Page 154 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes What is the distance limitation for packet radio?

Since packet radio is most commonly used at the higher radio frequencies (VHF), the range of
the transmission is somewhat limited. Generally, transmission range is limited to “unobstructed
line-of-sight” plus approximately 10-15%. The transmission range is influenced by the transmitter
power and the type and location of the antenna, as well as the actual frequency used and the
length of the antenna feed line (the cable connecting the radio to the antenna). Another factor
influencing the transmission range is the existence of obstructions (hills, groups of buildings ,etc).
Thus, for two-meter packet (144 - 148Mhz), the range could be 10 to 100 miles, depending on the
specific combination of the variables mentioned above.

What do you mean we can all use the same channel?

Packet radio, unlike voice communications, can support multiple conversations on the same
frequency at the same time. This does not mean that interference does not occur when two
stations transmit at the same time, known as a collision. What ‘same time’ means in this sense is
that multiple conversations are possible in a managed, time shared fashion. Conversations occur
during the times when the other conversations are not using the channel. Packet radio uses a
protocol called AX.25 to accomplish this shared channel.
AX.25 specifies channel access (ability to transmit on the channel) to be handled by CSMA
(Carrier Sense Multiple Access). If you need to transmit, your TNC monitors the channel to see
if someone else is transmitting. If no one else is transmitting, then the TNC keys up the radio,
and sends its packet. All the other stations hear the packet and do not transmit until you are
done. Unfortunately, two stations could accidentally transmit at the same time. This is called a
collision. If a collision occurs, neither TNC will receive a reply back from the last packet it sent.
Each TNC will wait a random amount of time and then retransmit the packet. In actuality, a
more complex scheme is used to determine when the TNC transmits. See the “AX.25 Protocol
Specification” for more information (ARRL, 1988).

What is AX.25?

AX.25 (Amateur X.25) is the communications protocol used for packet radio. A protocol is a
standard for two computer systems to communicate with each other, somewhat analogous to
using a business format when writing a business letter. AX.25 was developed in the 1970’s and
based on the wired network protocol X.25. Because of the difference in the transport medium
(radios vs wires) and because of different addressing schemes, X.25 was modified to suit amateur
radio’s needs. AX.25 includes a digipeater field to allow other stations to automatically repeat
packets to extend the range of transmitters. One advantage of AX.25 is that every packet sent
contains the sender’s and recipient’s amateur radio callsign, thus providing station identification
with every transmission.
Networking and special packet protocols: This is a sample of some of the more popular networking
schemes available today. By far, there are more customized networking schemes used than listed.
Consult your local packet network guru for specific network information.
Are there any other protocols in use other than AX.25?
!

Caution AX.25 is considered the standard protocol for amateur radio use and is even
recognized by many countries as a legal operation mode.
However, there are other standards. TCP/IP is used in some areas for amateur radio. Also, some
networking protocols use packet formats other than AX.25. Often, special packet radio protocols
are encapsulated within AX.25 packet frames. This is done to insure compliance with regulations

148 LOVELY PROFESSIONAL UNIVERSITY