Specifications

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The Information Frame is used when actual data is being
transmitted. It is also used to provide sequencing, flow and
error control functions. The sequence number bits are used to
indicate the number of the frame that will be sent/received next,
and are used by both the primary and secondary nodes. The
Poll Final bit is used by the primary to tell the secondary whether
or not an immediate response is required. The secondary uses
the Poll Final bit to indicate whether the current frame is the last
in its response, or whether more frames are coming.
The Supervisory frame is used to control the communication
network. It can request or suspend data transfer, report status,
and acknowledge receipt of data. Note that since Supervisory
frames are used exclusively for control, they do not have data
fields.
The Unnumbered frame is unsequenced, can contain one or two
bytes, and is used to provide miscellaneous control commands.
For instance, it might be used by a primary node to activate the
secondary nodes in the network.
Another bit-oriented synchronous communication protocol, High
Level Data Link Control (HDLC) which is based on SDLC, also
uses the frame format described above. However HDLC, which
was approved by the International Standards Organization (ISO)
in 1979, differs from SDLC in several ways. With HDLC, 32-bit
checksums can be used, thereby providing an advantage over
SDLC in the sophistication and accuracy of error checking, and
thus data integrity. Unlike SDLC, HDLC protocols cannot operate
using loop or hub go-ahead configurations (see page 27).
The largest difference between the two, is that SDLC uses only a
single transfer mode, while HDLC provides three choices. Both
use Normal Response Mode (NRM) in which a secondary node
is precluded from communicating with a primary node until the
primary gives permission. The two additional HDLC modes are
Asynchronous Response Mode (ARM) and Asynchronous Balanced
Mode (ABM). In ARM mode, any secondary can initiate
communication without receiving permission from the primary.
ABM mode requires that all devices be configured as combination
nodes that, depending on the situation, can assume the role of
primary or secondary in the network. In such a system, any
device can initiate communication at any time without permission.
Data Buffers
Though synchronous communication enables transmission of
large amounts of data at high speed, it puts in place extensive
control and error-checking mechanisms to prevent data
corruption. However, in full-duplex networks using bit-oriented
protocols, the transmitter is most likely sending frame B before it
knows if frame A was received successfully. (This is not as much
of a problem in slower byte-oriented protocols where data flows
in only one direction at a time.) To maintain the highest possible
data rates, synchronous hardware must contain sufficient data
buffers to store transmitted data (for resending if necessary) until
a successful transfer is confirmed.
Quatech synchronous serial PCMCIA (see page 54-55) and PCI
(see page 68) cards use a 1024-byte FIFO for data buffering.
Our ISA synchronous cards (see page 81-83) use DMA. All support
both bit and byte protocols, and point-to-point and multipoint
full- and half-duplex networks. We also supply SYNCDRIVE
software (see page 56) with all synchronous cards. SYNCDRIVE
provides hardware specific device drivers, DLLs and APIs to simplify
incorporating Quatech boards into your BiSync, SDLC and HDLC
applications.
Isochronous
A steady data stream
Unlike asynchronous and synchronous communication, which
both involve elaborate error checking mechanisms, the driving
force behind isochronous communication is a fast, steady,
uninterrupted data stream. Isochronous clocking information is
derived from or included in the data stream, and the delay factor
is dependent on a channel's characteristics and can be logically
determined. Communication can be disrupted if the transmitter
does not maintain a constant transfer rate, or if the receiver has
an insufficient buffer to store data at the rate it is arriving and
then hold it until it can be processed by software. To maintain
data transfer speed, error checking is often omitted. Though
software can be written to track errors, there is no hardware
mechanism by which to request retransmission of corrupted data.
Isochronous communication is best suited for applications where
a steady data stream is more important than accuracy. A good
example is video conferencing where infrequent small “blips" in
the data stream are tolerable, however, long pauses between a
transmission and a response are not.
To ensure that isochronous transfers are not bogged down by
other devices, the USB specification sets aside bandwidth for
them. IEEE 1394 also uses isochronous communication, as it is
ideal for the high-speed video and audio applications for which
the bus was designed.
Synchronous and Isochronous Communication