Internet Telephony Lecture Notes

Outline

The U.S. phone network before 1984 was a five-level hierarchy:
End (central) office (smallest, lowest, most common)
toll center
primary center
secondary center
regional center (largest, highest, least common)
- End offices were grouped into exchange areas, within which calls were free (toll free).
Calls were routed as low in the hierarchy as possible.
Extra-hierarchial links provide more efficient routing.
- Trunks between centers at the same level skip routing through next higher-level centers.
  - Trunks may also skip hierarchy levels.
- Tandem offices share trunks among centers.

On 1 January 1984 the U.S. phone network was split into long-distance and local parts.
- Technological, regulatory, and economic factors implied long-distance access (exchange areas) was no longer a natural monopoly.
- Exchange areas still represented natural monopolies.
Exchange areas were organized into local access and transport areas (LATAs).
- LATAs were grouped into seven regions, each region under the control of a regional Bell operating company (RBOC).
  Then: Ameritech, Bell Atlantic, BellSouth, Nynex, Pacific Telesis, Southwestern Bell, and U.S. West.
  Now: AT&T, Verizon, and Qwest.
Inter-exchange carriers (IXCs) provided long-distance service (then including AT&T, MCI, and Sprint).
Each LATA had point of presence centers (POPs) where IXC and LATA networks interconnected.

Until the early 1960s, the phone network was end-to-end analog.
- The first digital trunk (T1) appeared in Chicago in 1962.
- The first digital microwave link appeared in NYC (between Brooklyn and Staten Island) in 1972.
- The first short haul fiber-optic link appeared between Atlanta and Smyrna in 1980.
  - Long haul fiber appeared between Washington D.C. and NYC in 1983.
Ditial switching showed up in Chicago in 1976 (4ESS in a primary center).
- Digital switching in the end office appeared in 1977 in Georgia.
By the early 1990s all paths between end offices were digital.
- “The last mile” between end offices and customers remained analog.
Integrated Services Digital Networks (ISDN) provided a digital path between customers and end offices.
- Introduced in the late 1980s, it didn't do too well in America.

An analog line terminates at an end office and is converted to (or from) a 64 kbt/sec (= 8k samples/sec * 8 bts/sample) signal.
24 64 kbt/sec signals are combined into a 1.544 Mbts/sec signal.
- A frame consists of 24 8-bit samples (= 192 bits) and an extra framing bit.
- 8k frames/sec * 193 bits/frame = 1.544 Mbt/sec.
- This Digital Signal 1 (DS1) signal is carried by a T1 carrier line.
Further multiplexing creates higher rate digital signals carried by corresponding higher rate T transmission lines (more or less).
- A DS2 is four DS1 signals multiplexed.
The ITU defined a similar, but more regular, set of rates (the E hierarchy).
- A 2.048 Mbit/sec E1 signal contains 30 voice circuits and so on.

Each 8-kHz clock tick (= 1/8kHz = 125 μsec) produces 24 eight-bit samples grouped into a frame.
- The frame is divided into 24 slots, with one sample per slot.
- The ith 64-kbt/sec channel gets the ith frame slot.
This data-multiplexing scheme is known as time-division multiplexing (TDM).
- Each input (output) channel transmits (receives) an 8-bit sample during its slot and at no other time.
- Every input slot requires an 8-bit sample from the input channel.

Network signaling conveys control and status information about the network.
- Supervisory signaling: off-hook, dial tone, hook flash.
- Information signaling: calling and called numbering, toll charges.
Network signaling can pass between the network and end-points or stay within the network (inter-switch signaling, for example).
In-channel signaling merges the bearer and control planes; both signals are carried by the same network.
- In-band signaling carries both signals over the same circuit.
  - Touch-tone (dual tone multifrequency (DTMF)).
- Out-of-band signaling uses separate circuits in the same network for control and data signals.
  - T-line transmission framing; digital subscriber line (DSL).
Common-channel signaling uses a separate network to route signals.
- Simpler and more secure than in-channel signaling.
- Each network (control and bearer) optimized to its traffic.
- Control-signal routing is independent of bearer-signal routing.

Background and Terminology (Chapter 1) in Digital Telephony (third ed.) by John Bellamy, Wiley, 2000.

This page last modified on 17 June 2008.