IP Packet Switching in Telecom - Part 2 - TelecomHall

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Posted by leopedrini Sunday, February 05, 2012 10:06:00 AM Categories: Course

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So far so good Like promised, lets start our journey about IP

networking in the telecom context from the ceiling and going down. So

lets understand what the heck is an application.

Note: My blog Smolka et Catervarii(portuguese-only content for the moment)

Technically we call an application any program that runs under control and taking advantage of the

services of the operating system. Thats a fairly reasonable definition for our purposes, since allnetworking architectures are devised to allow communication between applications, not people. Each

application has its own way of human-machine interaction handling (if it exists at all). Were not

concerned with this here. All we want to explain is how applications can reliably exchange data among

them.

And here we arrive at the first paradigm-breaking aspect of the change from the circuit-switching-based

plain old telephony service (POTS) and the IP-packet-switching-based next-generation network (NGN).

POTS networks are organized in such a way that youve got dumb (and reasonably cheap) user terminals

connected through a smart (and very, very expensive) network. Everytime the user wants to use network

services and for a very longtime thered be only one: telephony he/she has to ask the network for it.

By means of sound-based network-to-user signaling and key-pressing user-to-network signaling (seeDTMFand ITU-T recommendation Q.23 and Q.24) the user says I want to talk with this user, and the

network makes the arrangements to provide the end-to end circuit which the communicating parties will

use.

IP-based networks, of which the Internet is the major example, were built assuming the user terminals are

smart (and not overwhelmingly expensive) and the network doesnt have to have more smartness than

necessary to perform a single function: take the data packets from one side to the other with reasonable

reliability. All the aspects of communication that telephony engineers are used to name as call-control

are negotiated directly between user applications. This is the function of the so-called application-layer

protocols.

So we have, so to speak, two different philosophies to handle the call control (which is another way to

say session control): the network-in-the-middle approach, and the end-to-end principle. The schematic

call-flow diagrams below give an example of the differences between them.

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Generally speaking there are two ways of application interaction, both widely used: peer-to-peer and

client-server. On peer-to- peer sessions the communicating parties have the same status, and any of them

can request or offer services to the other. Client-server sessions, on the other hand, have a clear role

distinction between the parties: one requests services (the client) and the other fulfill the services

requests (the server).

Most of Internet applications use the client-server model, and that goes quite well with the end-to-end

principle. Otherwise NGN telecommunication services go both ways. Theres services that are a clear fit to

the client-server model, like video or audio streaming, and theres services that use peer-to-peer, like

voice and video telephony (by the way, videoconferencing can go both ways).

This and a few other issues (security, mostly) forced NGN call-control architecture to use client-server

interactions for signaling, and peer-to-peer or client-server for data exchange, according to service

characteristics. The diagram below is an example of this.

The packet routers between the elements are not shown. And this picture is a gross oversimplification of

NGN architecture. I will not go into details about this, but if you want to get a more rigorous approach tothis subject I recommend you strt reading ITU-T recommendations Y.2001 General overview of NGNand

Y.2011 General principles and general reference model for Next Generation Networks .

Roughly speaking, the AAA (authentication, authorization and accounting) server role goes to the IP

Multimedia Subsystem (IMS), which was initially standardized by 3GPP/ETSI (see ETSI TS 123 228 V9.4.0

IP Multimedia Subsystem), and later adopted by ITU (recommendation Y.2021 IMS for Next Generation

Networks). Actually it does much more than simply AAA functions. Its the entry door to all NGN signaling

which are based on Session Initiation Protocol SIP, and Session Description Protocol SDP (see ETSI TS

124 229 V9.10.2 IP multimedia call control protocol based on SIP and SDP ; IETF RFC 3261 SIP: Session

Initiation Protocol; and IETF RFC 4566 SDP: Session Description Protocol).

On the next part of this article series well take a closer and more formal look at IMS, SIP and SDP.

Hope itll be soon.Auf wiedersehen.

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