Telephony

Network Configuration
Signal Types and Characteristics
Terminals

Mobile Telephony Earth Stations Used As Fixed Earth Stations

Telephony on Interactive VSAT Networks

Typical Dedicated Telephony VSAT Equipment

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Many different earth station configurations can be used to carry telephony, though, in Europe, until relatively recently, telephony has only been carried by satellite on international trunk links between large earth stations owned by the PTOs.

This implementation continues to be used, but in the last few years, with the opening of the old Eastern bloc, satellites have proved to be a very cost effective way of rapidly introducing additional telephony circuits into these regions. Satellite telephony has become particularly important in the old Soviet Union.

Deregulation also now allows satellite networks to carry telephony in some countries. This is encouraging the introduction of satellite telephony technologies only used before in the Americas and Asia.

Several techniques as follows are available for the implementation of telephony links and networks over satellite connections, some of which have already been described in other sections of this Handbook:

This was the original major application that was foreseen for communications satellites when they were first introduced, though telephony has now been overtaken by television as the main user of satellite capacity.

Most European PTOs still have a substantial number of international trunk telephony circuits carried in this manner by satellite.

A recent development has been the use of international satellite telephony trunks to provide the external connectivity for a number of digital overlay and mobile telephony networks that are being established in the ex-Communist countries of Eastern Europe.

Further detailed information is provided on the implementation of telephony trunks on the Eutelsat system and on the Intelsat system in their respective sections.

Three types of transmissions are used to carry telephony trunks between large earth stations:

Frequency Division Multiplexing (FDM) is an analogue technique which is now obsolete. Existing installed kit is being rapidly replaced by digital equipment as earth stations are refurbished. FDM transmission will not be discussed further.

SCPC channel pairs are the basis of Intelsat's IDR services and of similar services offered by other satellite operators. This technique is used for low and medium capacity trunks between large earth stations. One or more digital SCPC channel pairs are established between pairs of large earth stations to provide trunks between the earth stations which can carry telephony, data or a mix of the two. Transmissions are continuous (duty cycle 100%) with channel pairs between earth stations being permanentlty assigned.

TDMA services provided by Intelsat and Eutelsat are used for high capacity telephony trunks. Several large earth stations share the same frequency capacity transmitting in bursts in dynamically assigned time slots which are allocated according to demand. The transmission rate is usually 120 Mb/s with a single TDMA carrier occupying a whole transponder.

Telephony circuits are generally connected to the PSTN through an international gateway exchange.

Both SCPC and TDMA are used with large earth stations and are generally added to an earth station which is used for other services as well.

SCPC circuits require the addition of a modem per duplex link (per channel pair) at the earth stations at each end of each link. IBS/IDR modems, which are generally used for this type of service, cost approximately 10 kEuro each.

TDMA services require TDMA equipment at each participating earth station. A set of TDMA equipment for an earth station currently costs of the order of 100 kEuro.

Inmarsat Std A terminals have been used for some time as "fixed mobile phones" where their use is authorised and where the terrestrial infrastructure is lacking for whatever reason. More recently Inmarsat Std B and M terminals have also been used in this role.

Further detailed information on Inmarsat telephony earth stations is provided in the section on the Inmarsat system.

Most of the satellite PCS systems (Iridium, Globalstar, ICO etc) also intend to have fixed versions of their mobile satellite phones for applications ranging from public call boxes to rural telephony.

Interactive VSAT networks are described in detail in the section on interactive VSATs.

Some interactive VSAT manufacturers have included interfaces which allow their primarily data VSATs to carry telephony. On most interactive VSAT networks this should not be seen as a primary function of the network, and indeed would lead to blocking on many networks.

A number of manufacturers have recently developed interactive VSATs specifically for telephony applications, particularly rural telephony. These are TDM/TDMA hubbed systems in which inbound carriers are assigned for the duration of each call. The main effect of this is to change the equipment fit at the hub and the amount of spectrum required. Otherwise, the design is exactly the same as for interactive data VSATs.

Interactive telephony VSATs are usually used in rural environments where there are a large number of villages which make calls to (or via) a regional population centre or city.

These links are often designed to carry permanent, relatively high data rates (often up to 2.048 Mb/s) between two locations. Such data rates can easily cater for several telephony channels in addition to a substantial quantity of data. The diagram below indicates how this can be achieved over a 2 Mb/s link using readily available hardware. Note that if PCM coding is used and that 64 kb/s data channels are provided, a total of 30 voice and data channels can be supported on a 2.048 Mb/s point to point link. Other voice encoding techniques such as ADPCM and linear predictive coding allow much more efficient transport.

Most meshed VSATs were originally designed to carry exclusively telephony, though most now focus on a mix of data and voice or just data. The operation of meshed VSATs is described in detail in the section on meshed VSATs.

Meshed systems are usually used when there will be traffic between remote sites and not just between remote sites and a gateway site. In situations where a substantial proportion of the traffic will be to a single remote site it is often possible to equip that site with a much larger antenna than the other sites and to make it into a gateway site.