Wilan LIBRA 5800 AP, User Manual

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About Point-to-Multi-Point (P-MP) Systems
June 2003 Rev 0 17
The Libra 5800 LCPE consists of three parts: 1) LCPE, 2) Ethernet Power Inserter with CAT-5 cable
(bought separately) and weatherproofing kit (included), and 3) the External Antenna and cable (both
bought separately).
• LIBRA 5800 LCPE – The LCPE is the main piece of equipment. It is designed for outdoor
installation but can also be installed indoors if needed. The LCPE is equipped with an N-type
connector so that the external antenna can be connected to it. Thus the range of the P-MP
system can be significantly increased by use of higher gain antennas. Also, in situations
where very severe conditions may be encountered outdoors the LCPE can be installed
indoors with cabling to the antenna outside.
• Ethernet Power Inserter – This piece of equipment is a small box that connects between the
LCPE and the P.C. This box also provides power for the LCPE equipment to run. A CAT-5
outdoor cable is used to connect the Power inserter to the LCPE. The weatherproofing kit is
used with standard RJ-45 connector to ensure reliable connection for outdoor systems.
• Antenna and Cable – In order to accomodate different range requirements for P-MP links,
the LCPE is designed to be used with an external antenna. Antennas and cables are selected
by the user based on the network requirements.
Radio Operation Background
The Libra 5800 communicates using a technique call Time Division Duplexing (TDD) in both the P-P
and P-MP configurations. TDD uses one frequency for both the Down Link (DL) Transmission (Base
to Remote in P-P, or AP to CPEs in P-MP), and for the Up Link (UL) (Remote to Base in PP or CPEs
to AP in P-MP). The DL and UL transmissions are performed at different times, therefore the system
is known as a Time Division Duplexing system. The available frequency band is therefore separated
into multiple TDD channels allowing for use of the whole frequency bands for very high capacity.
Time Division Duplexing Channels
In addition to using TDD, in a P-MP system, the AP and CPE also use Time Division Multiplexing
(TDM). TDM is a process of using time slots to allow the AP to transmit to multiple CPEs during a
single transmit cycle. During the Up Link cycle each CPE is polled and if it has data it transmits in
turn. This is known as Time Division Multiple Access (TDMA). All CPEs thus share the bandwidth
available by allocating time slots in turn to each of the units on both transmit and receive channels.
The following diagram shows TDM in a DL and TDMA in the UL. Each slot is allocated to a different
CPE. In the Wi-LAN system each slot may vary in time depending on traffic destined for each of the
CPEs. CPEs that are not very active will also be polled less frequently thus reducing the latency of
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