Specifications

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26
Synchronous Communication
not only that all devices in the system be synchronized with
each other, but also that each individual device have its
transmission and reception lines synchronized as well.
There are three clocking methods by which to achieve
synchronization: internal, external, and recovered clocking. All
three methods derive the clock signal for the reception line from
the incoming data. The clock signal for the transmission line will
always be generated by the devices internal oscillator, but the
phase reference used by the internal oscillator differs for each of
the clocking methods. When internal clocking is used, the transmit
clock is phase locked to the device's own internal oscillator. For
external clocking, the transmit clock is phase locked to the phase
of the oscillator belonging to another device in the network. For
recovered clocking, the transmit clock phase is locked to the
clock derived from the incoming data.
In general, the DCE device (such as a modem) uses internal
clocking, while the DTE device (such as a PC) uses external
clocking and synchronizes around the DCE device. (See page 30
for a detailed description of DTE and DCE devices.) In cases
where DTE-DTE or DCE-DCE connections are necessary, one device
must be configured atypically, or a device such as a modem-
eliminator or tail-circuit buffer must be placed between the two.
However, in large networks with multiple devices this is not always
possible. One solution for such networks is to have all devices
synchronize around a single modem's clock source. However,
this solution has the tendency to result in clock drift, and thus
can potentially corrupt data. The other solution is to use recovered
clocking so that a modem can derive the clock from data on its
reception line then send that information out on its transmit line
to be used by the next modem in line, etc.
Byte-Oriented Synchronous Protocols
Synchronous communication can be implemented for full and
half-duplex networks using bit- or byte-oriented protocols. Half-
duplex networks, whether point-to-point or multipoint, can only
support communication in one direction at a time. The most
commonly used protocol for such networks is IBM's Binary
Synchronous Communication Procedures (BiSync). BiSync is a
byte oriented protocol, which means that it approaches transmitted
data as “blocks" that must each be decoded and tracked to
determine what they are, and what they are telling the receiver
to do.
In a BiSync system one computer is designated as a control station.
It is responsible for initiating all data transfers, and thereby
controlling the direction of flow on the communication line. Byte-
oriented communication begins with establishing synchronization,
Synchronous
Coordinated Speed
As its name implies, synchronous communication takes place
between a transmitter and a receiver operating on synchronized
clocks. In a synchronous system, the communication partners
have a short conversation before data exchange begins. In this
conversation, they align their clocks and agree upon the
parameters of the data transfer, including the time interval
between bits of data. Any data that falls outside these parameters
will be assumed to be either in error or a placeholder used to
maintain synchronization. (Synchronous lines must remain
constantly active in order to maintain synchronization, thus the
need for placeholders between valid data.) Once each side knows
what to expect of the other, and knows how to indicate to the
other whether what was expected was received, then
communication of any length can commence.
The theory behind asynchronous and synchronous
communication is essentially the same: Point B needs to know
when a transmission from Point A begins, when it ends, and if it
was processed correctly. However, the difference lies in how the
transmission is broken down. Think of the difference in terms of
a friendly chat. With asynchronous communication you would
need to stop after every word to make sure the listener understood
your meaning, and knew that you were about to speak the next
word. With synchronous communication, you would establish
with your listener that you were speaking English, that you will
be speaking words at measured intervals, and that you would
utter a complete sentence, or paragraph, or extended soliloquy,
before pausing to confirm understanding. Further, you would
establish with your listener beforehand that any extraneous noises
you make during the speech or between speeches (coughing,
burping, hiccupping) should be ignored. Clearly the second
approach is much faster, even though initializing communication
may take slightly longer. In fact, by replacing the start, stop and
parity bits around individual words with start, stop and control
(processing instructions and error checking) sequences around
large continuous data blocks, synchronous communication is
about 30% faster than asynchronous communication, before any
other factors are considered.
Clock Synchronization
In order to initiate a successful synchronous communication link,
several distinct pieces of hardware must be configured around a
common clock. This configuration must take two data lines into
account, the transmission line (the line it uses to send data) and
the reception line (the line it uses to receive data). It is essential