- LAN technology can't economically support very large numbers of
hosts or long-distance connections.
- Analog voice communication over long
distance is problematic because signals have to be amplified at
repeating stations and this introduces distortion.
- It's possible to deliver better audio at the receiving end if one
digitizes the audio for transmission and reconstructs the analog
from the digital after it arrives. Telephone companies have been
interested in this since long before data networks became
prevalent.
- To digitize audio, we sample the analog
voltage at regular intervals and assign one of 256 discrete
integral values to the voltage.
- We then transmit binary codes for the integers one after another
in the order that we make the measurements.
- Attenuation of the digital information is
not nearly as problematic as attenuation of analog data.
As long as the receiver is able to determine the value of each
number, there is no real loss.
- To get enough fidelity in the audible range,
the standard (Pulse Code Modulation) calls for 8000 digital samples
per second (one sample every 125 micro-seconds [μs]) and
digitized levels ranging from 0 to 255.
- Telephone companies lease analog voice lines to people who build
data networks. The 'networkers' can use the leased lines to
transmit digital data.
- Telephone companies also have equipment for
transmitting digital information in sophisticated ways.
- The technology for digitized voice is synchronous -- the
voice data is carefully sent at a specific rate by telephone
technology -- it has to be heard coming out just as it sounded
going in -- that requires that the data come out of the system at
exactly the same rate that it entered.
- People who build long-haul networks often
use leased digital voice lines as described above for
point-to-point links.
- There are radical differences in the digital standards used for
digital computer data and digital voice data.
- The network connections need to have a special converter called a
Data Service Unit/Channel Service Unit (DSU/CSU) installed at
each end of the line. The DSU/CSU
converts between two incompatible digital data formats.
- One family of digital voice lines
belonging to the telephone companies have names that begin with
T-. The T-series is a popular choice for network links,
especially the T1.
- Some data rates for T-series are:
T1 ----- 1.544 Mbps 24 voice circuits T2 ----- 6.312 Mbps 96 voice circuits (4 X 24) T3 ----- 44.736 Mbps 672 voice circuits (28 X 24)
- One voice channel requires 8000 8-bit samples per second. That
is 64,000 bits per second. 1.544Mps is just over 24 times
64,000. Therefore a T1 line has capacity
for 24 simultaneous voice channels plus a little overhead.
The T2 line is equivalent to 4 T1 lines -- so in
particular the telephone company can "implement" four T1 lines
with a T2 line. Similarly, a T3 line is equivalent to 28 T1
lines.
- Another family of names for these same lines: DS1, DS2, and DS3
is perhaps more proper, but the distinction is seldom made.
- The T-names stand for the carrier system
- The DS-names stand for the rules about multiplexing multiple
phone calls on the connections.
- T1 lines are expensive --
thousands of dollars per month. T3 costs several tens of
thousands of dollars per month.
- Telephone companies using time division multiplexing can lease
fractions of a T1 line.
- 64 Kbps is a popular fractional rate.
- For customers that need more capacity than
T1, but less than T2 or T3, there is a technology called inverse
multiplexing.
- Under inverse multiplexing, multiple lines, say five T1 lines,
are ganged together and operated as if they were one line with
five times the capacity of a T1 line.
- The computer on one end of the connection sends digital
information to an electronic device called an inverse multiplexor
(inverse mux). The inverse mux alternates sending parts of the
data over each of the multiple lines. An inverse-mux at the
other end reassembles the multiple data streams into one.
DSU/CSU function is required for the ends of each T1 line. The
inverse-mux'es may do that or the DSU/CSU's may be separate.
- Telephone companies have Synchronous Transport Signal (STS)
digital circuits for major (trunk) communication lines.
Capacities of STS greatly exceed capacities of T-series digital
circuits:
STS-1/OC-1 ---------- 51.840 Mbps 810 voice circuits STS-3/OC-3 ---------- 155.520 Mbps 2430 voice circuits STS-12/OC-12 -------- 622.080 Mbps 9720 voice circuits STS-24/OC-24 ------ 1,244.160 Mbps 19440 voice circuits STS-48/OC-48 ------ 2,488.320 Mbps 38880 voice circuits
- It's interesting that data rates to be found on cheap LAN's are
extremely expensive to obtain from telephone companies.
- The higher STS data rates require optical
fiber. The OC-X codes refer to that.
- If the code above has a C suffix (concatenated), it means the
line is not inverse-multiplexed. For example and OC-3C is just a
single line operating at 155,520 Mbps, while a OC-3 is three OC-1
circuits operating at 51.840 Mbps each.
- Operationally, there is not much difference but data network
designers tend to prefer the concatenated circuits.
- The SONET standard for transmission over STS circuits is geared
to synchronized voice transmission, but data networks leasing
these lines are often required to use SONET encoding on the
circuit.
- The slow analog voice connections running
from the local telephone central office to person's homes is a
bottleneck.
- Modems can't get much faster using the old
style analog circuit standards.
- There are alternatives that have become prevalent.
- Integrated Services Digital Network (ISDN) is a standard for
running digital voice, data, and compressed video over the same
twisted pair phone lines used for analog voice.
- ISDN offers two B channels at 64 Kbps each plus a control D
channel at 16 Kbps.
- ISDN had some advantages at one time but
never really got into mainstream use, and nowadays the cost is not
worth the minor speed advantage it offers.
- This is a rather new technology that provides very fast downstream bit rates (as much as 6.144
Mbps) and bit rates upstream which are quite adequate for the
needs of typical Internet users (up to 576 Kbps).
- ADSL is designed to run over ordinary
analog voice lines in a frequency range where it does not
interfere with the analog telephone service. Both
services can use the line simultaneously.
- A pair of special ADSL modems have to be installed on the line,
in parallel to the existing circuit.
- When you run ADSL, it adapts to the unique characteristics of the
circuit. It uses a combination of
frequency division multiplexing and inverse multiplexing
technologies (Discrete Tone Modulation: DMT). ADSL
finds frequencies with high
signal-to-noise ratio on the line. It pushes more bits
per second into those frequencies.
- There is no guaranteed data rate with ADSL. ADSL does as well as the line conditions allow.
Downstream rates vary tremendously, from 32 Kbps to 6.4
Mbps, and upstream varies from 32 to 576/640 Kbps.
- (It would be interesting to get information on what are the
averages and variances in data rate experienced with this type of
service.)
- Since the computers of DSL users can stay up and connected to the
Internet continuously, they can be more vulnerable to hackers.
It's advisable to use a firewall.
- SDSL is symmetric. It is like ADSL, but provides equal data
rates in both directions.
- On some lines, SDSL works well but ADSL does not.
- "Reverse ADSL" is rare -- a service that is faster upstream than
downstream.
- HDSL is High-Rate Digital Subscriber Line. It provides the speed
of T1/DS1 -- 1.544 Mbps in both directions. You need two twisted
pairs and you have to be very close to the CO.
- HDSL2 is like HDSL, but requires only one twisted pair.
- HDSL technology can be used on a circuit containing a bridge tap
-- some DSL technologies cannot.
- VDSL is very-high bit rate DSL: upto 52 Mbps. It requires
"intermediate concentration points" in neighborhoods, with fiber
connections the rest of the way to the CO.
- ADSL notwithstanding, telco twisted pair
wiring has inherent limitations: speed and protection from
interference.
- The TV coax connected to many
homes is a very good alternative. Such cables can carry many channels (not sure how many -- more
than a few hundred?). Each channel can support around 36 Mbps,
which is enough to give approximately 500
Kbps service to 75 active hosts.
- Cable modem users too may be vulnerable to hackers. They also
are advised to use a firewall.
- Cable companies that deliver TV are alredy well equipped to
deliver downstream digital data service, but upstream is a
different matter. To provide upstream
service, cable companies have to revamp a lot of equipment,
such as uni-directional broadband amplifiers.
- There are some hybrid schemes that use telephone for upstream and
cable for downstream.
- Cable companies that add upstream
capability can also provide interactive television and video on
demand (thought to be the "killer application" for home
networks -- "Please show me Once Upon a Time in America
now."
- HFC connects the cable company office via
optical fiber trunk lines to neighborhood. Ordinary coax runs
from neighborhood stations to subscribers' homes.
- HFC would improve service, but it requires the cable company to
purchase and install a lot of expensive equipment.
- Fiber trunk lines and twisted-pair to the subscriber.
- Getting good data connections to "rural"
areas is a special challenge. DSL is problematic or
impossible in areas that are far from a telco central office or on
lines that have coils, taps, or repeaters. This is likely in rural
areas. Cable TV is unlikely to exist in rural areas.
- In such environments, one can use something like cell-phone
technology for data networking.
- There is also a broadband 155 Mbps technology called WiMAX
(Worldwide Interoperability for Microwave Access - IEEE 802.16) with
a range of about 30 miles.
- A satellite system offers high bandwidth and no need for terrestrial
communication links.
- Disadvantages:
- Delay: about 1/4 second from ground to ground.
- Uplinks are large and expensive
- Low-cost satellite-based local loop systems employ:
- A broadcast mechanism - like ethernet
- An alternative uplink path - e.g. conventional dialup
- Someday low earth orbit satellite arrays may provide good
connectivity to people widely dispersed over the earth.
- One hopes that there will remain some way to "get away from it
all."