POTS: Plain Old Telephone Systems
Also some basic background about telecommunications
Peter Dordal, Loyola University CS Dept
How does a basic telephone system work?
What is telecommunications? How is it different from data communications?
The Bell Telephone Co was founded in 1877, making the phone system almost
140 years old.
Public internet: not 25 years old (started 1991)
How many songs, comedy bits or movie scenes involve telephones in crucial
ways? How many involve computers? Only count scenes with emotional content!
To some extent, the telecommunications system is all about real-time
traffic. Except some of it is not even digitized, and much of the rest is
not packetized. And voice traffic
is low-bandwidth. (The tReadingrend seems to be towards greater and greater
Characteristics of voice traffic:
- minimal delay
- even smaller variation in delay
- modest bandwidth: 64kbps voice channel (when digitized)
The magic 64kbps number comes from 8-bit
samples taken 8000 times per second. This is known as the DS0
rate for digitized lines. This is in universal use in North America.
Sampling at 8000 times/second means, as we shall see, that frequencies up
to 4000 Hz are preserved. When we look at ultra-fast SONET protocols,
we'll see that no matter how large the bandwidth, the basic frame rate is
8000 frames/second (frames simply get larger as the bandwidth goes up), so
one single byte in each consecutive frame always represents a singld DS0
Also: voice is loss-tolerant and error-tolerant! (loss-tolerance unites
cell-phone users everywhere!)
Why TCP is bad for voice: the problem is
that if a packet is lost, TCP stops and waits for the lost data to time
out and be retransmitted. This can introduce very large random delays.
Note that Skype does use TCP,
though. (Of course, Skype's sound quality is often bad.)
Turnover delay: when you stop
talking, and the other person replies immediately, what delay do you
perceive? This equals one full RTT (round-trip time). 200ms is annoying
here. But sometimes it rises to ~500ms, or more!
Jitter: this is variation
in the delay. Suppose most voice packets have a delay of 10 ms, but a few
are delayed 100ms. To recreate the original sound, voice packets have to be
played back at a uniform rate, with the time lag from creation to playback
constant for all packets. You can
Fill time: with packet-based
telephony, the largest source of delay is packet fill time. Voice data
accumulates at 8 bytes/ms. If a packet is 1000 bytes, it will take 125 ms to
fill, resulting in a minimum RTT of 250 ms. This is objectionably large.
Typically, voice packets contain much less data, as little as 48 bytes
(ATM). Skype packets are variable-sized (in part due to compression), but
most packets have less than 120 bytes of data.
- Wait 100 ms for all packets,
so the late packets are included
- Wait 10 ms, thus ignoring the 100-ms packets and introducing voice dropouts instead
Plain Old Telphone System
(plugboard model); circuit switching (see Stallings chapter 10.2, figure
Note the following features of classic-POTS:
- Connecting exchanges
- trunk lines, long-lines
- circuit switching
- fixed bitrate / analog capacity
- reserved channel (even if you
- need for amplification
- how it differs
- telecom use of packetization
FXS (Foreign Exchange Subscriber) and FXO (Foreign
Exchange Office) ports (4-wire modular jacks)
FXS ports are the ones you plug (analog) phones into; they connect to subscribers
FXO ports are the ones you connect to the line; the connect to the office
Mixing them up can lead to mayhem
(or at least overvoltage), and they have exactly the same shape.
A traditional phone has an FXO port; the wall jack is the FXS port. A
typical modem card has an FXO port you connect to the wall jack, and an FXS
port that you plug your phone into so you can still call when the modem is
not in use.
A modem itself is a special kind of sound card: it translates between "modem
sounds" and digital serial-line data.
The term POTS suggests a somewhat old-fashioned technology. The term PSTN,
for Public Switched Telephone Network, is sometimes used to describe
"modern" voice networks.
Mostly from www.telephonetribute.com:
1793: first commercial SEMAPHORE system, between line-of-sight towers (cf
Terry Pratchet's SF novel Going Postal)
1844: Samuel FB Morse demo of telegraph: "What hath God wrought?"
1860: end of commercial semaphore
1875: first speech by Alexander Graham Bell: "Mr. Watson, come here, I want
1876: first patent; offered to Western Union for $100K. Western Union
refuses: "Furthermore, why would any person want to use this ungainly and
impractical device when he can send a messenger to the telegraph office and
have a clear written message sent to any large city in the United States?"
Edison invents electric motor & phonograph
1877: AG Bell and Thomas Watson form Bell Telephone Co
1880: 30,000 telephones in US
1882: Bell buys Western Union
1884: phone service between NY and Boston
1890: Herman Hollerith 1st punchcard contract for census
1892: Almon Strowger, St Louis undertaker, develops dial phone
Phone service between NY
and Chicago. You went to a special ATT office to place your call.
1894: Bell patents expire
1899: Brown Telephone Co founded; later to become Sprint
1906: Lee deForest invents the vacuum tube, the first way to amplify audio
1907: Theodore Vail returns as CEO of ATT; founder of end-to-end policy
(control everything from phone to phone)
1913: 1st US antitrust suit against Bell system
1915: 1st use of vacuum-tube amplifiers on trunk lines; 1st transcontinental
1922: Alexander Graham Bell dies
1949: US sues ATT to divest Western Electric, ATT's manufacturing unit
1951: First direct-dialed long-distance call
1956: consent decree: ATT could keep Western Electric but could only make
telephone equipment, and had to license its patents
1956: Hush-a-phone decision: allowed customers to add
things that did not affect the network (like acoustic couplers)
1968: Carterphone decision: customers can add "customer
premise equipment" (own phone, fax, etc)
1973: Robert Metcalfe invents Ethernet, at Xerox
1978: final implementation of Carterphone decision
1981: Bell divestiture trial
begins; Judge Greene presiding
1982: Bell settles suit
1984: divestiture: ATT spins off the "seven sisters":
Ameritech - acquired by SBC,
GTE was also a full-sized phone company.
Bell Atlantic - acquired GTE, changed name to Verizon
Bell South - acquired by SBC/ATT in 2006
Nynex - acquired by Bell
Pacific Telesis - acquired by SBC in 1997
Southwestern Bell: changed name to SBC
in 1995, bought ATT in 2005 & changed name to ATT
US West - now owned by Qwest
1986: Sprint "pin drop" advertising campaign, announcing an era of
long-distance quality equaling local-call quality. Sprint had the first
2005: ATT bought by SBC, which changes its own name to ATT
- reservations of capacity
- "virtual" circuits
- physical transmission issues (signal degradation)
- TDM, FDM (note that there are some subtle encoding issues)
- Asynchronous Transfer Mode (ATM)
- Fiber optics, SONET
Digitization; encoding of voice
Sampling rate, 8-bit v 16-bit encoding
Selling data-transmission lines (ISPs, WANs)
Echo is a constant problem with phone systems. One source of the echo that you hear is your voice coming out the other party's handset, into their
microphone, and back over the line to you.
If Alice and Bob talk to each other using skype, and Alice hears a lot of
echo while Bob hears none, a likely possibility is that Bob is using his
speakers, and his microphone picks up a lot of speaker sound, while Alice is
using headphones and a mike that does not pick up any headphone sounds. That
is, the echo Alice hears when speaking is from her voice traveling to Bob's
speakers, into Bob's microphone, and back to her. It is "Bob's fault". This
kind of echo is called acoustic echo.
There is another, more unavoidable, form of echo: hybrid
echo. There are two wires run to an analog phone. At some point relatively
near the phone, the two wires go into a special transformer called a hybrid and the output is two pairs
of wires: one pair for each direction. The signals then travel separately in
each direction, until they are recombined by the hybrid at the other end.
Hybrids create a certain amount of echo, due to electrical reflection of the
analog voice signal.
As with the skype situation, the echo you hear is the result of "leakage" at
the hybrid at the other end. The downside of hybrid echo is its
inevitability (headsets don't help); the upside is that the echo delay and
amplitude is absolutely fixed, and so echo cancellation is
relatively straightforward (or at least a lot easier; there are now
real-time acoustic-echo cancelers). The echo can be removed by adding back
in a copy of the original signal with the appropriate delay, appropriate
amplitude, and reversed sign.
Acoustic echo cancellation
The most serious problem with acoustic echo is that the delay and amplitude
are highly variable; we have to adopt a continuous-update strategy.
One approach to addressing acoustic echo is to use earphones, or a very
directional microphone, so as to minimize the sound pickup.
Failing that, though, digital removal of acoustic echo is difficult and
CPU-intensive. The first option is to try adding back a delayed copy of the
original signal, trying different delay times and different amplitudes until
you discover a combination that reduces the overall signal power.
There are a lot of delays and
amplitudes to try, although some power-reduction can be seen when the delay
is within a millisecond (8 µ-law sample times). Once you have an initial
guess, you can implement an adaptive-update mechanism by trying delays and
amplitudes "close to" whatever worked most recently.
There is also a second option that takes into account the fact that we're
doing this in real time, and we see both the original signal and the
reflected (echoed) signal. We can introduce pulses in the outbound signal,
and see if we can detect them on return; then we can simply measure the
delay and amplitude. Alternatively, we can watch for specific peaks in the
outbound signal and measure the delay and amplitude of those upon return;
this avoids injecting additional noise.
Acoustic echo cancellation is essential for speakerphones. It is essentially
the same technology as is used in noise-cancelling headphones, too.
Software acoustic echo cancellation is cpu-intensive; this is best done in
Some Telecom Business Nomenclature
LATA: Local Access and Transport Areas. The US is divided
into LATAs. These were originally defined in the AT&T divestiture
judgment of 1984, separating AT&T from the seven "baby bells" (Regional
Bell operating companies, or RBOCs). LATA boundaries are now determined by
the FCC. Since then, new US LATAs have been created, and Canada has adopted
the LATA structure.
IntraLATA calls are calls that start and end within the
same LATA; traditionally, these were the cheapest (though they may still be
toll). Now, it is common for intraLATA (within one LATA) calls to be billed
at a higher rate than interLATA calls. This has to do with
Ideally, one tries not to have LATA boundaries bisect heavily populated
There is some confusion about calls in the following categories:
The latter category, especially, confuses the heck out of people.
- made within one LATA but to another state,
- made within one state but to another LATA
LEC: A LATA is a tariff region; the actual carrier (which
may not be related to the original RBOCs) is your Local Exchange
Carrier, or LEC. Verizon, for example, is still a LEC in some
markets. Frontier Communications is another (currently buying many Verizon
subscribers, such as me, though I then dumped them for Flowroute).
IEC: Non-RBOC LECs are sometimes called Independent
Exchange Carriers, or IECs. (This is not to be confused with ILECs,
or Incumbent LECs, the phone company with the "natural monopoly"
in a given region; competitive LECs or CLECs are the johnny-come-lately
Note that another notion of local calls may be defined by your LEC
to be calls of very short distance (commonly ~8 miles), and not subject to
per-minute billing at all.
Exchange: the office serving phone numbers with a common
area code and first three digits of the 7-digit number (eg the 508 exchange
in Rogers Park). Due to Local Number Portability, some numbers in an
exchange might not share the exchange prefix.
IXC: Inter-eXchange Carrier: a long-distance provider.
Carries calls not only between exchanges but between LATAs. Once upon a
time, Sprint and MCI were the best-known IXC alternatives to ATT in the US.
Your LEC is required to let you choose your IXC.
IXCs (and maybe LECs too) have Carrier Identification Codes. You can ask for
a certain IXC to carry your call by dialing 10-10 followed by the Carrier
Identification Code, eg 10-10-288 for ATT. (Strictly speaking, 101 is the
prefix and the 4-digit CIC is 0288; some IXCs have a CIC with non-zero first
In the US, phone numbers are 10 digits (soon we will need 11 digits, but
there are no concrete plans yet for coping with the change). Many LECs
enforce "1+" dialing, where you have to dial a 1 before any long-distance
calls (where long-distance here means any measured call). The 1 here is the
international-calling prefix for North America, but its primary role is to
remind subscribers that the call is a toll call, billed by the minute.
Sometimes the leading 1 is an indication that the number following will have
10 digits. However, some LECs don't even require that. Distinguishing 7
digits from 10 digits is done with timing, anyway; if you don't dial the
last three digits soon enough, your 10-digit number might be treated by your
LEC as a 7-digit number.