Side_1_360

Two examples of attribute lines are the frame

rate and a quality indicator:

a=framerate:<frame rate>, giving

the number of video frame rates per second.

a=quality:<quality>, giving a value

from 0 to 10 (10 being the best, 5 the default,

and 0 the worst but still usable).

7.4.3 Multimedia Conferencing

Architecture

The multimedia conferencing architecture elabo-

rated by IETF is depicted in Figure 27.

As shown, SIP may use UDP (unlike HTTP).

Then, several SIP messages can be put into the

same UDP message. When TCP is used, several

SIP transactions can be carried on the same TCP

connection. If the server leaves the TCP connec-

tion open after returning the reply, the client

may use the connection for later SIP messages

(or even other protocols, like HTTP).

A similar architecture has also been described

for H.323.

7.4.4 Interworking

Supporting the telephony service, more functions

must be available, such as servers for handling

the service (and supplementary services) as

described above. An additional element is a gate-

way used whenever two domain types are to be

traversed. Commonly a gateway is separated into

a Media Gateway (MG) and a Media Gateway

Controller (MGC) function, see Figure 28. The

protocols between MGs could be any of the

packet formats for traffic flows. The signalling

protocols between the MGCs might be SIP,

Bearer Independent Call Control (BICC), or

belong to the H.323 family. Megaco is a protocol

that may be used between the MG and the MGC.

Similar to SIP, H.323 also defines mechanisms

for call routing, call signalling, capability ex-

change, media control and supplementary ser-

vices. Work is undergoing to specify interwork-

ing between the two protocols, e.g. within the

ETSI TIPHON project.

7.4.5 Performance Issues

In order to support transport of real-time traffic

flows over an IP network, one must be able to

handle:

• Timing and synchronisation of, and between,
  individual samples of traffic flows for the
  same applications;


• Effects of packets being lost;


• Effects of packets being delayed;


• Packets arriving in a different order at the
  receiver than they were sent;


• Multiple traffic flows and different types of
  traffic flows;


• Monitoring and flow control.

There are multiple ways of implementing the

voice transfer service, in the network, but per-

type description type description

v protocol version o owner/creator and session

identifier

s session name i * session information

u * URI of description e * e-mail address

p * phone number c * connection information

b * bandwidth information z * time zone adjustments

k * encryption key a * zero or more session

attribute lines

t time the session is active r * zero or more repeat times

m media name and i * media title

transport address

Table 2 Type identifiers for
SDP (* indicates optional
type)

Figure 27 IETF multimedia
conferencing architecture

SMTP

SDP

UDP

IP and IP multicast

integrated services forwarding

TCP

SAP SIP HTTP

conference set-up

and discovery

conference course

control

audio/

video

shared

applications

RSVP distributioncontrol RTCPRTP/ multicastreliable

UDP

conference management media agents