Let There Be
Light!
by Ken
Freed.
.
Understanding
how T1, DS3, ISDN and DSL copper wire-line
telecommunication services can support digital video
transmission.
ViBroadcast
content often contains video and audio that at some point
was carried point-to-point by telephone networks. For
acquisition, production or distribution, television needs
telecommunication lines. How and where do TV operations
use digital telecom services?
Consider the
infrastructure in which we're operating. For your typical
phone call, an analog signal goes from the telephone
handset over POTS lines to the "central office" of the
local exchange carrier. From there, your call is
digitally encoded, switched and multiplexed with other
calls for transmission on fiber backbones to destination
cities where a central office decodes your signal,
switching your call to an analog telephone at the other
end &emdash; where you enter voicemail.
What constitutes
digital telecom service? We've been sending analog audio
over plain old telephone service (POTS) since Marconi and
Bell. The pioneer's dream of live pictures over twisted
pairs of copper wires finally is practical with ADSL
video at VHS quality, but the TV industry's utilization
of POTS mostly remains limited to phone tag. Instead,
broadcasters today undertaking digital operations rely on
upgraded telecom plants that provide the needed
bandwidth.
For those entering
digital telecom services like a swimmer testing the water
with a toe, dive in knowing that digital supports higher
data rates than analog can deliver, which equates to
higher quality signals. Also, the shorter the telco line,
the faster the bitrate. Engineers can benefit from
understanding the kinds of digital telecom services now
available, and by identifying how to gain access to them.
T1 and
DS3
A good place to
start may be the telephone company's four major classes
of digital lines &emdash; DS1 (T1), DS3, and two forms of
ISDN &emdash; each involving paid carriage over multiples
of twisted pairs. The line classes vary chiefly by
bandwidth capacity. As our benchmarks, uncompressed NTSC
needs 270 Megabits per second (Mb/s), yet MPEG-2
compression only needs a pipeline able to carry content
at 3 Mb/s up to 30 Mb/s.
In telco parlance,
a line able to handle one digital signal (one voice
telephone call) is labeled a "DS-Ø," commonly
written as "DSO." Each 8-bit DSO travels at 64 kilobits
per second (Kb/s). When 24 DSO 64 Kb/s channels are
multiplexed into one bundle, the resulting spine of
twisted pairs is a "DS1, the telecom link we commonly
call a "T1."
Most digital lines
carry multiples of a single DS call. Short and sweet: A
single "T1" channel at 1.5 Mb/s is a DS1. DS2 combines
four T1 channels for a throughput of 6.3 Mb/s. DS3
provides 28 T1 pipes at 45 Mb/s. And DS4 offers 168 T1
channels at 274 Mb/s.
If expressed as a
hierarchy, Cross-connecting DS1 (T1) at 1.5 Mb/s, the DS1
line can be multiplexed into a DS2 (T2) line at 6.3 Mb/s
or into a DS3 (T3) line at about 44.7 Mb/s. A DS3 line
can be multiplexed into a DS4 (T4) line at 274.1 Mb/s.
Digital microwave service can access any DS1, DS2, DS3,
and DS4 line, yet to access a fiber optic system (e.g.
ATM/SONET), you need at least a DS4 line.
The TV industry
mostly uses T1 and DS3 lines.
T1 equals a DS1
line capable of carrying a multiplexed set of 24 DS0
calls, each at 64 Kb/s, which collectively handles 1.544
Mb/s to deliver at least "approval quality" video. T1
supports cable modems, low-end Motion-JPEG (512 kb/s to 3
Mb/s) up to MPEG-1 (1-2 Mb/s). T1 offers variable
non-realtime video acquisition, production and post
applications, including digital effects systems (1.4-10
Mb/s) when bundled,. Multiple T1 lines can handle low-end
MPEG-2 (3-20 Mb/s).
DS3 is the result
of 28 multiplexed T1 lines (28 x 24 DS). A DS3 line can
handle 44.736 Mb/s, sufficient bandwidth for realtime and
non-realtime acquisition, production and broadcast of
compressed NTSC at 45 Mb/s for full-motion video at 30
fps. DS3 can handle one or two MPEG-2 channels with room
for instructions.
Bundled DS3 can
handle transport of digital video recordings in formats
like D1 (270 Mb/s). Bundled T3 now serves the film
industry, TV network production studios, antenna and
satellite broadcast centers, cable and wireless headends,
and tests of telco "Open Video Systems."
DS lines like T1
and DS3 lines are available mostly in the larger cities.
In contrast, one
still finds working installations of a 56 Kb/s line,
"Switched 56," an early digital data line deployed by
telcos well before 56k modems arrived on the scene.
Switched 56 can stream low-end video like QuickTime and
AVI for computers applications.
ISDN
Apart from T1 and
DS3 lines, the most ubiquitous digital telecom service is
ISDN (Integrated Services Digital Network). Deployed in
reply to demands from Internet Service Providers (ISP's)
as well as enterprise networks needing high-speed data
services, ISDN has two classes:
Basic Rate ISDN
(BRI) is equal to two 64 kb/s "B" channels for content
plus a 16 kb/s "D" channel for address signaling and
other call-related data. Designated "2B+D," Basic ISDN
operating at 128 Kb/s is quite suitable for "desktop
video" applications, such as low-end video
teleconferencing at 15 frames per second. Basic ISDN also
supports non-realtime Motion-JPEG for television
production and post-production.
Primary Rate ISDN
(PRI) provides 23 DS0 64 kb/s "B" channels plus one 64
kb/s "D" channel, designated "23B+D." Primary ISDN
handles bitrates from 1.28 Mb/s to 1.47 Mb/s. PRI
provides high-end "viewphone" services at 30 fps. Primary
ISDN permits realtime M-JPEG in production and post.
Primary ISDN also supports MPEG-1 for multimedia
production.
Responding to
demands by PC users, most local telcos in the USA now
offer ISDN lines to homes and businesses.
The above digital
services and bitrates apply in North America, Japan,
South Korea, and the Republic of China. Digital line
standards adopted by the European Conference of Postal
and Telecommunication Administration call for a
progression of 64 kb/s, 2.0 Mb/s, 8.4 Mb/s, 34.3 Mb/s,
and 139.2 Mb/s. Only two of these bitrates permit direct
interconnections with American dataline services. The 64
Kb/s is the same as DS0, and the 139.264 Mb/s line is the
same as a "DS4E" line in North America (a subset of DS4).
Otherwise, you need a translator.
Digital telecom
services initially appear quite affordable. Basic ISDN
service in the USA runs from $25 to $100 per month,
depending on the local carrier and the distance to a
central office. Primary ISDN may range from $600 to
$1,200 per month or more. T1 lines go for $400 to $1,000
per month. Monthly DS3 service starts at $2,000 and goes
up to $10,000, depending on available bandwidth and its
usage.
Before you pencil a
budget, however, add to your calculations the per-minute
charges on top of the monthly rate. And if any
long-distance transport is utilized, pile that LD
carrier's charges atop the local carriage costs. Digital
telecom service can quickly get expensive, especially for
the installation. Call diverse providers of digital
telecom services for competitive bids specific to your
particular requirements.
Digital
Subscriber Lines (DSL)
Another class of
telecom service permits sending video streams over plain
old twisted pairs of copper wires.
Digital subscriber
lines take advantage of existing POTS plant. Various
forms of digital subscriber lines (xDSL) include
Asymmetric (ADSL), Single (SDLS), High data rate (HDSL),
and Very high data rate (VDSL). Most DSL applications are
within the computer arena, but ADSL does carry video
&emdash; perhaps better than many inside and outside
telephony seem willing to admit. VDSL, while still a
young technology, may rapidly grow up to handle MPEG-2
for HDTV.
Of available
digital subscriber lines, ADSL seems the best suited for
video applications because most of the bandwidth is
devoted to the downstream traffic. ADSL offers just
enough upstream bandwidth for sending instructions on
what to send downstream. ADSL downstream capacity yields
at least "VHS quality" video at 30 fps, which may be
deemed "approval quality" for some production and post
applications.
ADSL initially
gained attention when an east coast regional Bell
operating company (RBOC) announced ADSL video tests as a
means of entering the video business before cable or
wireless could deploy cable modems or begin "interactive
TV" services with digital set-tops. ADSL apparently
remains a quiet strategy for telco video service
providers.
ADSL deployments to
date vary from ISP computer modems to video-on-demand
trials. Meanwhile, 9 Mb ADSL modems are able to compete
with 10 Mb cable modems for Internet access customers.
An ADSL circuit on
a twisted pair utilizes frequency division multiplexing
and echo cancellation to create three asymmetric channels
for downstream, upstream and POTS traffic. The downstream
channel handles 1.5 Mb/s at 18,00 feet to 6.1 Mbs at
12,000 feet (or 9,000 feet, depending on the wire gauge).
Most recently, ADSL service has upgraded to 9 Mb/s
downstream. The upstream duplex channel handles 16 kb/s
to 640 kb/s over the same distances, shorter circuits
yield faster carriage. ADSL circuits also have one 64 kbs
POTS channel &emdash; in case ADSL fails.
As telco
infrastructure matures, ADSL modems will accommodate ATM
transport for MPEG-2 (see table). Applications then may
expand far beyond Internet access to include interactive
multimedia, video-on demand, interactive games,
electronic commerce, and the host of other services that
comprise "interactive television."
As DSL itself
matures, expect broadcast applications to increase. For
instance, VDSL7 at 13 to 52 Mb/s can transport both NTSC
and HDTV. For local stations wanting to offer wide-screen
digital TV, the use of existing POTS lines might help
lower implementation costs. VDSL circuits then could be
modified for asymmetric upstream traffic, which would
support interactive HDTV. Do telcos have this in
mind?
On the off chance
the "must-carry" rule ever does get reversed,
broadcasters could use POTS to transmit programming to
DSL set-tops with phoneline returns. Will rooftop
antennas ever become obsolete?
Fiber
Services
Where operating
budgets allow, the best video transport choice is an
optical fiber line. High-bandwidth glass (or clear
plastic) fiber-optics provide unsurpassed signal
integrity and video resolution. Optical fiber is the most
reliable means available for cleanly and safely
delivering at point B exactly what was transmitted at
point A.
Fiber sustains the
encoding and decoding of digital data packets better than
any other transmission medium. A widely adopted open
standard for fiber network operation is called SONET
(Synchronous Optical NETwork). SONET permits compliant
devices from competing manufacturers at point A and point
B to communicate clearly. Users of one encoder, for
instance, need not worry about optical "mid-span meet"
mishaps when the signal reaches another vendor's
lightwave decoder at the other end of the line.
Standardizing the characteristics of light pulses allows
SONET to support "multivendor interworking."
A SONET advantage
is synchronized, single-stage multiplexing and demux,
which permits individual signals to become "visible" for
manipulation. SONET switches point-to-point transactions
within a multipoint-to-hub LAN or WAN configuration,
which exponentially expands the possible number of
interconnections inside the network. Acquisition,
production, post, traffic, and master control all can
have simultaneous point-to-point access to any piece of
video in the system. High speed keeps the digital signal
remarkably robust in the process.
One SONET
synchronous transport signal (STS-1, or its optical
equivalent of OC-1) has a base rate of 51.48 Mb/s. SONET
is 5 Mb/s faster than DS3 and starts just below the top
speed of today's VDSL. One HDTV signal needs about 50
Mb/s (with headroom), so each STS channel has the
capacity for one digital TV channel. Multiples of STS add
to the bitrate. Imagine multichannel service on STS-3 at
155 Mb/s
Because of its 51
Mb/s speed, SONET is the preferred network standard for
the 45 Mb/s digital packet switching technology, ATM
(asynchronous transfer mode). A brainchild of the Bell
system, ATM is being adopted by telcos and cablecos and
many others to enable the switched multimedia services
integral to interactive TV deployment.
Here's where
synchronicity enters the scene. Telecommunications
companies often lay fiber in anticipation of demand.
Sometimes unused fiber already exists between where you
are and where you need to be. Any unlit optical link is
called a "dark fiber," and dark fiber produces no
revenues. Telcos and other telecom service providers with
dark fiber may be willing to make cost-effective deals to
light up their lines. Does opportunity need to knock
twice?
Where can you find
dark fiber? Start with your local exchange carrier and
long distance company. Check the Yellow Pages for
third-party SONET providers. Search the "World Wide Wait"
(unless you're using T1 or faster) to find encoders and
other tested equipment for establishing links over a
previously dark fiber. Ask them all if they know of any
dark fiber in your neck of the woods.
Closing the
Circuit
We've now covered
your principal options for transporting video from
point-to-point over the main classes of digital telecom
services.
For applications
where lower speeds suffice, Basic and Primary ISDN can
work wonders for multimedia and even low-end television
production. T1 and DS3 can do the job when digital signal
acquisition, production and distribution demands higher
bandwidths. For HDTV television applications, for end-to
end uniformity, VDSL can serve where bundled DS3 and DS4
have not yet been supplanted by fiber.
Strands of silicon
fiber increasingly are available to meet the TV
industry's existing and anticipated demands for digital
telecom services. As fiber interlinks our facilities, let
there be light. 
.
