Introduction to Networking for NonStop S-Series Servers

Device-Specific Connections
Introduction to Networking for HP NonStop S-Series Servers—520670-005
6-2
Synchronous Data Transmission
asynchronous, half-duplex (one way at a time) transmission of data across point-to-
point links.
Synchronous Data Transmission
In synchronous data transmission, the data is transmitted in blocks (also called
frames in some protocols). The beginning and end of the block are marked by special
synchronization characters, but individual characters are not separated by special bits.
Byte-Synchronous Protocols
Byte-synchronous protocols use synchronous transmission; some similar byte-oriented
protocols can also use asynchronous transmission. Among the typical characteristics
of byte-synchronous protocols are two-way alternate (one way at a time) operation and
the presence of control characters in the data stream. For instance, control characters
delimit messages and message headers. The most common byte-synchronous
protocol is IBM Binary Synchronous Control (BSC), of which there are three versions:
3270 BSC, 2780/3780 BSC, and conversational BSC.
Bit-Synchronous Protocols
Bit-synchronous protocols are the most efficient and flexible: they are synchronous and
often operate in two-way simultaneous mode. (That is, messages can be traveling in
both directions at the same time.) A message is a stream of bits delimited by bit
patterns called flags. Control information, addresses, and data occur at known
locations in a frame, so there is no need for control characters to label the information.
Bit-synchronous protocols are typically used for high-speed communications.
Asynchronous, byte-synchronous, and bit-synchronous data-link control protocols vary
in several essential characteristics, and a given protocol can offer a variety of options
for
•
The method that a device uses to gain access to the link, such as polling by the
host system
•
Whether the device is a peer to all other stations on the link or a secondary station
to the host
•
The method that a device uses to detect and recover from errors in transmission
•
The format of the data for transmission on the link
•
Whether transmission on the link is half-duplex or full-duplex
HP supports many industry-standard data-link control protocols. Still, there are devices
that do not follow any of the protocols provided. HP offers interfaces that let you
develop support for such devices; see Other Devices on page 6-13 for more
information.