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 packetization.)

Characteristics of voice traffic:

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 line.

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 either:
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.


Plain Old Telphone System
(plugboard model); circuit switching (see Stallings chapter 10.2, figure 10.2)
plugboard diagram
Note the following features of classic-POTS: 
Some topics

packet switching 

POTS today:
    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 you"
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 signals
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 call
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, 1999
Bell Atlantic - acquired GTE, changed name to Verizon
Bell South    - acquired by SBC/ATT in 2006
Nynex           - acquired by Bell Atlantic, 1996
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
GTE was also a full-sized phone company.
1986: Sprint "pin drop" advertising campaign, announcing an era of long-distance quality equaling local-call quality. Sprint had the first all-fiber network.
2005: ATT bought by SBC, which changes its own name to ATT
Telephony issues:
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 ยต-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 hardware.

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.

Map: http://upload.wikimedia.org/wikipedia/commons/1/1f/LATAs.png


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 competition.

Ideally, one tries not to have LATA boundaries bisect heavily populated areas.

There is some confusion about calls in the following categories:
The latter category, especially, confuses the heck out of people.

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 alternative providers.)

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 digit.)

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.