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622 Mb/s SDH fiber links can be broken into smaller 'pipes' of various sizes, and Internet routers can use links of between 64 kb/s and several hundred Mb/s. Telephone exchanges typically connect to each other with multiple 2 Mb/s 'pipes' (or 1.5 Mb/s in North America). One exchange may link to another via dozens of such 2 Mb/s links, and they may all be carried by one fiber, or more likely they may be carried by two separate fibers (or via microwave), to provide 'diversity' and so reduce the impact of any one fiber failing.
The national and global fiber networks are now carrying several other types of traffic in addition to the telephony for which they were, until recently, primarily intended. The fastest growing traffic type is the IP packets of the Internet. Corporate private networks and video links for television stations also are important types of traffic. These different traffic types are generally combined as separate 'tributaries' to the river of data that the fiber carries. For instance a 622 Mb/s fiber may carry several 34 Mb/s datastreams of Internet traffic, each terminating at a router of a particular Internet Service Provider. Another 34 Mb/s stream might carry ATM (Asynchronous Transfer Mode) traffic between two offices of a large corporation, and so carry a variety of traffic types within those ATM cells. Other streams might be leased to other carriers. Multiple 34 Mb/s streams may consist of separate 2 Mb/s sub-streams, each of which is connected, via the SDH fiber equipment at each end, to telephone exchanges in the same building - using short fibers or twisted pair copper wires.
The splitting of the capacity of fiber networks in this way is on a long-term, 'pipe'-like basis. The traffic in a 34 Mb/s stream cannot exceed that data rate, and is kept completely separate from the other data streams, although the bits of the various streams take it in turns to travel down the fiber as a series of pulses of infra-red light.
These relatively permanent 'pipes', either the complete capacity of a fiber, or some fixed subset of it, are 'circuits', and they are assigned on a monthly or yearly basis, and connected physically by the SDH fiber terminating equipment which provides the various sub-streams on individual fibers. These are effectively 'leased lines': permanent bi-directional 'circuits' for digital data. They can be expensive to rent. For instance the monthly rental of a 2 Mb/s circuit on one of the PacRim fibers between Australia and California was approximately US$60,000 in 1998, simply because demand is high and there are no alternatives, other than satellite links which have higher time delays.
The long-distance and submarine cable fiber links are expensive and impressive pieces of engineering which carriers must pay for via the tariffs they charge for leased lines on the fiber and for the telephone traffic they carry.
Both the Internet and the telephone network represent separate functional networks - with very different functionalities. They both use the same long distance fibers and satellite links. Whereas the switching points of the Internet are rack mount devices costing perhaps US$100,000 at most, telephone exchanges resemble 1960s mainframes, cost millions of dollars, and occupy floor-space equivalent to a small house.
A large telephone exchange must provide tens of thousands of continuous, bi-directional, 'pipes', without a glitch, between tens of thousands of 64 kb/s channels within hundreds or thousands of 2 Mb/s fiber links. In addition it must run complex software to manage the call and generate accurate billing information. To achieve this while any one part of the exchange fails or is upgraded, especially with technology which was designed in the early 1990s or before, explains the very heavy and relatively inflexible nature of traditional telephone exchanges.
An Internet router has a much less demanding task. It receives packets, stores them briefly, sends those it can to the appropriate port (typically one of three to eight ports) and simply forgets packets which cannot be sent because their destination port is busy. It can perform some elaborate filtering and billing operations, and it must do some elaborate processing to decide which packets should be routed to particular ports, but its work is not so critical as a telephone exchange, because all Internet applications cope gracefully if some packets are lost.
Voice Calls
Analog POTS Telephone Calls
The most common type of telephone call is from one analog phone to another. The audio signals received at the exchange (or its Remote Access Unit) and sampled in time at 8,000 samples per second (8 kHz) and measured with an 8 bit accuracy (one of 256 possible voltages) and so converted into a 64 kb/s (64,000 bit per second) data stream which is suitable for switching and transmission via the digital exchanges and fiber links. This 'digitizing' process is performed by an Analog to Digital Converter - a chip on a circuit board which connects to four or perhaps eight twisted pair lines.
A 'Remote Access Unit' is street cabinet or larger piece of equipment installed in a building, which is connected to its parent exchange via digital fiber or microwave links. It may be dozens or hundred of kilometers distant from the exchange, and drive hundreds or thousands of analogue or BR-ISDN lines, just as the exchange would drive them. However the Remote Access Unit does not in itself perform full exchange functions.
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