Introduction to Serial Transmission 

Serial transmission is a method of data transmission in which bits of data are transmitted over a single channel. For example, assume that an octet has the value shown in the following graphic. The left-most 0 bit is the most significant bit (MSB) and the right-most 1 bit is the least significant bit (LSB). The eight bits in the octet are converted into a linear series of bits, one at a time, with serial interface operation. The interface transmits the MSB first.

This one-at-a-time transmission contrasts with parallel data transmission that passes several bits at a time. For the octet in the preceding graphic, each of the eight bits using a parallel interface has an output element. Instead of one bit at a time, the parallel bit stream can carry all eight bits at a time. Keeping the parallel transmission of all eight bits synchronized succeeds over short distances. Parallel transmission occurs within a single device or between devices physically near to each other (for example, a PC and a local printer). 

For long distance communication, WANs uses serial transmission. To carry the energy to represent bits, serial channels use a specific electromagnetic or optical frequency range. The preceding graphic shows that many media can move information across a WAN with serial transmission.

Although all of the media are used to carry serial data, most of the serial traffic today is multiplexed with other traffic for transport across long distances on copper or fiber-optic cables. Multiplexing is discussed in more detail for narrowband transmission—T1, E1, Y1, and J1—later in this guide. Serial data is most often carried on copper cable for shorter distances, for example, a multipin cable between a dumb terminal and the control port on a router or switch.

Frequencies are described in terms of their cycles-per-second (or Hertz) function as a band or spectrum for communications. For example, the signals transmitted over voice-grade telephone lines use 3 kHz (kilo or thousand Hertz). This band is called bandwidth.

Another way to express bandwidth is the amount of data in bits per second (bps) that the serial channel can carry, in digital WANs data is usually expressed in kilobits per second (kbps). The following graphic shows some examples of transmission bandwidth sizes. 

Even though a telephone network may multiplex several calls (or groups of calls) as the transmission hierarchy is used, asynchronous services themselves occur in the voice grade range: 300 and 1200 bps at the minimum speed, and 19.2 kbps to 38.4 kbps at the high end, often with leased lines.

With a leased line, a dedicated point-to-point connection is available for private use only. There is no need to dial any numbers in order to connect—the line is always up. Economies occur if sufficient traffic exists to warrant the additional expense of the leased line.

ISDN lines have fast call setup and provide good response time for multimedia traffic such as World Wide Web sessions. ISDN Basic Rate Interface (BRI) sends user traffic over one or two 64-kbps bearer (B) channels.

Fractional T1/E1 subrate transmission consists of multiple low-speed data logical connections submultiplexed into a single 64-Kbps DS0. DS0 subrate data rates cover the range of 2.4, 4.8, 9.6, and 56 kbps. Common multiplex standards include:

• DS0A and DS0B for T1
• X.50 for CEPT E1

There are two transmission modes for subrate data connections:

• Interpretive mode—This mode expects the data stream from the customer to include some supervisory bits.
• Transparent mode—This mode does not look for any supervisory or control information in the data stream; it assumes all bits are data bits.

Asynchronous transmission usually encapsulates individual characters in control bits called start and stop bits. The following graphic shows three eight-bit units of asynchronous data transmission. This transmission could be from a serial interface in a PC modem. 

Each character is separately framed in an asynchronous bit string sequence:

• A start code occupying one bit-time signals the beginning of the frame, which helps the receiver know that, in the overall stream of bits transmitted, the next few bits can be treated as a unit of information (for example, an ASCII character).
• When the frame of character data information is finished, a stop code indicates the end of frame and provides a simple checking mechanism.
• Variable time can occur between each frame transmitted.

Some examples of asynchronous data include the following:

• Key strokes between a keyboard and a PC
• Modem transmissions
• Print code between a PC and a serial printer

Asynchronous transmission has a low maximum bandwidth. The most commonly used asynchronous interface is EIA/TIA-232 (formerly called RS-232). This 25-pin interface standard has been in effect since 1969 when the U.S. Electronics Industry Association (EIA) published how serial binary data could be interchanged at speeds below 20,000 bps.

In Europe, the ITU-T adopted virtually the same specifications as the V.24 standard; it consists of V.24 describing the interchange circuits, V.28 covering electrical signals, and ISO 2110 on 25-pin assignments.

Using DB-25 connectors, the EIA/TIA-232 cable is the most common interface for asynchronous communications. The EIA/TIA-232 male and female cable connectors are shown later in this chapter.

Synchronous transmission uses more expensive clocking to operate at faster rates. It combines much more data into the information payload carried in the bit stream of each serial frame. The following graphic shows synchronous transmission. 

Each synchronous frame has:

• A header that contains addressing and control information.
• The information field that contains upper-layer protocol data (for example, data from the end-user application).
• An ending with a cyclic redundancy check (CRC), which is a series of algorithms that calculate a number based on the content of all the bytes in a block and a polynomial value.
• Once a CRC has been calculated at the sender's end, the result of the calculation is appended to the end of the block. Then, at the receiving end, the CRC can be checked against the content of the block. If the results do not match, the block can be rejected. With a ITU-T standard CRC test, devices at each end of a synchronous link can detect up to 99.997 percent of the transmission errors.

Between synchronous frames, at least one instance of idle code (also called a flag) is inserted to delimit the frames. Unlike asynchronous transmissions, synchronous transmissions must always be active, in other words, bits must always be present. Multiple flags occur between frames during idle periods. A commonly used idle flag is a hexadecimal 7E (binary 01111110).

Some examples of some synchronous protocols include the following:

• HDLC
• SDLC
• Frame Relay
• X.25

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