The overall protocol architecture that makes up the capability to deliver multimedia over what was originally a pure data network, is surprisingly not so very different from the original Internet Architecture.
The protocol stacks for Internet multimedia are show in figure 1.1 above. Most of the protocols are not deeply layered unlike many other protocol stacks, but rather are used alongside each other to produce a complete session. It is possible to use multimedia applications over the Internet without some (or all) of the additional protocols (e.g. omission of RSVP, or omission of SDP, and so on) depending on the performance or functionality required. Later chapters will show how each new protocol adds value to the basic IP packet delivery model.
In the next section, we discuss the underlying unicast and multicast delivery model, which must be added within the IP layer to give maximum benefit to the network.
Packet Switched Data networking adds value to circuit switched networks by sharing the capacity amongst multiple users in time. The data stream from each user is broken into chunks to form packets, which only require capacity when each packet is being sent. The capacity required is thus the sum of the average bandwidth, rather than the total peak bandwidth. This ``statistical multiplexing gain'' comes at the expense of having sometimes to queue packets for access to the transmission channel.
The statistical multiplexing gain is quite large for traditional bursty data applications such as WWW access. For multimedia traffic, this gain is harder to achieve (depending on the compression algorithms used, as discussed in later chapters, it can even become even harder) and yet we can get a spatial gain in use of the network by using group communication carefully. A simple way to send data to multiple recipients is to send it multiple times from the source. Such ``multiple unicasts'' make very poor use of the links near the sender, and potentially incur a lot of delay before a single transmission is completed. The Internet offers a mechanism to avoid these overheads called IP multicast.
Multicast is essential in the context of Internet technologies that may replace television (streaming services, as discussed in chapter nine), but it is also highly relevant in telephony services, especially in the business community: it is extremely common for one phone call to spawn another - the ability to teleconference is not widespread in the traditional plain old telephone system except at some commercial cost, or in a restricted subset of telephones. The Internet ability to provide this service is quite powerful.
We discuss some of the limitations of multicast as a service in chapter three. Some of the problems that vendors have had in supporting large scale multicast as well as service guarantees in Router products have prompted the design of a new generation of routers coming on to the market as we write. Similarly, the understanding of service guarantees and multicast has been slow to diffuse into the commercial Internet Service Provider community until relatively recently, but this is changing rapidly.