Eion LibraPlus 5845, Руководство пользователя, Страница: 20 / 140

Fig. 2.3. 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 EION 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 the
system. Once they are ready to transmit or receive they will move up the polling list and will be polled
more often.
Fig. 2.4. Time Division Multiplexing/Time Division Multiple Access (TDM/TDMA)
Antenna characteristics and placement are critical. Because of OFDM’s excellent Non-Line of Sight
performance and its resistance to frequency selective multipath fading CPE directional antennas do
not have to be pointed directly at the AP antenna. Having a clear line of sight is always preferable, but
is not necessary with the LibraPlus series. There are cases in which the optimal performance is achieved
when the CPE antenna does not point directly to the AP (e.g. when using reflection off a nearby
structure to avoid an absorbing obstruction).
2.1.4.7. Quality of Service (QoS)
In the LibraPlus 5845 Quality of Service QoS is achieved through the provisioning of the WMM
(Wireless MultiMedia Extensions). WMM prioritizes traffic according to four Access Categories (AC)
- voice, video, best effort, and background. WMM creates QoS for traffic priority management to time-
sensitive applications such as voice, video and multimedia traffic. A WMM QoS scheme can be further
enhanced through the use of VLAN customer separation.
2.1.5. About Point-to-Point (P-P) Systems
For P-P systems LibraPlus comes in two versions, the Rapid Deployment (RD) and the Extended
Range (ER) units. P-P links are used when only two locations are connected, for example for backhaul
purposes between P-MP Base Stations and the Network Operating Center for connection to the Internet
backbone, or in situations where throughput requirements between two locations are such that the
bandwidth can’t be shared.
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