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Compression of Voice Signals
For overseas calls, it is common to 'compress' the 64 kb/s raw data of the call to a lower rate, such as 32 or 16 kb/s, in order to carry more calls over the expensive international links carried by submarine fiber or geostationary satellite. This compression (also known as data reduction) involves a small extra time delay for encoding and decoding, and results in less clear, but hopefully still intelligible, speech. Such compression may severely limit the speed with which fax machines and modems can communicate, so a carrier's equipment may be programmed not to compress such calls, by detecting the 2,400 Hz tone which modems and fax machines begin each call with.

Voice compression is also used in all digital mobile phone technologies, such as GSM, which is discussed below. All voice compression for telephony is 'lossy' - it results in a loss of signal quality. Silence removal is also a form of compression, since each person typically talks for only a third of the time.

ISDN Voice and Data Calls
There are two methods by which customers can connect to the network for ISDN (Integrated Services Digital Network) calls: Basic Rate ISDN (BR-ISDN) and Primary Rate ISDN (PR-ISDN). BR-ISDN provides two 64 kb/s 'B' (bearer) digital links to the exchange, and one 6 kb/s 'D' signaling channel, for setting up, ending and receiving calls. PR-ISDN is conceptually similar, but involves 30 B channels (or 23 in North America) and one 64 kb/s D channel.

A sophisticated protocol on the D channel enables multiple items of customer equipment to initiate and accept calls, using multiple telephone numbers, and to perform other functions - such as notifying the exchange that a call received on one of the B channels is malicious and should be traced.

Customer equipment on a BR-ISDN service is most likely to be a computer interface card or router, which can make two separate calls using the two B channels. PR-ISDN is typically used by large organizations for connecting their ISDN PABXs. PR-ISDN is increasingly important to Internet Service Providers, who need large numbers of telephone lines in a reliable, compact format, and to support the 56k modems of their customers, which can only be achieved via a digital interface to the telephone network, rather than using an analogue twisted pair and modem.

Charging for ISDN calls
Whether each B channel is handling a digital call, for instance to a B channel of an ISDN service in the same country, or perhaps anywhere in the world, or whether it is handling a voice call - to a POTS, ISDN or mobile destination - there is no difference in the operation of the ISDN link between the customer and the exchange.

There is however typically a significant difference in how the carrier routes and charges for the two kinds of call. The customer equipment initiating the call (by sending a number and other items of data to the exchange on the D channel) specifies whether the call is to be a voice or a data call. If it is a data call, then the carrier will always route it through exchanges and inter-exchange links which provide a clear 64 kb/s data path. (The exception is in North America, where for historical reasons many inter-exchange paths only reliably transport 7 of the 8 bits, and ISDN data calls are therefore limited to 56 kb/s.) If it is a voice call, then the exchange may route it through analogue exchanges and may subject the call to compression on international links.

While many customers are happy with a POTS service, and so only some will pay the premium usually charged for a BR-ISDN service - and pay for the equipment to interface to that service, since analogue phones do not connect to it - the aim of ISDN is not to require separate type of exchange for the two types of service. While ISDN has in the past been introduced as an 'overlay' network - a special subset of exchanges which were ISDN capable, which interworked with, but which were separate from the non ISDN exchanges - ISDN was always intended to be more than just a specialized service. ISDN involves making the entire telephone system ISDN compatible - and for each exchange or Remote Access Unit to be provisioned with the mix of analogue and BR-ISDN line cards, and PR-ISDN interfaces (typically by fiber, although with dual twisted pairs in the past) according to customer requirements. In the long term, this is the most efficient, reliable and flexible way of providing the mix of voice and data circuit-switched services which customers are expected to require.

Telephone Exchanges
Telephone exchanges are also referred to as 'switches', and in North America, it is common to refer to either the exchange/switch, and/or the building it is located in, as the 'Central Office' (CO). The telephone network consists primarily of:

• Telephones and the Customer Access Network which connects them to their local exchange.
• 'Local' exchanges. These connect to customers, either directly - with POTS or BR/PR-ISDN line cards in the exchange itself - of via Remote Access Units. These exchanges may have over 100,000 services attached to them. An exchange which manages AMPS or GSM base-stations is effectively a local exchange.
• 'Transit' exchanges. These do not connect directly to customers, but provide links between the other exchanges.
• Other specialized exchanges, for instance to manage international calls, or to interface to the networks of other carriers in the same country.
• Inter-exchange links of 2 Mb/s, typically carried by SDH/Sonet fiber links of 155, 622 or greater capacity. Within each 2 Mb/s link there are thirty two 64 kb/s bi-directional 'pipes' which can be used for are used for voice calls or for carrying SS7 signaling information.