Modem modulation protocols

Modulation protocols are the standards used to define the various ways of modulating carriers used in modem to modem links.

The two modems at the ends of the link must both be working to the exact same standard, or else they will not be able to demodulate correctly the data streams.

However they could be manufactured by completely separate rival companies, who have no contact with each other.

The standards are therefore defined by an independent body formerly known as CCITT, now known as ITU-T.

Standards are created based on the current technology available - as technology advances, new standards are created; the existing standards are not changed or removed, the list of standards merely gets longer.

This has implications for manufacturers of modems, since a new modem may end up "talking" to a much older modem. This means that new modems must not only be compatible with the most recent standards, but must also be backward compatible with several older standards as well.

Typical protocols include :-

	V.21 bis	300 bps
	V.22	1200 bps
	V.22 bis	2400 bps
	V.23	1200/75 and 75/1200 bps
	V.32	9600 bps ( + 4800 bps )
	V.32 bis	14,400 bps ( + 12000, 9600, 7200, 4800 bps )
	V.34	28,800 bps

There are now modems available that run at 56k bps. These are made by two different companies, who each invented their own protocol - and these two protocols are different.

A modem from one company will not "talk" to a modem from the other company.

However the ITU is seeking to solve this problem by producing a new standard for 56k working, which is known as V90 - this standard is different from either of the existing 56k protocols, and presumably all manufacturers will migrate to V90 in due course.

As a final point, having a faster modem will not neccessarily speed up access for the end user to the internet.

Although data can be shifted faster betwwen the end user and the internet service provider ( ISP ), it will make no difference to the delays and propagation speed that the ISP experiences with its connection on to the internet.

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