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M3SR Series 4100
5.3.1.1
General Constraints
•
All radios belonging to the same air network must have the same settings in the under-
lying waveform, i.e. SECOM-H settings like data rate, search pattern etc.
•
The same applies to connection and operation mode, these settings must be identical
on all radios belonging to the same network.
•
Such radios must also belong to the same air IP network.
•
IP addresses belonging to the same IP network, be it Ethernet, air or PPP, must be
unique.
•
Ethernet, air and PPP IP networks shall not overlap.
•
Several settings are in the form of an IP address or netmask.
•
If not stated otherwise, all parameter changes (concerning radio settings or IP settings
described below) have immediate effect, i.e. no reboot of the radio is necessary.
5.3.1.2
Recommendations for the Transport Layer
As mentioned above, the radio channel has different characteristics when compared to an
Ethernet network:
•
Very slow data rate
•
Very high round-trip-time (RTT)
•
Very high delay variation
•
High bit error rate
These characteristics can have serious effects on the upper layer protocols like TCP.
Drawbacks Using TCP-Based Applications over Radio Channels
The TCP protocol itself is designed to adapt to the characteristics of the transmission chan-
nel or error situations that might occur. Typically it is used in wired LAN (Local Area Network)
environments built up with packet switching routers, dial-up connections, etc.
TCP measures the Round-Trip Time (RTT) that its packets need to be transmitted from the
client to the server and back. Based on that RTT timing, it adapts its internal protocol timers,
e.g. the retransmit timers. With the help of this mechanism it is possible to use TCP over a
wide bandwidth range (very slow dial-up connections or high speed LANs).
The error case which occurs most often in packet-switched networks is congestion. This hap-
pens if more packets arrive at a router than the router can forward e.g. over a slow link. If the
router discards some packets which could not be sent because of this, TCP will recognize
the packet loss. As a result it will retransmit the unacknowledged packet(s) when the retrans-
mission timer exceeds the preset time. At the same time it will reduce its local transmission
rate to help the congestion situation to be resolved. The TCP's transmission window (the
amount of yet unacknowledged data that is allowed to be sent) will be reduced abruptly and
5.36
IP over Air (IPoA)
Operating Manual 6175.4760.02 – 01
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