Below is a diagram indicating the format of Remote Access Service
(RAS) frames:
+---+---+---+---+------ --------+---+---+---+
RAS 1.0 frame |SYN|SOH| LENGTH| <--> Data <--> |03h| CRC |
asybeui frame +---+---+---+---+------ --------+---+---+---+
^
|--SOH_BCAST or SOH_DEST
+---+---+---+---+---+------ --------+---+---+---+
RAS 2.0 frame |SYN|SOH| LENGTH|COH| <--> Data <--> |ETX| CRC |
asybeui frame +---+---+---+---+---+------ --------+---+---+---+
with compress ^
|--(SOH_BCAST|SOH_COMPRESS)
or (SOH_DEST|SOH_COMPRESS)
+---+---+---+---+---+------ --------+---+---+---+
RAS 2.0 frame |SYN|SOH| LENGTH|COH| <--> Data <--> |ETX| CRC |
asybeui frame +---+---+---+---+---+------ --------+---+---+---+
with compress ^ ^
with XON/XOFF | |--(SOH_BCAST|SOH_COMPRESS)
| or (SOH_DEST|SOH_COMPRESS)
|
+-------(SYN | SYN_XONXOFF)
+---+---+---+---+---+---+----- -------+---+--+--+
RAS 2.0 frame |SYN|SOH| LENGTH| E-TYPE|<--> Data <-->|ETX| CRC |
with TCP/IP +---+---+---+---+---+---+----- -------+---+--+--+
no compress ^
|--(SOH_BCAST|SOH_TYPE) or (SOH_DEST|SOH_TYPE)
+---+---+---+---+---+---+---+---- ------+---+--+--+
RAS 2.0 frame |SYN|SOH| LENGTH| E-TYPE|COH|<-> Data <->|ETX| CRC |
with TCP/IP +---+---+---+---+---+---+---+---- ------+---+--+--+
and compress ^
|--(SOH_BCAST|SOH_TYPE|SOH_COMPRESS)
or (SOH_DEST|SOH_TYPE|SOH_COMPRESS)
NOTE: Compression is performed after COH and before ETX. This does
mean that E-TYPE is not compressed. Also, when CRC is performed,
SOH through ETX (inclusive) is checked. ETX is included because RAS
1.0 also checked ETX.
SYN = 0x16
SOH_BCAST = 0x01
SOH_DEST = 0x02
SOH_TYPE = 0x80
SOH_COMPRESS = 0x40
SYN_XONXOF = 0x20
CRC = 16 bit CRC polynomial (Big-Endian format)
LENGTH = length of Data part of frame (two-byte Big-Endian)