Untitled Essay, Research Paper
By: Scott CrandellTelecommunications – T1’sDigital transmission is what T1s are all about. T1s are the wanted access in the
local loop. They provide multiple services simultaneously over the same medium with
the ability to have WATS, international, virtual networking, private networking and
switched data services. T1 circuits have grown from point to point links for private
networks to switched and dedicated access services. T1s have been used for decades
with their attributes and functionality keeping them still in use in the industry today.
Starting in 1960 multiplexing started with only two users that were multiplexed on one
“carrier facility.” More and more copper was added to the local loop till
1970 when a line per subscriber became too expensive and industry turned toward the
technology of the T1. The name T1 came from AT&T in the 1960’s and was
something just to name the technology till the name was picked up to describe the digital
transmission line. What the T1 did was allowed “24 voice channels onto a single
four-pair wire”(Minoli). The industry decided to switch to T1s because the cost
savings of having 24 channels with less lines used and the room for expansion.
Though the switch between massive amounts of copper to T1 solved some problems the lines
were not very usable due to technology still using analog signals. Digital finally
started being used in the 1970’s because it helped eliminate noise in the circuit
followed quickly by PBX that realized the potential of the digital technology.
Multtiplexing for analog and digital were both used but analog used space division where
digital used time division. Since price between the two types of multiplexing was
similar industry started to switch to digital time division multiplexing. Instead of
using cross-point technology with analog digital multiplexing was used which allowed an
eight contact circuit connection be converted to a single serial datastream and
transmitted over the same copper wire (Minoli).
Digital multiplexing uses something called pulse coke modulation. The signal is
sampled 8,000 times per second and made into 8 bits. Every channel contains 64K bps with
is figured by 8000 X 8. Transmission can go as high as Gigabits per second and have
two or more conversations happening at once. T1 have capable speeds of 1536 Kbps due
to the 24 channels multiplexed into 64K bps channels.
Synchronous and asynchronous are two types of time division multiplexing.
Synchronous multiplexing has a system where a mechanism determines where each bit belongs
whereas asynchronous has information in packets that have a label as to where to send the
data. One data bit has to be used to determine the end and beginning of each
packet. Multiplexing is used for data, voice and a combination of both. T1
transmits over a copper wire and is DS1 formatted. It can be transmitted over fiber
and is better for longer runs to eliminate noise and repeaters on the line. 672
channels can be achieved with a T3, which is equivalent to 28 T1s. So during the
1970’s and the 1980’s T1s and T3s were used for the backbone of the transmission
network (Bezar 200).
In the CO either one or two interface cards are used to receive the 24 individual
telephone circuits. The T1 cards have the ability to convert the analog to digital
with the 8.000 per second sampling required to fit the 8 bit compressed sample.
After it is converted to digital and it is transmitted it has to be converted back to
analog in order for the DTMF signaling to be effective. If the conversion of analog
to digital and vise versa does not want to be made 7 bits instead of 8 can be used which
only transmits 56K bps opposed to the 64K bps with 8 bits. Other types of signaling
are also possible.
T1 cable gauge ranges anywhere between 16-26 and is transmitted with repeaters put roughly
every mile to preserve the signal. When using a pair-gain repeater the requirement
is only every five miles. Repeaters are either pole mounted or placed underground
accessed through sewer plates. By the signal being digital re-creation is simple
compared to analog with noise and distortion kept to a minimum (Bezar 201). Data
transmission needs a clean circuit with the least possible problems on the line.
Various problems can be stopped with equipment such as echo suppressers proper grounding
and the elimination of load coils. T1’s should fall within the standard of less
than 10-6 errors per bit in order to provide good service. Fiber is becoming popular
due to the lack of errors and noise. Fiber doesn’t need as many repeaters and
doesn’t provide power do electric induced noise is not a worry. Further more
fiber doesn’t require as much maintenance but it costs more and instillation has to
be properly done including good splices and no bends larger than two inches in the wire
which will cause attenuation.
T1s can be used for analog voice, data command and control, video teleconferencing or
anything else that can be transmitted as digital data (Minoli). T1s are often used
to link offices or campuses together or to link private networks over short distances.
An interconnection can be made between two businesses PBX’s so the telephone
system will act like one unit. By doing this cost can be cut with the use of the
same outgoing trunk lines.
With the demands of larger capacity transmission lines T1s might be the solution to the
problem. The technology of fiber, prices of T1s have already decreased. Users
are requiring more bandwidth with the use of video conferencing, connection of networks
and the desire for faster service. When choosing what to purchase an assessment of
the industry need to be made with all the new advancements planning and implementing
something new could be a difficult job.