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Video Transport on Digital Private Lines
How T1 and T3 lines can be used for security camera video through broadcast video transmission.

By: John Shepler

High speed digital private lines, such as T1 and T3 carriers, got their start transporting telephone conversations. Over the last 20 years or so, businesses have pressed this technology into service to expand their local area networks and to tie their PBX telephone systems together. One more recent trend has been using digital networks to converge or combine data and VoIP telephone services. Another is transporting video signals.

The classic video standard is NTSC (National Television System Committee) which is what the U.S. adopted for over the air broadcast of black and white pictures in 1941. It has an analog bandwidth of 4.2 MHz and, with audio, is transmitted in a 6 MHz TV channel. Some clever engineering allowed color information to be squeezed into the same channel size.

Sixty some years later, most TV sets are still analog and still use the NTSC standard. But TV broadcasting is migrating to a new digital high definition standard, HDTV, and satellite and many cable TV signals are delivered digitally. They can then be used as digital inputs to high resolution monitors or converted to analog for monitors and TV sets that need that format.

One nice thing about digital video signals is that digital line services can be used to transport them between offices or cross-country. Digital video can be standard or HDTV broadcast programs, two-way video conferencing, medical imaging, security cameras and even digital cinema. Yes, it may not be that long before your local movie theaters ditch their massive film reels in favor of DVDs or even downloaded video streams stored on local hard disk drives.

The process of transmitting video digitally is quite a bit like what is done for digital telephony and digital broadcast audio. It all starts with a device called a CODEC or Coder/Decoder. This is the component that converts from analog to digital and back again. It also performs whatever compression is needed to fit the original signal through a particular size channel.

Why is that important? Raw digital conversions of analog signals can use up a lot of bandwidth fast. The standard telephone PCM (Pulse Code Modulation) codec called G.711 spits out a stream of 64 Kbps for each telephone conversation. Phone conversations are hardly what you'd call high fidelity. A real Hi-Fi stereo signal needs about 1.5 Mbps, which fits nicely on a T1 line. Video needs even more. A raw HDTV video signal needs 1.5 Gbps. That's 1,000 times what you need for audio. Even the standard NTSC video consumes about 143 Mbps. If video signals stayed in that format, you could forget transmitting them outside the studio or even recording them on DVDs.

The magic of the CODEC is compression of the converted signals. For video, the standard compression format is called MPEG (Motion Picture Experts Group) and the most popular format is MPEG-2. That's what's used for HDTV broadcasting and satellite TV. MPEG-2 does its magic by monitoring what is changing in the scene and only transmitting that. In practice, one frame of video doesn't really differ dramatically from the next, which comes only 1/60 of a second later. Get rid of all the "redundant" information in the pictures and the bandwidth requirement drops dramatically. That 1.5 Gbps raw HDTV becomes 19.4 Mbps for HDTV broadcast using MPEG-2 compression.

DS3 service running over a T3 line or multiplexed on a OC3 or higher optical carrier works well for transporting digital video signals using reasonable amounts of compression. A couple of HDTV signals will fit into the 45 Mbps bandwidth of DS3. Less compression can be used to transmit a single higher quality video stream for video production work, digital cinema or medical imaging use. If truly raw video is needed, OC3 at 155 Mbps, OC12 at 622 Mbps or OC48 at 2.488 Gbps are available. Otherwise, DS3 offers an excellent price point, wide availability, and enough bandwidth for most video applications.

Competition from Carrier Ethernet
In recent years, a newer technology called Carrier Ethernet has started replacing both T1 lines, DS3 and SONET OCx fiber. Ethernet over Copper (EoC) uses the same twisted pair wiring at T1 and is available from 2 to 45 Mbps. Ethernet over Fiber (EoF) is highly scalable in the range from 10 Mbps to 10 Gbps, with 100 Mbps and Gigabit Ethernet being very popular service levels. Ethernet is far more scalable than earlier technologies and enjoys a cost advantage as well.

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