Two-way communications
Sharing bandwidth

Two-way communications

Once the concepts behind a one-way link are established, it is relatively straightforward to visualise a two-way link as two one-way links operating back to back. Each earthstation is equipped with a both a transmitter and a receiver, often integrated into one physical unit called a transceiver.

Where data from one source is uplinked as a single carrier frequency that is not shared by any other sites, the system is classified as Single Channel Per Carrier (SCPC). Such systems are most akin to terrestrial leased circuits, and are used for point-to-point communication between any two places in the satellite footprint.

It is common to invest in high performance Earth station equipment to save money on space segment. The earthstation, known as an SCPC terminal, will typically be around 1.8m to 4.5m diameter. Links of tens of Mbit/s are achievable, up to E3 speeds and beyond.

The Earth station equipment is commonly located directly on the customer's premises. Alternatively, a service provider might provide access to the satellite from a shared antenna, or teleport, or might house the customer's terminal at their premises (telehousing).

The SCPC link is essentially two one-way links. The characteristics of each direction of transmission can usually be set quite independently to create asymmetric systems.

Permission must be obtained from the authorities in each country to allow transmission to take place. The application process for such a licence will usually be facilitated by the service provider. Because the link travels such a long distance to the satellite and back in each direction, there is an unavoidable delay of about 0.25 seconds in each direction.

Consideration must be given to where the satellite link derives it timing. If a clock signal is provided to the satellite modems they will need to buffer data to allow it to remain in synchronisation with the external clock throughout the range of motion, and hence path delay, of the satellite. This buffer will add a few milliseconds of delay to the link if an external clock source is provided. If the customer's equipment can synchronise to the satellite system, which is common in solutions involving routers and FRADs, this additional buffering may not be required. If sufficiently powerful, one Earth station may transmit more than one carrier. This allows two or more SCPC links to terminate at a single Earth station and allows a variety of network topologies to be created, known as point-to-multipoint systems.

Multiplexers and routers can be used at each end of the satellite link to combine several types of data, voice and video onto a single SCPC link. BT's SCPC product is called SatLink, and offers a full range of point-to-point and point-to-multipoint services between locations around the globe.

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Sharing bandwidth

Space segment is often the most costly part of the satellite system, and represents the network's information carrying capacity. Satellite systems are able to make good use of the bandwidth by sharing this resource between many sites. This can be achieved in a number of ways.

A one-way broadcast carrier may have several information streams multiplexed into it using, perhaps, frame relay or IP. Each receiver picks out the information it requires and discards the remainder. The amount of bandwidth assigned to each site varies according to its need. Rules to determine how bandwidth is shared can often be adopted, for example using committed information rates in frame relay links. As there are now two or more channels of information multiplexed onto a single carrier, this is known as a Multiple Channel Per Carrier (MCPC) system.

A system may have divided the available bandwidth into a number of two-way channels plus a signalling channel. Sites wishing to communicate will signal their requirements to a control station on the signaling channel. If bandwidth is available, the control station, usually implemented in software on a PC or workstation, will alert the recipient station that another station has requested a connection and allocate bandwidth for the connection. Each site will tune their modem to the frequencies sent out on the signaling channel by the control station and, once synchronised, may begin communication. tearing down the connection is similarly supervised by the control station. Once the sites have finished communicating the bandwidth is made available for re-use in a different connection. Once allocated to a pair of sites, the bandwidth is not available to other sites until released back to the control station for re-allocation. Such a system is known as SCPC Demand Assigned Multiple Access (SCPC DAMA). It is commonly used in a mesh configuration for switching telephone voice channels between sites on demand. Although the mesh topology is common, it is also possible to use the SCPC DAMA equipment in a star topology for carrying voice, video or data. Typically each terminal will be equipped with a number of channels so that it can connect with more than one site at a time. This system can also be augmented with multiplexers and routers.

Frame relay broadcast systems are similar to broadcast MCPC systems. Each site is equipped with a transmitter and one or more receivers. The receivers tune into the transmissions from selected other sites. Data transmitted from each site comprise multiple data streams combined together, usually using frame relay. All sites equipped to receive the signal accept information addressed to the site, forwarding it to the customer's DTE. Any information that is received but not addressed to the site will be silently discarded. The advantage of this style of system is that bandwidth in a site's transmit carrier can be dynamically allocated to whichever service requires it, up to the maximum speed the carrier will support. In addition, because each site only uplinks one carrier, the RF equipment can be operated in or near to saturation, which makes it more efficient.

There are no major disadvantages. However; for every site added to the system, the central site must be expanded to enable it to receive the new carrier if sending control information via a third station is to be avoided. Also, each site always uplinks a carrier of a fixed bandwidth, even if a site has little or no traffic to send.

MCPC systems that combine many information streams onto a single, permanent or semi-permanent channel are known as Time Division Multiplex (TDM) systems. If several sites are to share a single carrier frequency, they may do so using an access technique known as Aloha or one of its derivatives, and is known as a Time Division Multiple Access (TDMA) system.

Aloha was developed for use with terrestrial radio networks, where radio transmitters on the Hawaiian islands shared a single frequency to communicate with a host computer. Each site would transmit a burst of information whenever it needed to do so in an uncoordinated manner. If two transmissions were sent at the same time, they would both be destroyed. To overcome these limitations:

Data communications protocols were used to detect and re-transmit the missing packets

The channel was lightly loaded to keep the possibility of two sites sending at once sufficiently low to give a good quality of service. The TDMA technique was also successfully adopted in shared bus local area networks, like ethernet. A variation on the basic principle, called slotted Aloha, is commonly used in satellite systems. in the Aloha protocol any site can send whenever it chooses. In slotted Aloha sites must only send in pre-defined time slots. This brings two advantages:

It can be shown mathematically that the traffic carrying capacity of the channel is doubled under some special circumstances

The slots can be given numbers and certain sites can be granted exclusive rights to transmit in a particular slot. This guarantees a collision free throughput to sites that hold reserved slots. The bandwidth management features of the hub control system often allow a flexible approach to be taken, it is often possible for a site to dynamically reserve slots or release the reservation as its need for bandwidth varies.

TDMA systems come in two distinct variants. Mesh TDMA systems, where every site transmits on a single carrier (though some systems can frequency hop). These are relatively new and have yet to become commonplace. Star TDM/TDMA systems; these systems are centered on a single hub site, that transmits a single carrier onto which information for all sites is multiplexed. At each site the receiver discards information that does not bear the correct site address. In the return direction information from all of the sites is combined onto a single carrier using TDMA. This system is well established, with most of the cost and complexity in one place, the hub. The remote sites are usually very much smaller than the hub and built to be cost competitive and compact for ease of installation. This type of network is more worthwhile when each site has a very modest data communications requirement, as the cost of establishing the hub is then more easily justified by the savings gained from avoiding connecting the sites to expensive terrestrial services for the sake of a few credit card verifications or online transactions per hour.

Both mesh TDMA and start TDM/TDMA solutions can be expanded to multiple carriers. In a TDM/TDMA system it is common to associate several inbound (site to hub) carriers with one outbound (hub to site) carriers depending on the analysis of the traffic. In order to correctly size the network, quite detailed analysis of the traffic and applications used on the network is required.

BT offers TDM/TDMA services within its SatStar product portfolio for several major customers in Europe, Asia, the Americas and Africa.

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