Understanding the network.pdf - Back to Home

Asynchronous transmission involves the generation and transmitting of characters
that have a specific start and stop sequence associated with each character. This is
known as character framed data (CFD). CFD will sound familiar to you. It starts with
a single bit (start bit). Data is then sent, and data flow is ended with a stop bit
sequence. The receiving hardware must be able to determine the stop bit sequence.
CFD data can be sent continuously or in varying intervals as long as it follows the
start and stop sequence. Async data is transmitted one bit at a time, and async data
transmission equipment looks at each bit that is transferred. The highest practical
speed for async transmission is 1,800 bps. Data rates above this can be transferred
with asynchronous equipment. This data, however, is really transmitted
synchronously. The equipment converts the outgoing data to sync, and it is
converted back to async on the receiving end. Synchronous data transmission
moves both character-oriented (async) and bit-oriented data, known as binary
stream data. Where async data transmits data one bit at a time, synchronous data
transmits data as messages, and so sync data is known as message framed data
(MFD). Sync data is buffered because the messages are transmitted one at time in
a continuous stream. Sync data uses a separate TX (transmit) and RX (receive) path.
Synchronous transmissions are commonly used for data transfers above 2,000 bps.
An easy way to distinguish between synchronous and asynchronous transmissions
is that synchronous transmissions are dependent on a clock or timing source,
whereas asynchronous transmissions only require that the ends agree upon a
means of transmission flow (that is, start and stop bits, flow control, and so on).
There are many varieties of WAN transmission protocols available. These protocols
operate over the AT&T/Bell Labs Digital Carrier System/Digital Transport System or
Synchronous Optical Network (SONET). Examples of WAN transmission protocols
include the following:
• HDLC
• ATM (Asynchronous Transfer Mode)
• X.25/Frame Relay
• PPP (Point-to-Point Protocol)
These protocols will be covered in Chapter 5, "WAN Internetworking Technologies."
LAN Transmission Protocols
In LANs where the medium bandwidth is shared, there are two mechanisms used to
control access to the network:
• Contention-based mechanisms
• Non-contention-based mechanisms