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The CompuServe B Plus Protocol
May 16, 1988 2:10 PM
by
Russ Ranshaw
TABLE OF CONTENTS
A. Introduction.
B. Notation.
C. B Plus Packets.
1. B Plus Packet Structure.
2. Quoting of characters.
3. Check Value.
4. B Plus Packet Types.
a) Packet Type `+': Transport Parameters.
b) Packet Type `T': File Transfer.
c) Packet Type `N': Data.
d) Packet Type `F': Failure.
e) Optional `T' Packets.
1) Packet `Tr' : Download Resume.
2) Packet `Tu' : Upload Resume.
3) Packet `Tf' : Failed CRC.
4) Packet `TI' : File Information.
D. B Plus Control Sequences.
1. Enquire.
2. Positive Acknowledge.
3. Wait.
4. Negative Acknowledge.
5. Panic Abort.
E. Negotiation of Transport Parameters.
F. Terminal Program States.
G. Wait for Acknowledge.
H. Packet Send Ahead
I. Implementation Considerations
1. Time-Out
2. Packet Size.
3. Controlling Excessive Retransmissions.
J. B Plus Transport Layer.
K. Initiation of a B Plus Session.
L. Supporting the Various B Protocol Versions
M. Check Value Calculation.
1. Standard B Protocol Checksum.
2. XMODEM-Style CRC-16.
N. Interrogation.
A. Introduction.
The CompuServe B protocol was developed in 1981 to provide
support for a special purpose Vidtex terminal manufactured
by the Tandy Corporation. It was the outgrowth of a
proposed Bi-Sync oriented protocol, but with a different
packet structure and provision for more than even and odd
packets. The file transfer capability was added in 1982 to
replace the CompuServe A protocol with a more robust
protocol which was in keeping with the over-all B Protocol
design.
Some of the underlying assumptions made in designing the B
Protocol were due to the capabilities of personal computers
which were available at the time. Such machines were
generally limited in the amount of available memory, 64
kilobytes being a large capacity. Other factors, such as
the lack of a true UART for data communications, resulted in
the send/wait nature of the protocol where only a single
protocol packet at a time was sent.
The explosive growth of the Personal Computer industry has
given us a plethora of machines, most of which have far
exceeded the early limitations of memory and communication
ability. This growth has been accompanied by a multitude of
file transfer protocols, such as XMODEM, KERMIT, and ZMODEM.
CompuServe, realizing the need for enhancement, has
developed the B Plus Protocol to meet the increasing demands
being made upon its communication network and host