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TIME DIVISION MULTIPLEXED SYSTEMS Almost all long-haul digital communication channels are leased from telephone carriers. Digital telephone systems use synchronous frames of bits that repeat 8000 times per second. These usually have a one or a few bits that form a pattern to enable the system to "synch", and locate the start of the frame. Blocks of data are then identified by counting bits or groups of 8 bits from the start of the frames; this scheme is called Time-division Multiplexing . This repeating frame is far more efficient than a teletype-style UART , because there is less Overhead . Telephone systems range from T-1 lines, which have a frame of 24 bits, and one synch bit, to SONET frames, whose frame contains several thousand bits, and has about 1% synch bits distributed in the super frame. The advantage of these systems is that the bit pattern can be repeated by any type of Modem , making the data independent of the transmission medium. In telephone systems, these data formats travel in physical media arranged in loops. The loops may cover many cities, or a loop may be a single transmission link from one region to another. Routers transfer blocks as small as a single phone conversation from link to another. Generally, each channel of each node on the loop is allocated block(s) of the repeating frame. When a router receives data for a channel, it consumes data from this block of the frame. When it transmits data, it fills this block with the data it generates. In this way, a single fast channel is broken into several slower channels. These long-haul systems usually transmit Internet protocol data using a technique called Frame Relay . ASYNCHRONOUS TRANSFER MODE In the 1970 s, the Doelz company developed a semi-synchronous serial communication system. It used short synchronous frames on a physical loop, but the routing equipment was permitted to remove, store and delay lower priority data if it had higher-priority data. The crucial advantage of the Doelz network design was that low priority, high reliability digital network data from computers could be inexpensively combined with low reliability high priority data such as pulse-code modulated voice. AT&T researchers appear to have borrowed significant concepts from Doelz in order to develop a similar system, " Asynchronous Transfer Mode " (ATM). This also uses repeating small frames of fixed-size packets. The packets likewise included routing information in the form of channel identification indexes, and optional error correction data. As a primary protocol or transmission protocol, ATM has lost the competition to the internet protocols, which have substantially lower overhead per packet (<1% for IP). ATM is widely used as a backbone protocol in routers. On long-haul networks, ATM is often reformatted into frame relay. TELETYPE SYSTEMS Standard Teletype systems evolved as an automated Telegraphy system called Telex . Originally, a rotating mechanical commutator (a rotating switch) was started by a "start bit". The commutator would distribute the other bits to set Relay s that would pull on Solenoid s which would cause the mechanism to print a figure on paper. The routing was automated with rotary electromechanical dialing systems like those used in early telephohe systems. When Computer s became commonplace, these serial communication systems were adapted using I/O devices called Serial Port s that used UART s. The development of communications hardware had a deep continuing impact on the nature of software and operating systems, both of which usually arrange data as sequences of characters. SERIAL BUSES Integrated Circuit s are more expensive when they have more pins. To reduce the pins, many ICs use a Serial Bus to transfer data when speed is not important. Some examples of such low-cost serial busses include SPI , I2C , and 1-Wire . SERIAL VERSUS PARALLEL The communications links across which computers—or parts of computers—talk to one another may be either serial or parallel. A parallel link transmits several streams of data (perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed circuit tracks, optical fibres, etc.); a serial link transmits a single stream of data. At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit less data on each clock tick. However, it is often the case that serial links can be clocked considerably faster than parallel links, and achieve a higher data rate. A number of factors allow serial to be clocked at a greater rate:
In many cases, serial is a better option because it is cheaper to implement. Many ICs have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore cheaper. EXAMPLES OF SERIAL COMMUNICATION ARCHITECTURES
SEE ALSO EXTERNAL LINKS |
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