Last time we saw packet switching vs. circuit switching. Neither of these technologies is directly suitable for multimedia traffic. Multimedia requires performance guarantees, but it also tends to be bursty.

ATM (Asynchronous Transfer Mode) is a hybrid network that offers performance guarantees and flexible switching.

Statistical multiplexing is good for multimedia traffic.

Now, let's take a look at some of the major information networks in use today.

The Cable Television Network is a much simpler network capable of delivering high bandwidth in one direction to a user. The following diagram illustrates the architecture:

In general, one Fiber Node serves a neighborhood of roughly 2000 homes. The fiber optic lines that go from the Head End to the Fiber Nodes are much better signal carriers than the coaxial cables used in the feeder and drop lines (Notice there are fewer amplifiers -- the triangles in the diagram -- on the fiber lines). Sometimes the fiber optic lines carry a digital signal instead of an analog one. Instead of using something like MPEG-2, the lines simply carry a digitally sampled version of the analog signal that is converted back into analog when it reaches the fiber node.

The Head Ends are connected together in a ring fashion as seen below:

This is called a Hybrid Fiber-Coax Network or HFC. There is another system called FTTC, which stands for Fiber-To-The-Curb. This system uses fiber optic lines all the way into the users residence, providing a much higher quality signal.

The advantage of the Cable TV network is very high bandwidth to the user.

The problems of the Cable TV network are:

1. Switching: The network is designed for one-way communication.
2. Low Bandwidth back channel to the head end.
3. Homes do not have individual network addresses, any type of signal must be broadcast to the entire network. Scrambling/Descrambling technology can help provide the illusion of individual network addresses, but this is not very secure.

The telephone network looks like the following:

It has several telephones (TEL) connected to a PBX switch (You know, those big boxes you see around your neighborhood). These PBX switches are connected to the End Office (EO). Finally, the End Offices are connected to the Area Office (AO) that is responsible for an entire area code. It is important to note that if a user from TEL_A wants to call a user at TEL_B, the call still gets routed through the EO even though the two users share the same PBX switch.

There are various types of telephone service provided:

POTS - Plain Old Telephone Service

ADSL - Asymmetric Digital Subscriber Line

This is a simple enhancement for the telephone service. It is a high-bandwidth one-way line. It is implemented with a box between the customer and the network switch. The box receives video from one network, and provides telephone service over another network and internet service on its own network. (Integrated Access)

HDSL - High-Speed Digital Subscriber Line.

This is an enhanced version of ADSL that transmits all three types of network service over the same line, but later they are separated and switched independently. (Integrated Transmission)

There are three levels of integration:

1. Integrated Access

   ADSL and ISDN both provide integrated access. This is a type of system that provides a single access (e.g. a box on the wall) to several different networks. However, behind this box each network is transmitted indepently and also switched indepently.

2. Integrated Transmission

   HDSL provides Integrated Transmission, the next step in multimedia integration. It provides integrated access, and in addition transmits each of the various network streams along the same line. However, at some point the networks must be separated and switched independently.

3. Integrated Switching

Getting high-speed wires in a network is not difficult or expensive. Switches are the difficult part of a network. ATM is a technology for high-speed switching. In order to make ATM fast, the designers tried to make it as simple as possible. ATM uses fixed size packets (called cells) that are fixed to 53 bytes each. Why did they choose 53 bytes? A committee was formed to decide what the best cell size was. This committee consisted of two primary communities. The computer community wanted 64 byte packets. The telephone community wanted 42 bytes. After trying and trying to come to a resolution, they just split the difference and went with 53 bytes. Of these 53 bytes, 5 are used to carry the header information and 48 are used to carry payload (data).