Multimedia Traffic on Existing LANs Anton Kos, Sašo 7RPDåLþ Faculty of Electrical Engineering University of Ljubljana 7UåDãND/MXEOMDQD6ORYHQLD E-mail: anton.kos@fe.uni-lj.si Abstract The intention of this paper is to analyse the possibilities for multimedia communications over existent LAN tech- nologies. Types of traffic which form a multimedia traf- fic stream are explained in short. The most widespread LAN technologies, their division, basic properties and their multimedia capabilities are also discussed. Special attention is given to the Ethernet technology and the transmission of real-time voice, which is an integral part of multimedia traffic stream. Its suitability for the transmission of G.729 coded speech is determined by simulation and guidelines for using Ethernet as a net- work for multimedia communications are given. 1 Introduction Multimedia applications have started their march a few years ago and at that time everybody predicted that they would quickly make their way into business and peo- ple's homes. But by now their march has been slowed down considerably. What are the reasons for this? One of the reasons is certainly slow deployment of ATM technology, especially in LANs. Users are reluc- tant to change their "old" LAN technology, which is in most cases Ethernet, for a "new" one, which would be better suited for the transmission of multimedia traffic. Because of all that we have decided to study the possi- bilities for the transmission of multimedia traffic over existing LANs with emphasis on Ethernet and the transmission of packetised voice over it. 2 Traffic Types There are three basic types of traffic: data, voice and video. Each of them has its own set of properties and demands on the transmission network. Delivery of data traffic is not time critical and can therefore bear higher delays. We can label the following as data: documents, mail, files, still images, non real- time voice and video, etc. Data traffic has little demands on the network. What is really important is delivery without errors. Delay, network capacity and other pa- rameters are less important. Transmission of speech, audio and facsimile in real-time is considered as voice traffic. It is delay sensitive and imposes stringent demands on the transmission network in the form of low delay and delay variation, constant transmission rate, error rate, etc. While a human ear can tolerate a certain level of errors in the transmitted voice, it is more sensitive to delay and delay variation. There- fore networks for voice transmission must be fast and should allow bandwidth reservation. Transmission of real-time video has very similar de- mands on the transmission network. The major differ- ence is in bandwidth, which the network must be able to reserve for the transmission of video traffic. While the telephone network is fully capable of voice transmis- sion, its capacity is too low for the transmission of good quality video. Multimedia traffic comprises some or all of the above traffic types and so the underlying network must fulfil a combination of demands of all traffic types. In other words, the properties of a network, which will be able to carry multimedia traffic, will have to be a combination of the best properties of now separate networks sup- porting particular traffic types and services. 3 LAN Technologies While most of the attention nowadays is paid to Wide Area Networks (WAN) and global connectivity less attention is paid to Local Area Networks (LAN). In most cases LANs represent the first and the last section of the end-to-end transmission path and are therefore a very important part of the network. In general, existing LAN technologies are not well suited for transmission of multimedia traffic. Their properties are just not good enough to be able to carry all types of traffic. Still, some technologies perform better than others do. LANs can be divided into groups considering different criteria. Based on a data transmission mode, they can be divided into isochronous and packet networks. The main LAN technologies with isochronous transmis- sion are DQDB, FDDI II and Isochronous Ethernet. The most widespread packet LANs are Ethernet, Token