at the destination, and the ingress switch causes a ringing tone to be played to the calling party. When the called party picks up the phone (going off-hook) messages are generated backwards through the SS7 network to tell the ingress switch to stop playing the ringing tone and the two parties can start to speak to each other. This process is referred to as call establishment or call set-up.

Once the parties start to speak their voice is coded into 64 Kbps using the ITU-T standard G.71 1. This may be done in the customer premises equipment (ISDN phone or private branch exchange (PBX)) if the customer is connected to the network over a digital access line or it may be done in the edge switch if an analog access line is being used. The G.711 coder/decoder or codec produces 8 bits of code 8000 times/s which maps neatly onto the time slots that have been reserved for its transport.

The customer can activate a voice feature, such as "redial on busy", by dialing more digits after the call set-up is complete. This activates software in the Class 5 switches at either end of the connection which provides the feature to the customer. In the case of redial on busy, the egress Class 5 switch monitors the destination line. When the customer goes on-hook, it signals the ingress Class 5 switch over the SS7 network to apply a special ring to the calling party line.

Class 5 switches are the edge switches in the PSTN and implement millions of lines of software to provide features of this type plus special private network management features for business customers. Moving towards the core of the network we encounter Class 4 (or tandem) switches, so called because the voice traffic traverses them (or tandems through them) without accessing any software. Next come the Toll switches, which represent the entry points into the long distance network where billing software monitors the call holding time and charges the customer accordingly. Customers can access databases in the SS7 network which, for instance, translate toll...

Details zur Ausgabe

• Verlag: Wiley
• Erscheinungsdatum: 2001
• Sprache: Englisch
• ISBN 10: 0471495166
• ISBN 13: 9780471495161
• Einband: Gebundene Ausgabe
• Auflage: 1
• Anzahl der Seiten: 270
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar