Digital signal 1
(DS1, also known as T1, sometimes
"DS-1") is a T-carrier signaling scheme
devised by Bell
Labs
. DS1 is a widely used standard in telecommunications in North America and Japan
to transmit
voice and data between devices. E1 is used in place of
T1 outside of North America, Japan, and South
Korea. Technically, DS1 is the logical bit pattern used over a
physical T1 line; however, the terms "DS1" and "T1" are often used
interchangeably.
Bandwidth
A DS1
circuit is made up of
twenty-four 8-bit
channels
(also known as
timeslots or
DS0s), each channel being a 64 kbit/s
DS0 multiplexed carrier circuit. A
DS1 is also a
full-duplex circuit, which
means the circuit transmits and receives 1.544
Mbit/s concurrently. A total of 1.536 Mbit/s of
[82133] bandwidth is achieved by sampling each
of the twenty-four 8-bit
DS0s 8000 times per
second. This sampling is referred to as 8-
kHz
sampling (See
Pulse-code
modulation). An additional 8
kbit/s of
overhead is obtained from the placement of one framing bit, for a
total of 1.544 Mbit/s, calculated as follows:
- \left( 8\,\frac{\mathrm{bits}}{\mathrm{channel}} \times
24\,\frac{\mathrm{channels}}{\mathrm{frame}} +
1\,\frac{\mathrm{framing\ bit}}{\mathrm{frame}} \right)
\times 8000\,\frac{\mathrm{frames}}{\mathrm{second}}
=1544000\,\frac{\mathrm{bits}}{\mathrm{second}}
=1.544\,\frac{\mathrm{Mbit}}{\mathrm{second}}.
DS1 frame synchronization
Frame synchronization is
necessary to identify the
timeslots within
each 24-channel frame.
Synchronization takes place by allocating a
framing, or 193rd, bit. This results in 8 kbit/s of framing data,
for each DS1. Because this 8-kbit/s channel is used by the
transmitting equipment as
overhead, only 1.536 Mbit/s is actually
passed on to the user. Two types of framing schemes are
Super Frame (SF) and
Extended Super Frame (ESF). A Super
Frame consists of twelve consecutive 193-bit frames, whereas an
Extended Super Frame consists of twenty-four consecutive 193-bit
frames of data. Due to the unique bit sequences exchanged, the
framing schemes are not compatible with each other. These two types
of framing (SF and ESF) use their 8 kbit/s framing channel in
different ways.
SF framing
In SF Framing, aka Super Frame, the framing channel is divided into
two channels of 4 kbit/s each. One channel is for terminal frame
alignment; the second is used to align the signaling frames. The
terminal frame and signaling frame bits are interleaved, rather
than consecutive (they are switched in Figure 2). (correction per
ANSI T1.403 Section 7.2 "A frame is a set of 192 digit time-slots
for the information payload preceded by one digit time-slot
containing the framing (F) bit, for a total of 193 digit
time-slots." Meaning the first bit of the frame is a framing bit
and not the last bit.)
The terminal frame alignment channel is carried in odd-numbered
frames inside the super frame and occurs with the
DS0 channel synchronization. Since the framing bits
occur only once per frame, in the 193rd position, the bit placement
of each
DS0 can be calculated. After the framing
bit is sensed, the first DS0 timeslot is taken as the next 1-8
bits. Timeslot 2 is bits 9-16, timeslot 3 is 17-24, through to
timeslot 24. See Figure 1. The Terminal frame alignment pattern is
carried in odd-numbered frames, inside the super frame, and
consists of alternating 1s and 0s: 1–0–1–0–1–0.
Signaling frame alignment channel is carried in even-numbered
frames inside the super frame and is used for signaling frame
alignment. The signaling frame alignment pattern consists of a
0–0–1–1–1–0. Signaling frames are identified by the framing
signal's transition from 1 to 0 and from 0 to 1; thereby frames six
and twelve carry signaling information. See Figure 2.
The SF format uses
bit robbing to pass
signaling
information.
Bit robbing modifies the
least significant bit in each
user data timeslot twice per Super Frame. (See also
A&B). The two modified frames are the sixth (A)
and the twelfth (B). Using two bits, four possible signaling states
can be passed in each direction (0–0, 0–1, 1–0, 1–1). In order for
A/B signaling to work, the exact placement of the bits must be
known by both sides. Information on the frame sequence is necessary
to "pick out" the A and B bits. Channel information must also be
known in order to pick out the last bit of each channel. If the
proper alignment (timing) did not occur, the wrong bit could be
modified or read as the robbed bit. This method of signaling is
also commonly referred to as
Channel Associated Signaling or
CAS. See Figure 2.
The SF format is also known as D4 framing and D3/D4 framing
format.
ESF framing
In ESF, aka Extended Super Frame, twenty-four frames make up the
(extended) super frame. ESF divides the 8 kbit/s framing channel
into three segments. The frame pattern uses 2 kbit/s, and a
Cyclic redundancy check
(CRC) uses 2 kbit/s. The remaining 4 kbit/s make up an
administrative data link (DL) channel. The framing pattern occupies
the 4th, 8th, 12th, 16th, 20th and 24th frames. The pattern
consists of a 0–0–1–0–1–1 sequence. This is the only pattern that
is repeated in the ESF format. See Figure 3.
Figure 3.
The CRC algorithm checks a known segment of data and adds the
computed value to it. The combined data and CRC blocks are both
transmitted. The receive circuitry will run the same CRC algorithm
against the data portion and compare the calculation to the
transmitter's CRC value. In this manner, corrupted data can be
flagged as "CRC errors". The CRC checksum is passed in the 2nd,
6th, 10th, 14th, 18th, and 22nd frames. (See also
Error-correcting code).
The administrative channel provides a means to communicate within
the DS1 stream (
sub-channel).
Statistics on CRC errors can be requested and sent from one end to
another. The data channel occupies the twelve odd numbered frames.
Signaling and other information passes over this channel.
Provisions in the ESF standard would allow the normal A/B bit
robbed signal to be enhanced. The A/B bits can be extended to four
bits (ABCD). This provides 16 distinct states. An improvement from
A/B, which provides 4. To overcome incompatibility with A/B
signaling, equipment repeats the A&B bits (e.g. C = A and D =
B). These additional signaling bits will offer new features as
equipment is built to support it.
CRC errors can be detected and counted in at least one of four
different registers. The registers are for transmit (in and out)
and receive (in and out). Using recovered CRC data, it is possible
to segment and isolate the direction of problems.
Connectivity and Alarms
Connectivity refers to the ability of the digital
carrier to carry customer data from either end to the other. In
some cases, the connectivity may be lost in one direction and
maintained in the other. In all cases, the terminal equipment,
i.e., the equipment that marks the endpoints of the DS1, defines
the connection by the quality of the received framing pattern.
Alarms
Alarms are normally produced by the receiving terminal equipment
when the framing is compromised. There are three defined
alarm indication signal states,
identified by a legacy color scheme: red, yellow and blue.
Red alarm indicates the alarming equipment is
unable to recover the framing reliably. Corruption or loss of the
signal will produce “red alarm.” Connectivity has been lost toward
the alarming equipment. There is no knowledge of connectivity
toward the far end.
Yellow alarm indicates reception from the far end
of a data or framing pattern that reports the far end is in “red
alarm.” Red alarm and yellow alarm states cannot exist
simultaneously on a single piece of equipment because the “yellow
alarm” pattern must be received within a framed signal. For ESF
framed signals, all bits of the Data Link channel within the
framing are set to data “0”; the customer data is undisturbed. For
D4 framed signals, the pattern sent to indicate to the far end that
inbound framing has been lost is a coercion of the framed data so
that bit 2 of each timeslot is set to data “0” for three
consecutive frames. Although this works well for voice circuits,
the data pattern can occur frequently when carrying digital data
and will produce transient “yellow alarm” states, making ESF a
better alternative for data circuits.
Blue alarm indicates a disruption in the
communication path between the terminal equipment. Communication
devices, such as
repeaters and
multiplexers must see and produce line activity
at the DS1 rate. If no signal is received that fills those
requirements, the communications device produces a series of pulses
on its output side to maintain the required activity. Those pulses
represent data “1” in all data and all framing time slots. This
signal maintains communication integrity while providing no framing
to the terminal equipment. The receiving equipment displays a “red
alarm” and sends the signal for “yellow alarm” to the far end
because it has no framing, but at maintenance interfaces the
equipment will report “AIS” or
Alarm Indication Signal. AIS is also
called “all ones” because of the data and framing pattern.
These alarm states are also lumped under the term Carrier Group
Alarm (CGA). The meaning of CGA is that connectivity on the digital
carrier has failed. The result of the CGA condition varies
depending on the equipment function. Voice equipment typically
coerces the robbed bits for signaling to a state that will result
in the far end properly handling the condition, while applying an
often different state to the customer equipment connected to the
alarmed equipment. Simultaneously, the customer data is often
coerced to a 0x7F pattern, signifying a zero-voltage condition on
voice equipment. Data equipment usually passes whatever data may be
present, if any, leaving it to the customer equipment to deal with
the condition.
Real world use
Before the jump in Internet traffic in the mid 1990s, DS1s were
found mostly in larger businesses and telephone company central
offices as a means to transport voice traffic between locations.
DS1s have been and still are the primary way
cellular phone carriers connect their central
office switches (
MSC)
to the
cell sites deployed throughout a
city.
Today, many smaller companies often use an entire DS1 for
Internet traffic, providing 1.544 Mbit/s of
shareable synchronous connectivity (allowing for 1.536 Mbit/s of
usable traffic, and 8 kbit/s of framing overhead). However, DS1 can
be ordered as a channelized circuit, and any number of channels can
be reserved for non-data (for example, voice) traffic.
Many radio stations also use this technology in their broadcasting.
A T1 telephone line can be used as a link to convey the broadcast
audio from the studio to the transmitter/tower site, a distance
that can be quite a few miles in length. T1-based solutions, as
opposed to IP-based, remain very attractive to broadcasters because
the data is transported in effective real-time.
Inband T1 versus T1 PRI
Additionally, for voice T1s there are two main types: so-called
"plain" or Inband T1s and PRI (
Primary Rate Interface). While both
carry voice telephone calls in similar fashion, PRIs are commonly
used in call centers and provide not only the 23 actual usable
telephone lines (Known as "B" channels) but also a 24th line (Known
as the "D" channel for Delta) that carries signaling information.
This special "D" channel carries:
Caller
ID (CID) and
Automatic Number
Identification (ANI) data, required channel type (usually a B,
or Bearer channel), call handle, DNIS info, requested channel
number and a request for response.
Inband T1s are also capable of carrying CID and ANI information if
they are configured by the carrier to do so but PRIs handle this
more efficiently. While an Inband T1 seemingly has a slight
advantage due to 24 lines being available to make calls (as opposed
to a PRI that has 23), each channel in an Inband T1 must perform
its own setup and tear-down of each call. A PRI uses the 24th
channel as a data channel to perform all the overhead operations of
the other 23 channels (including CID and ANI). Although an inband
T1 has 24 channels, the 23 channel PRI can set up more calls faster
due to the dedicated 24th signalling channel (D Channel).
Origin of Name
The name T1 came from the carrier letter assigned by AT&T to
the technology. Essentially, the "T" is a part number that was
assigned by AT&T. Just as there is the generally known
L-carrier and N-carrier systems, T-carrier was
next letter available and T1 is the first level in the hierarchy.
DS-1 meant "Digital Service - Level 1", and had to do with the
service to be sent (originally 24 digitized voice channels over the
T1). The terms T1 and DS1 have become synonymous and include a
plethora of different services from voice to data to clear-channel
pipes. The line speed is always consistent at 1.544 Mbit/s, but the
payload can vary greatly.
Alternative Technologies
Dark Fiber:
Dark fiber
refers to unused
fibers, available for use.
Dark fiber has been, and still is, available for sale on the
wholesale market for both metro and wide area links, but it may not
be available in all markets or city pairs.
Dark fiber capacity is typically used by network operators to build
SONET and dense wavelength division
multiplexing (DWDM) networks, usually involving meshes of
self-healing rings. Now, it is also used
by end-user enterprises to expand
Ethernet
local area networks, especially since the adoption of
IEEE standards for Gigabit Ethernet and 10 gigabit
Ethernet over singe-mode fiber. Running Ethernet networks between
geographically separated buildings is a practice known as "
WAN elimination".
Semiconductor
The T1/E1 protocol is implemented as a "line interface unit" in
silicon. The semiconductor chip contains a decoder/encoder, loop
backs, jitter attenuators, receivers, and drivers. Additionally,
there are usually multiple interfaces and they are labeled as dual,
quad, octal, etc., depending upon the number.
The transceiver chip's primary purpose it to retrieve information
from the "line", i.e., the conductive line that transversed
distance, by receiving the pulses and converting the signal which
has been subjected to noise, jitter, and other interference, to a
clean digital pulse on the other interface of the chip.
Examples
The
global telephone
network (also known as the Public Switched Telephone Network or
PSTN).
Notes and references
- "How Bell Ran in Digital Communications" September 1996,
webpage: BYTE-Bell: Bell Labs scientists developed a
time-division multiplexing scheme, T1.
- Just Circuits - T1 Made Simple [1]
- Versadial, Call recording encyclopedia, last accessed 19 Apr
2007
- Newton, H: "Newton's telecom dictionary", page 225. CMP books,
2004
See also
Wikis
Quiz
Map
Search
