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<p>The 802.11a and .11g standards provide for physical layer bit rates of up to 54 Mbit/s. At first glance, this looks like a large improvement over the existing 802.11b standard of 11 Mbit/s. Unfortunately, due to limitations at the MAC layer, achievable throughputs of only 20 Mbit/s have been demonstrated.</p>

In both TDD and FDD modes, the length of the frame can vary (under the control of the BS scheduler) per frame. In TDD mode, the division point between uplink and downlink can also vary per frame, allowing asymmetric allocation of on air time between uplink and downlink if required.

In general, designers have thought that it would be impossible to deliver 10-Gbit connections over existing copper cables. However, a new transceiver architecture has been developed that will allow designers to make this type of connection a reality.

In this article, we'll provide a quick look at the main challenges that designers must overcome in order to deliver 10-Gbit services over copper connections. The articles will then show how the proposed architecture solves these problems.

FXWYPE5E5YAF077_Datasheet PDF

Impairments to Communication There are a host of elements that degrade the communication channel over UTP wiring. These consist of impairments related to the propagation of the signals themselves, such as insertion loss and inter-symbol interference (ISI), which are caused by the limited bandwidth and real impedance of the cable itself, plus degradations due to interfering effects, such as echo, near-end crosstalk (NEXT) and far-end crosstalk (FEXT). In addition, background noise and other radiated signals, such as alien” NEXT (NEXT from other cables) can reduce the received signal-to-noise ratio (SNR).

Figure 1 provides an illustration of the main impairments designers will face when working with copper connections.

FXWYPE5E5YAF077_Datasheet PDF

Now that we've laid out the challenges, let's see how the proposed architecture handles these issues. We'll start by examining insertion loss, ISI, and equalization.

Insertion Loss, ISI, and Equalization Insertion loss is the measure of signal loss, over a length of cable, versus the frequency of the signal. Figure 2 shows an example of this from measurements made on CAT-5e cable.

FXWYPE5E5YAF077_Datasheet PDF

The LINE Env” curve (pink) in Figure 2 represents a model of the cable based on the measured data (the blue line) and is about 3 to 4 dB worse than the measured data at the higher frequencies.

At 10 Gbit/s, chromatic dispersion has cost implications for even the shortest lengths of single mode fiber. This is because the spectrum of the transmitted signal depends upon the transmitter technology used.

The least expensive transmitters are directly modulated lasers (DMLs), which introduce wavelength transients as the output power is switched. These produce a wide spectrum, resulting in severe pulse broadening due to chromatic dispersion. More expensive transmitter technologies such as electro-absorption (EA) modulators and Mach-Zehnder modulators produce narrower transmitted spectra, resulting in less pulse broadening.

By compensating for the effects of dispersion, EDC allows low-cost direct modulated lasers to be used in many applications which otherwise require more expensive modulators. At the longest distances, EDC can allow EA modulators to replace Mach-Zehnder modulators, which also represents a cost saving.

Telecom system designers typically use a dispersion penalty of 2 dB to determine the maximum distance for which a particular transmitter can be deployed. Figure 3 shows dispersion penalties for directly modulated and EA modulated transmitters, with and without EDC.

Multimedia vs. Data Companies have attempted to build multimedia networks based on computer communication protocols (e.g. 802.11x). The drawbacks of this approach are:

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