Introduction to Multicast

Imagine you could get any information you wanted from the Internet without having to ask for it. - No bottlenecks from low speed connections, no waiting for busy servers to get around to sending you data. Instead, all the data you need at your fingertips...

What's the catch? ­ To provide instantaneous access for all users using today's Internet technology will require a huge investment in upgrading the network infrastructure (routers and links) to ensure sufficient capacity is available whenever a user needs it. To respond to the increasing demand, many Internet Service Providers have started a major upgrade programme, as they install next generation terrestrial (cabled) networks.

Network capacity is however not the only obstacle, an increasing number of users, with improved network speeds, means that unless something changes, there is going to be a very significant increase in server load. Already, web servers are finding it difficult to keep up with the numbers of higher speed clients, the demand for popular Internet sites will soon exceed the performance of even the most sophisticated server hardware.

A communication technique called "Multicast" offers a potential solution to this shortfall, by changing the way information is sent across the network. Multicast systems deliver the same information to a group of users using a single transmit operation ­ saving network capacity and reducing server requirements. Although little used at the moment, this offers the promise of low cost Internet services, together with universal access, wherever a user may be.

Why Satellite Multicast?

Next generation networks will provide service guarantees to those customers willing to pay for the service, but there are complexities for Internet Service Providers (ISPs) wishing to deploy and operate wide scale multicast networks. A number of issues have hindered terrestrial deployment, and are likely to continue to do so, at least for the foreseeable future. These include:

Satellites may have a role in providing the multicast service to complimenting next generation terrestrial networks. Providing uniform accessibility is easy via satellite, because the signals sent via the satellite may be received by any receiver within the coverage area of the satellite (known as the satellite "footprint"). Typically, this allows the same signal to be received across a whole continent. A satellite solution also minimises the number of multicast-capable routers required in the core network, which will simplify initial deployment and operations/maintenance. Protocol development (see inset panel) remains an area where much work is required, but much fundamental work has now been done.

Satellite Service Provision

Design of a satellite Internet system requires integrating components from various system segments:

Each segment performs a specific set of related functions and is the concern of a specific group of specialist engineers. Communication between segments is controlled by protocols. These are normally organised in layers, roughly corresponding to the Open Systems Interconnection (OSI) reference model. In satellite systems the layers are often known as radio (physical layer), access (link layer), network (link/network layers) , and services (network layer/management).

Next Generation Satellites

Although many satellite service providers are now piloting multicast services, there is a strictly limited capacity available. The frequency bands normally used for satellite data services (12-14 GHz) have become very congested and there is not sufficient capacity for a major new service. To achieve wide area coverage will require a new generation of satellites operating in the Extra High Frequency (EHF) band (20-40 GHz), where there is currently abundant capacity available.

To efficiently utilise the EHF band requires a new type of satellite which provides On Board Processing (OBP) (where the satellite digitally regenerates the signal and performs satellite switching), and multiple spot beams (where the satellite may send data to one or more antennas directed at different parts of the satellite coverage area). This approach has recently been used in several next generation satellite systems.

OBP together with the use of spot beams presents fresh challenges to multicast service support. The challenge centres around finding efficient ways to inform the satellite which spot beams need to send each multicast packet, and to co-ordinate use of the satellite when there are large numbers of terminals simultaneously receiving the same multicast data (and transmitting appropriate responses using the multicast protocol).

The overall design methodology required for the multicast service is likely to differ from that of next generation TCP/IP satellite systems. The first goal will be to specify and test building blocks from each segment based on commercial needs. The protocols used, and technology developed, will then need to be promoted within standards bodies. There is also a need to involve ISPs and applications developers to ensure the system provides a useful and commercially viable service to the end user.

More information: