Serial communications

From Academic Kids

In telecommunications and computer science, serial communications refers to any data transmission scheme in which data is sent one symbol at one time, sequentially over a communications channel.

Serial communications is the normal method of performing all long-haul communications.

Serial communication is substantially different from a serial bus. Although the problems may be similar in some respects, long-haul commmunications is far more concerned with the robustness and expense of the wire, and remains a different engineering environment.


Time division multiplexed systems

Almost all long-haul digital communication channels are leased from telephone carriers. Digital telephonic 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 are fewer bits used for 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 genius of these systems is that the bit pattern can be repeated by any type of modem, making the data very 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 1970s, A company named Doelz 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 comuters 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." 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 relays that would pull on solenoids whcih 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 computers became commonplace, these serial communication systems were adapted using I/O devices called serial ports that used UARTs. This system is now generally obsolete, but it had a deep continuing impact on the nature of software and operating systems, both of which usually arrange data as sequences of characters.

Serial busses

Integrated circuits 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 buses include SPI, I2C, and one-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:

  • Clock skew between different channels is not an issue (for unclocked serial links)
  • A serial connection requires fewer interconnecting cables (e.g. wires/fibres) and hence occupies less space. The extra space allows for better isolation of the channel from its surroundings
  • Crosstalk is less of an issue, because there are fewer conductors in close proximity.

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

de:Serielle Schnittstelle


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