Video Transport on Digital
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
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
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
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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