Introduction to Packet in Connecticut

Chuck Rexroad AB1CR

Betsey Doane K1EIC

Overall Agenda

Session 1 – Introduction to Packet

Session 2 – Introduction to Packet Networks

Session 3 – Using the Flexnet network and FBB Bulletin Boards

Session 4 – Proper Setup and Operation of Your Packet Station

Session 1: Introduction to Packet

Introduction to Packet Agenda What is Packet and Why do I care? How does Packet work?

Why Use Packet?

Packet is clearly the leading digital communication tool in the VHF/UHF bands. While packet almost died out in the 1990s it is seeing a comeback in NTS and as a network for ARES work. Packet has a number of advantages: Many hams still have packet equipment, It is more difficult for non-hams to monitor packet than voice, It can handle long lists and other documents that would tie up a

voice frequency for an extended period of time, It is error correcting, which is to say whatever is typed at one end

will be received at the other, It is likely that packet will be setup at Office of Emergency

Management (OEM) offices, Red Cross Chapters, Hospitals, and temporary locations that generate significant amounts of information that needs passed.

Who Uses Packet?

NTS uses packet daily. ARES is assigning “packet people” in each

OEM region. CT ARES Packet team is another group of

skilled operators. Anyone who wants to enjoy packet on a

regular basis. Those who do not operate regularly will likely not succeed when an emergency arises.

Current Packet System

How does Packet Work?

Think about your Computer, modem, and Internet Service Provider. We are all used to this.

Now think about your Computer, a “Packet Modem” and a packet network that is your service provider.

The Packet Modem is called a Terminal Node Controller (or TNC) and has a microprocessor in it. It is much more than a modem.

Computer Computer

Modem

Telephone

Comm. Tower

TNC

Connectingto the Internet

Connectingto the

Packet Network

Radio

InternetServiceProvider

PacketNetwork

Comm. Tower

Radio

TNC

Server

Server

Modem

Telephone

PublicTelephone

Network

How Does Packet Work (#2)

Packet requires that a station “Connect” to another station in order to send and receive packets

Once this connection is made, the TNCs will send and receive messages, correcting for any errors that occur without the operator even knowing it.

When the stations are done, they disconnect, which places them in a “ready to connect” mode.

How Packet Works (3)

Packet is a protocol (using AX.25 for those who care)

The protocol describes: Connect Request Connect Accept Message Send Message Acknowledge (ACK)

You can see most of these packets if you monitor a busy frequency

Why is it called “Packet” The senders TNC:

Breaks up the message into individual packets no longer than 256 characters in length. (A message is whatever was typed before the <ENTER> key is hit.)

It then computes a checksum which is a unique signature for that packet.

The TNC then Transmits the packet and the checksum. The receiving TNC:

Receives the packet and checksum, Computes the checksum of the received message, Compares the two checksums and either acknowledges the

packet or asks for a retransmission.

How Does Error Correcting Work when I am Just Listening? It doesn’t. Only the stations that are “Connected” can

ask for retries based on checksums. APRS is based on unconnected packets and

thus is not error correcting.

Demonstration

Connecting to another station across the room.

Issue the Connect request See the Connect occur Send and receive packets Disconnect

Questions and Responses