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The ideal union of PAM and Ethernet

Posted: 16 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Ethernet? pulse amplitude modulation? PAM? physical layer? media access control?

The capacity for complex, highly sophisticated communication is a key component of the human experienceof being human. Play is likewise important to our intellectual, creative, and social development.

Today much of our daily communication and entertainment is conducted over Ethernet cables, which connect wired devices in local area networks. Indeed, the technological revolution is truly a communication revolution as well.

In electronic communication, a carrier signal is sent from one end of a system to another (source and destination) through a technique known as modulation. Modulation is nothing more than a carrier signal that varies in accordance with the message signal. Modulation techniques are used to change signal characteristics.

This article describes how various Ethernet speeds evolved through the utilisation of various pulse amplitude modulation (PAM) schemes.

At the speed of Ethernet
Ethernet was standardised as IEEE 802.3 by the Institute of Electrical and Electronics Engineers (IEEE), a professional association directed towards the advancement of the theory and practice of electrical, electronics, communications, and computer engineering. The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed. The speed ranges from 1 Mbit/s to 400 Gbit/s, while the physical medium can range from bulky coaxial cable to twisted pair and optical fibre (figure 1).

Figure 1: Layer Architecture in Ethernet 802.3.

Consequently, the IEEE 802.3 standard utilises various modulation schemes for transmission of data packets across various Ethernet speeds. Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fibre cable.

Unique media access control (MAC) addresses are used to specify both the destination and the source of data packets. Because a source may transmit data faster than the destination can receive it, Ethernet implements flow control mechanisms, such as PAUSE frame, that can temporarily suspend the transmission of data.

PAM and Ethernet: Coupling
Historically, multi-level signalling has played an important role in increasing the data rates that existing wire infrastructures could support, providing high-speed symbol-transfer rates at relatively low observed-line rates. The principle of multi-level signalling is to use a larger alphabet of m symbols to represent data, so that each symbol can represent more than one bit of data. As a result, the number of symbols that needs to be transmitted is less than the number of bits, hence the bandwidth is compressed.

Most forms of Ethernet use pulse amplitude modulation (PAM) constellations. In PAM signal modulation, information is encoded in the amplitude of a series of signal pulses. For example, a two-bit modulator (PAM4) takes two bits and maps the signal amplitude to one of four possible voltage levels (perhaps C2V, -1V, 1V, 2V) over a specified period, Tp. Demodulation of the signal is accomplished by detecting the amplitude level of the carrier at each period Tp.

Figure 2: A PAM Signal.

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