Link Speed
OC
(Optical Carrier) The transmission
speeds defined in the SONET specification. OC defines transmission by optical
devices, and STS is the electrical equivalent. See DS.
Service Speed (Mbps)
OC-1
STS-1 51.84 (28 DS1s or 1
DS3)
OC-3
STS-3 155.52 (3 STS-1s)
OC-3c
STS-3c 155.52 (concatenated)
OC-12
STS-12 622.08 (12 STS-1, 4
STS-3)
OC-12c STS-12c 622.08 (12 STS-1, 4 STS-3c)
OC-48
STS-48 2488.32 (48 STS-1, 16
STS-3)
OC-192 STS-192 9953.28 (192 STS-1, 64 STS-3)
OC-768 STS-768 38813,12 (768 STS-1, 256 STS-3)
DS
(Digital Signal) A classification of digital circuits. The DS technically refers to the rate and
format of the signal, while the T designation refers to the equipment providing
the signals. In practice, "DS" and "T" are used
synonymously; for example, DS1 and T1, DS3 and T3. See OC.
Voice
Service Channels Speed
DS0
1 64 Kbps
DS1
24 1.544 Mbps (T1)
DS1C
48 3.152 Mbps (T1C)
DS2
96 6.312 Mbps (T2)
DS3
672 44.736 Mbps (T3)
DS4
4032 274.176 Mbps (T4)
Voice
Service Channels Speed (Mbps)
E1
30 2.048
E2
120 8.448
E3
480 34.368
E4
1920 139.264
E5
7680 565.148
SONET CIRCUITS
Service Speed (Mbps)
STS-1
OC1 51.84 (28 DS1s or 1 DS3)
STS-3
OC3 155.52 (3 STS-1s)
STS-3c OC3c 155.52 (concatenated)
STS-12 OC12 622.08 (12 STS-1s, 4 STS-3s)
STS-12c OC12c 622.08 (12 STS-1s, 4 STS-3c's)
STS-48 OC48
2488.32 (48 STS-1s, 16 STS-3s)
SONET
(Synchronous Optical NETwork) A
fiber-optic transmission system for high-speed digital traffic. Employed by telephone companies and
common carriers, SONET speeds range from 51 megabits to multiple gigabits per
second. SONET is an intelligent system that provides advanced network
management and a standard optical interface. It uses a self-healing ring
architecture that is able to reroute traffic if a line goes down. SONET
backbones are widely used to aggregate lower-speed T1 and T3 lines.
SONET is specified in the Broadband ISDN (B-ISDN) standard. The European
counterpart is SDH. Following are the levels of service. OC (Optical Carrier)
refers to the optical signal, and STS (Synchronous Transport Signal) refers to
the electrical signal, which is the same speed.
SONET uses time division multiplexing (TDM) to send multiple data streams
simultaneously. Its smallest increment of provisioning is VT-1.5, which
provides 1.7 Mbps of bandwidth. The next increment, STS-1, jumps to 51.84 Mbps.
Any data stream that does not fill that channel goes wasted.
Bellcore's GR-2837 standard maps ATM cells onto
SONET, turning a SONET pipe into a cell-switched (packet-switched) transmission
carrier that utilizes the full bandwidth of the medium without waste.
SONET is built in a self-healing ring architecture which uses at least two
transmission paths in the event one fails (see SONET ring).
SONET CIRCUITS
Service Speed (Mbps)
VT-1.5 1.7
OC-1
STS-1 51.84 (28 DS1s or 1 DS3)
OC-3
STS-3 155.52 (3 STS-1s)
OC-3c
STS-3c 155.52 (concatenated)
OC-12
STS-12 622.08 (12 STS-1, 4
STS-3)
OC-12c STS-12c 622.08 (12 STS-1, 4 STS-3c)
OC-48
STS-48 2488.32 (48 STS-1, 16
STS-3)
OC-192 STS-192 9953.28 (192 STS-1, 64 STS-3)
OC-768 STS-768 39813,12 (768 STS-1, 256 STS-3)
Transporting IP
|
In a WAN or
over the Internet, IP traffic is widely carried over SONET lines, either
using ATM as a management layer or over SONET directly. In the future, IP is
expected to travel directly over DWDM fiber (rightmost diagram). |
WDM
(1) (Wavelength Division Multiplexing) A technology
that uses multiple lasers and transmits several wavelengths of light (lambdas)
simultaneously over a single optical fiber. Each signal travels within its unique color band,
which is modulated by the data (text, voice, video, etc.). WDM has dramatically
increased the carrying capacity of the fiber infrastructure of the telephone
companies and other carriers. Also known as "dense WDM" (DWDM),
vendors have introduced systems that can support hundreds of wavelengths, each
carrying 10 Gbps. That means terabits of data per
second can travel over one optical strand, thinner than a human hair. Contrast
with TDM.
See CWDM
and fiber
optics glossary. See also FDM.
DWDM
(Dense WDM) The term given to wavelength
division multiplexing (WDM) when significantly more channels were being added.
Since WDM is increasingly more "dense" all the time, both terms are
used synonymously. See WDM.
TDM
(Time Division Multiplexing) A technology that transmits
multiple signals simultaneously over a single transmission path. Each lower-speed signal is time sliced
into one high-speed transmission. For example, three incoming 1,000 bps signals
(A, B and C) can be interleaved into one 3,000 bps signal (AABBCCAABBCCAABBCC).
The receiving end divides the single stream back into its original signals.
TDM enabled the telephone companies to migrate from analog to digital on all
their long distance trunks. The technology is used in channel banks, which
convert 24 analog voice conversations into one digital T1 line. Contrast with FDM. See circuit switching, channel bank and T1.
From FDM to TDM to Packet Switching
|
TDM has
replaced FDM (frequency division multiplexing) for telco
backbone networks, but packet switching is expected to become the norm for
voice and data in the 21st century. |
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FDM
(Frequency Division Multiplexing) A technology that
transmits multiple signals simultaneously over a single transmission path, such
as a cable or wireless system. Each signal travels within its own unique frequency range
(carrier), which is modulated by the data (text, voice, video, etc.).
In the 1930s, the telephone companies began to combine multiple analog voice
signals over one line using FDM. This was later replaced with digital methods
(see channel bank). For years, cable TV
companies have used FDM to transmit many channels over the same wire. The
set-top box or TV tuner locks onto a particular frequency (channel) and filters
out the video signal for the TV screen. Contrast with TDM. See circuit switching and WDM.
FDM Is Where It Started
|
Since the
1930s, the analog telephone system has used FDM to combine multiple voice
conversations onto one line. The analog telephone backbones have given way to
digital, thus FDM has been replaced with TDM for backhaul trunks. |
CWDM
(Coarse Wavelength Division Multiplexing) CWDM is a network technology that is
synergistic with Metro DWDM, not in competition. Standardization of CWDM
technology; and simple, cost effective service interoperability between CWDM
and DWDM systems will enable its adoption.
Because they need to serve smaller bandwidth applications than DWDM systems,
CWDM based systems are characterized by wider channel spacing than DWDM optical
networks. The frequency separation between each individual color of light on
the actual fiber is significantly further apart, which allows the system
designers to use lasers that have looser tolerances on spectral width and
thermal drift, therefore less expensive.
T1
(1) A 1.544 Mbps point-to-point dedicated, digital circuit provided by
the telephone companies.
The monthly cost is typically based on distance. T1 lines are widely used for
private networks as well as interconnections between an organization's PBX or
LAN and the telco. The first T1 line was tariffed by AT&T in January 1983. However, starting in
the early 1960s, T1 was deployed in intercity trunks by AT&T to improve
signal quality and make more efficient use of the network.
A T1 line uses two wire pairs (one for transmit, one for receive) and time
division multiplexing (TDM) to interleave 24 64-Kbps voice or data channels.
The standard T1 frame is 193 bits long, which holds 24 8-bit voice samples and
one synchronization bit with 8,000 frames transmitted per second. T1 is not
restricted to digital voice or to 64 Kbps data streams. Channels may be
combined and the total 1.544 Mbps capacity can be broken up as required. See DS, T-carrier, bipolar transmission, D4 and ESF.
(2) See Type 1 font.
64 Kbps
T-Carrier Total Speed Channels
T1
1.544 Mbps 24
T2
6.312 Mbps 96
T3
44.736 Mbps 672