ATM based Wireless (Mobile) Broadband Multimedia Systems

Contributed by Ramjee Prasad and Luis M. Correia
Edited by Jean-Paul Linnartz

With enormous complexity of managing and operating the many different types of networks that are now in use, the door is open for finding a common platform - a network on which all established services can be supported and which will allow new services to be introduced without needing new networks to run them on. The answer seems to be asynchronous transfer mode (ATM). This is the technology being defined and standardised for broadband integrated services digital network (B-ISDN). Thus, ATM, when adequately modified, is also an answer for the future mobile wireless broadband multimedia systems.

ATM is a packet oriented transmission scheme. The transmission path of the packets of constant length, the so called ATM-Cells, will be established during connection set-up between the two end-points by assigning a virtual channel. At this time, the necessary resources are provided and the logical channels are assigned. All packets of a virtual channel are carried over the same path. The transmission capacity of virtual channel are characterised by the parameters mean bit rate and peak bit rate during connection set-up. ATM-Cells are generated according to the need of data source. Thus, ATM is a very good method to meet dynamic requirements of connections with variable data rates.

MBS will be the interface between the fixed ATM-Net at the base station side and mobile ATM-Net at the mobile station side. Normally, the ATM-Net at the mobile station side will only consist of one end system. For every end station it is possible to operate several virtual channels with different data rates at the same time.

Most important benefit of ATM is its flexibility, it can be used for the new high bit rate services which are either variable bit rate (VBR) or burst traffic. Several factors that tend to favour the use of ATM cell transport in MBS, can be given as follows:

flexible bandwidth allocation and service type selection for a range of applications,
efficient multiplexing of traffic from bursty data/multimedia sources,
end-to-end provisioning of broadband services over wireline and wireless networks,
suitability of available ATM switching for inter-cell switching,
improved service reliability with packet switching techniques,
ease of interfacing with wired BISDN systems.

Adoption of ATM compatible fixed length cell-relay format for MBS appears to have advantages. A 48 byte ATM cell payload will become the basic unit of data within the MBS network. Within MBS, specific protocol layers (e.g., data-link and medium access control layer) will be added to the ATM payload as required, and replaced by ATM headers before entering the fixed network. The use of ATM switching for intercell traffic also avoids the crucial problem of developing a new backbone network with sufficient throughput to support intercommunication among large numbers of small cells. ATM multiplexers may be used to combine traffic from

Figure: ATM-compatible MBS approach

For a seamless internetworking mechanism with the wired broadband network it is vital to have the MBS protocol layering harmonised with the ATM stack. In the protocol reference model, new wireless channel specific physical, medium access control and data-link layers are added below the ATM network layer. This means that regular network layer and control services such as call set-up, virtual channel identifier/virtual path identifier (VCI/VPI) addressing, cell prioritisation and flow-control indication will continue to be used by mobile services. While, the baseline ATM network and signalling protocol will have to be specified to specific mobility related functions such as address registration (roaming), broadcasting, hand-off etc.