Digital Video and IP Video are Not the Same by Phil Hippensteel

4/26/2012 1:56:49 PM
By PSN Staff

Dear Professor Phil,

I continually hear people talking about digital video and IP video as if they are the identical.  Are they the same?

Sam,  Hershey, PA

Sam,

They are not, but they are closely related terms. Consequently, you are correct that many people who are very familiar with analog video seem to confuse the two terms.  Let’s consider each one separately.

Digital video refers to encoding a signal from a source in ones and zeros, called binary representation.  That doesn’t mean that the signal has only two states or levels.  In fact, if it has eight levels, each level can represent three  bits (ones and zeros).  The usual form of digital representation of an analog signal is pulse code modulation (PCM).  In this process, the signal is measured and the level is designated as one of 256 levels.  Each level corresponds to 8 bits.  Hence, PCM is a form of digital representation.  If the result is transmitted in a stream exactly or essentially in the form it was created, we call the method a digital transmission.  T-1 may be the most common digital transport mode for PCM encoded voice.  Mpeg transport is the standard method of sending digital video streams. These streams may use IP but they do not need to use IP.

On the other hand, video or audio that is  transported in a method called IP transport is packetized.  That is, the digital bit stream is separated into blocks of bits and inserted into IP packets with a source address, destination address, and generally an additional UDP or TCP header.  These packets can carry a fixed number of bits or a variable number of bits.  However, each individual packet can be transferred by the network on any path the network routers  choose.  Non-packet digital streams almost always follow a prescribed path established at the start of the transmission and we say that they have a switched circuit.

Phil Hippensteel is a professor of information systems at Penn State Harrisburg.

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