Kantronics KAM 98, Руководство пользователя

Страница: 49 / 384

Kinds of Packets
As shown in the above diagram, the “control” field in an AX.25 packet includes a
code telling what kind of packet the current packet is. In addition to connected
information packets, the AX.25 protocol also defines two other kinds of packets,
each of which is nearly identical in structure to the connected information packet:
control packets (of which there are several kinds) and unnumbered packets. To
learn more about these, see the documentation for the KAM’98’s MCOM com-
mand.
As noted at the beginning of this section, the organization of amateur radio pack-
ets is defined by the AX.25 protocol. Now it is useful to give a brief discussion of
this protocol, and another that fits “within” it.
Packet Communication Introduction
KAM’98 v 8.3 31 User’s Guide
An AX.25 information packet (also called a "frame")
Flag Flag Address Control
Data
Checksum PID
Sing le c ha ra c te r, for
"be g inning of pa c ke t"
Sing le c ha ra c te r, for
"e nd of pa c ke t"
De stina tion sta tion,
Sourc e sta tion, a nd
up to 8 inte rm e d ia te sta tions
Kind of p a c ke t,
p a c ke t num b e r,
a nd othe r c ontrol
inform a tion
Protoc ol ID: Cod e
for p rotoc ol use d
for this p a c ke t
(e. g . , AX. 25 or
TCP/IP)
Cod e d e rive d
from this p a c ke t,
use d to c he c k
for e rrors in
tra nsm ission
(string of b its)
Data:
Information or message
being sent "inside" this
packet (1 to 256 bytes,
which may include
protocol-related data)
© Cop yrig ht 1996 b y Ka ntronic s Co. , Inc .