AEA PAKRATT PK-232, Technical Reference Manual

Page: 47 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-2 Page 47
AX.25 Layer 2 Protocol Specification
A.1 Scope and Field of Operation
In order to provide a mechanism for the reliable transport of data between two signaling termi-
nals, it is necessary to define a protocol that can accept and deliver data over a variety of types of
communications links. The AX.25 Link- Layer Protocol is designed to provide this service, inde-
pendent of any other level that may or may not exist.
This protocol conforms to ISO Recommendations 3309, 4335 (including DAD 1&2) and 6256 high-
level data link control (HDLC) and uses some terminology found in these documents. It also con-
forms with ANSI X3.66, which describes ADCCP, balanced mode.
This protocol follows, in principle, the CCITT X.25 Recommendation, with the exception of an ex-
tended address field and the addition of the Unnumbered Information (UI) frame. It also follows
the principles of CCITT Recommendation Q.921 (LAPD) in the use of multiple links, distinguished
by the address field, on a single shared channel.
As defined, this protocol will work equally well in either half- or full-duplex Amateur Radio environ-
ments. This protocol has been designed to work equally well for direct connections between two
individual amateur packet-radio stations or an individual station and a multiport controller.
This protocol allows for the establishment of more than one link-layer connection per device, if the
device is so capable. This protocol does not prohibit self-connections. A self-connection is consid-
ered to be when a device establishes a link to itself using its own address for both the source and
destination of the frame.
Most link-layer protocols assume that one primary (or master) device (generally called a DCE, or
data circuit- terminating equipment) is connected to one or more secondary (or slave) device(s)
(usually called a DTE, or data terminating equipment). This type of unbalanced operation is not
practical in a shared-RF Amateur Radio environment. Instead, AX.25 assumes that both ends of
the link are of the same class, thereby eliminating the two different classes of devices. The term
DXE is used in this protocol specification to describe the balanced type of device found in amateur
packet radio.
A.2 Frame Structure
Link layer packet radio transmissions are sent in small blocks of data, called frames. Each frame is
made up of several smaller groups, called fields. Fig.1 shows the three basic types of frames. Note
that the first bit to be transmitted is on the left side.
Fig. 1A – U and S frame construction
First Bit Sent
Flag Address Control FCS Flag
01111110 112/560 Bits 8 Bits 16 Bits 01111110
Fig. 1B – Information frame construction
First Bit Sent
Flag Address Control PID Info. FCS Flag
01111110 112/560 Bits 8 Bits 8 Bits N*8 Bits 16 Bits 01111110
Each field is made up of an integral number of octets (or bytes), and serves a specific function as
outlined below.