Technical Manual

Preliminary Release

6.0 Theory of Operation

The 2788 concurrently operates transparently using two communication interfaces: Ethernet

and RS-232 (Fig. 1). The 2788 encapsulates any protocol in a proprietary Alligator format to

encrypt and forward multiple packet fragments across a UHF radio link and reassemble them

seamlessly. It is most easily understood as a wire transmission with added latency due to time

taken for transmission over a 12.5 kHz channel at 9600 bps along with delays due to packet

screening, assembly and encryption.

6.1 RS-232 Communications

The RS-232 communications interface forwards all incoming serial data with no screening. It

operates at one of five programmable data rates: 1200, 2400, 4800, 9600, 19200 and 38400

bps. There is no RTS or CTS data flow control required because data is forwarded based on TXD

line activity. This permits a simple three wire interface consisting of TXD, RXD, and GND.

6.2 Ethernet Communications

All Ethernet packets (UDP, TCP/IP, Ping, ARP, . . .) are recognized and can be transparently

forwarded over the RF network.

6.2.1 Packet Forwarding

The Ethernet communications interface forwards incoming IP packets, provided that they pass

a user-programmable forwarding screening test. Packet forward screening is done to minimize

data traffic not intended for the radio network that could otherwise flood the system. The 2788

has four programmable forwarding modes:

1. No Screening

2. Host Screening

3. RTU Screening

4. RTU and Host Screening

The simplest forwarding mode does not screen any packets. This is referred to as the IP

promiscuous mode and is only used in small systems where flooding by unwanted packets is

improbable. It is not recommended in systems with either the master or remote ends tied to

LANs due to the flooding potential. The 2788 defaults in this mode for testing but is not

recommended for field deployment.

The host screening mode is highly recommended for packet forwarding on the master radio. At

the master radio, a host table with up to 16 entries is created by the user with a list of host

MAC addresses. Only packets originating by authorized hosts on the MAC table are forwarded.

The remote radio can also be configured for host filtering. This causes the remote radio to

forward only packets originating from the RTU back over the RF link that are destined for an

authorized host MAC address. This greatly reduces the probability of system flooding.

The RTU screening mode is used so that only packets destined for authorized RTU MAC

addresses are forwarded. At the master radio, an RTU table with up to 512 entries is created by

the user with a list of RTU MAC addresses. Only packets destined for authorized RTUs on the