CSE 228 Lecture 14

Multimedia Networking

As was stated in the first lecture, without the ability to transfer its information over a network, multimedia would be little more than an enhancement to the user interface of a PC. The NOSSDAV conference (Networking and Operating System Support for Digital Audio and Video) started 13 years ago in 1989. Even after 13 years, many of the issues from the first conference are still up to debate!

A Brief History of Networks

• 10 years ago, Cable TV networks began to become mainstream along with Telephone Networks. The telephone network is considered to be an intelligent, pervasive network.
• Roughly 20 years ago, computer networks began to appear. These networks used modems to convert data to voice and transmitted this voice signal over standard telephone lines. At the other end of the line, another modem converted this voice signal back into data for use.

 

Switching

As computer networks became more pervasive, more and more data and also less voice was transmitted over telephone lines. This begs a question, Instead of converting data to voice and sending voice signals, can we convert voice to data and send data? The networking community's initial reaction was no, saying that voice data and internet data are fundamentally different. To understand why, let's look at the two types of switching required for this data, circuit switching and packet switching.

Circuit Switching

The telephone network used circuit switching. Circuit switching operates by first reserving a complete route from the sender to the receiver. Each switch encountered along the path immediately reserves a specified bandwidth and CPU time necessary to allow the call to operate at full capacity for the entire duration of the call. This bandwidth does not change during a call, and remains reserved even if the connection is not transmitting any data but is still active. An example follows:

A fixed bandwidth will be reserved at S1,S2,S4,S5, and S7 for the duration

of this call.

Packet Switching

The other end of the network spectrum, and the method used by the internet, is packet switching. Packet switching sends small packets or chunks of data through the network. If the bandwidth required to send a packet to the next switch is not available, the packet is stored and forwarded to the switch as soon as possible. These switches are called store-and-forward switches for this reason. The problem with packet switching is that there are no performance guarantees available, packets can take highly variable amounts of time to be delivered.

Performance

The performance of Packet Switching is called Best Effort performance. If you transmit from sender to receiver, all the network will do its best to get the packet to the other end as fast as possible, but there are no guarantees on how fast that packet will arrive.

The performance of the telephone network is called Deterministic Performance. Deterministic performance gives us a very tight bound on network performance.

What is performance? There are three measures:

• Bandwidth: How much data can you transmit in one second over the network.
• Delay: What's the delay that a packet suffers from sender to receiver?
• Jitter: How much does the delay vary from packet to packet.

 

Remember that multimedia is very time dependent. Tight bounds must be met in order to keep the stream continuous and intelligible. Also, multimedia traffic tends to be bursty, with the speed of its traffic varying somewhat throughout a signal.

A performance method in between deterministic performance and best effort performance is called Statistical Guarantees. This means that a guarantee such as "at least 90% of the packets will arrive with delay < d" is possible. ATM uses statistical guarantees on performance.

Now, you might ask "Why give a statistical guarantee that says at least 90% of the packets will arrive on time? Why not assign a constant bandwidth channel that is 90% of the maximum bandwidth of the signal instead?" For constant bit-rate (CBR) traffic, this is true. There is no reason to have statistical guarantees unless you have variable bit-rate (VBR) traffic.

An Example

Let's say for a moment that we want to send a signal that has an average bit-rate of 1.2Mb/s and a peak bit-rate of 5Mb/s. If we want to send two of these signals over the same line, we would need a 10Mb/s line if both signals always reach peak bit-rate at the same time. If, however, one signal is at average while the other signal peaks and vice versa, we can get by with a 6.2Mb/s line. Both of these cases make strong assumptions about the data that will be sent along the line, however. If we allocate a certain bandwidth such that if the peaks coincide some data will be lost and we know this tends to happen < 10% of the time, we can say that 90% of the traffic will arrive on time -- a statistical guarantee.

ATM vs. GEE

Asynchronous Transfer Mode (ATM) networks were thought to be the solution to modern networking up until about one year ago. The other community, Gigabit Ethernet Everywhere (GEE) says that we can make Ethernet so fast that it can provide a flexible, fast networking solution at a fraction of the price of ATM.