Telephone System Physical Layer Notes

Introduction

The modern telephone system is a miracle of modern technology; it is ubiquitous, reliable and provides very good quality voice and data transmission. Of course, cellular telephone is providing much of the same services, but the potential data rates for cellular are much lower and it won't be discussed here.

A telephone system has both local and global parts and has the organization shown below:

The center of the system is the Central Exchange (CEX), which is the primary switching location for normal analog telephone lines. However, the telphone system has long used digital technology for long distance and high capacity communications, as evidenced by the Digital Cross Connect (DCC). The DCC's serve local ISDN and leased digital lines,and serve as a point in a global digital network that routes telephone calls world wide over SONET lines. This is obviously a complex system and we won't attempt to discuss it in detail, but the essential points will be discussed below.

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Basic Analog and Digital Methods

Analog

Historically, the telphone system used analog lines. Telephones converted sound to an electrical signal and transmitted it directly over the lines. The incoming line was connected to the CEX where the signal was switched to an outgoing line. It was all quite simple. The only characteristic of interest was that the signal was filtered to a maximum of 4000 Hz. This was primarily to avoid transmission problems with higher frequencies that are not necessary for adequate normal voice.

Digital

For all of the reasons discussed earlier, the telphone companies decided that digital services were superior to analog in the 1960's. The cost of converting the entire system was too expensive, but they did begin convering the long distance services to digital. The first problem is how to convert an analog signal to a digital form. The process is called, obviously, analog-to-digital or A/D conversion. It is accomplished by sampling the analog signal at some frequency, and at each point, representing the amplitude of the signal as a number. The method used by the telephone system is named Pulse Code Modulation (PCM) and looks like this:

In order to insure that 4000 Hz can be adequately represented, 8000 samples per second are required, or one sample every 125 microseconds (usec). Initially, each sample was a 7 bit sample, but later, 8 bits was used, particularly for data.

8000 samples/sec * 8 bits = 64,000 bits per second

So, a single PCM channel provides 64,000 bits of capacity, which is enough to carry one voice channel. Remember, to represent 4000 Hz, you need to sample 8000 times per second (2 samples per cycle).

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The "T" Standards The carriers that are used for distance communication have higher capacities, so they bundle multiple channels together into Digital Signaling (DS) carriers.

• DS0 - 64,000 bps, for carrying data
• DS0A - 56,000 bps, for carrying voice
• T1 carrier (DS1)
  24 PCM channels + 1 control bit = 193 bits * 8000 = 1,544,000 bits/sec = 1.544 Mbps
• T-3 carrier (DS3)   -   24 T-1 channels = 37.056 Mbps
  but framing information is added so that the total bit rate is 44.736 Mbps. The multiplexing of the channels is done bit-by-bit, so that the 193 bits of each T-1 frame is spread evenly throughout the T-3 frame.
• There are also T2 (4 T1's) and T4 (7 T3's) carriers, but they are seldom used.
• Signaling System 7 (SS7)   -   A protocol architecture stack to support packet network delivery of telecommunication data. It is designed to handle call setup/teardown, wireless services, call forwarding, caller id, conference calling, etc.

These DS carriers have been the mainstay of high capacity data communication since the 1970's. Many leased lines are T1 or T3 carriers.

It is also possible to fractionate these carriers and buy only part of a T1 or T3. If you buy one channel, you get 64 kbps and additional capacity comes in 64 kbps chunks.

T carriers use time-division multiplexing with fixed time slots. A T1 releases one frame every 125 usec, and each frame contains 24 channels. So each channel gets 1/24 of the capacity of the carrier and then 125 usec later, it gets 1/24 of the carrier again. Each channel is taking turns by sending one 8-bit word each 125 usec. If a channel is unused during one of the time slots, that space in the frame is wasted.

ISDN

Integrated Services Digital Network is a technology whose time didn't ever quite arrive. In theory, homes would have been connected to the CEX with ISDN lines, which consisted of two 64 kbps channels and one 16 kbps signalling channel. ISDN would have provided digital telephone to homes as well as other digital services, but the number of installations has always been very small .

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SONET

A more modern carrier is Synchronous Optical NETwork, which was designed to operate over fiber media at very high data rates - well over 1 Gbps. SONET uses synchronous framing, transmitting frames (are you ready?) 8000 times per second.

SONET is part of a set of standards called the Synchronous Digital Hierarchy (SDH) which is designed to provide high-speed backbone networks. The basic building block is called synchronous transport signal level 1 (STS-1) and it has the following frame format.

As you can see, each STS-1 frame contains 90 bytes * 9 rows = 810 bytes, with 27 overhead bytes and 783 payload bytes. At 8000 fps, the total capacity is

810 bytes/frame * 8 bits/byte * 8000 frames/sec = 51.84 Mbps

Each byte in the frame represents 64 kbps of capacity at 8000 times per second. SONET uses NRZ encoding of the overhead bytes and to insure timing information is present, the data is xor'ed with a known 127 bit string. This is called scrambling. SONET uses synchronous framing which means that a frame is delivered every 125 usec and each frame is the same length. This eliminates the need to worry about the length or sentinels and speeds up communications.

As with the DS standards, there is a hierarchy of carriers.

SONET Signal	Optical Signal	Bit Rate
STS-1	OC-1	51.84 Mbps
STS-3	OC-3	155.52 Mbps
STS-9	OC-9	466.56 Mbps
STS-12	OC-12	622.08 Mbps
STS-18	OC-18	933.12 Mbps
STS-24	OC-24	1244.16 Mbps
STS-36	OC-36	1866.24 Mbps
STS-48	OC-48	2488.32 Mbps
STS-192	OC-192	8853.28 Mbps

SONET uses the add-drop multiplexors (ADM) to rapidly add or remove tributary streams. For example, at an ADM, it might add some STS frames to be sent to other locations, while removing traffic destined for the local network. The ADM's are designed to do this without demultiplexing all of the traffic, which is very time consuming. This makes SONET a very efficient carrier for high speed traffic that also allows routing. The multiplexing is done column first within a frame, and the interleaving is byte-by-byte between STS-1 channels.

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Modem Technology

Everyone is familiar with modem technology, and it has been a part of computing since the earliest years. Early modems were 110 baud or 10 character per second contraptions that were acoustically connected to telephone lines. Modems have gone through a variety of evolutionary stages. Remember that at all times, the 3000 Hz bandwidth limit acts to limit that speed at which one can transmit.

One standard was known as Bell 103, which is a standard for 1200 baud or 120 character per second models.

• FSK encoding for 300 bps (and 300 baud)
• Originator uses 1070 HZ for 0, 1270 HZ for 1
• Answerer uses 2025 HZ for 0, 2225 HZ for 1
• Bandwidth = 2225 Hz

A more recent standard is called Quadrature Amplitude Modulation. It uses phase-shift keying at 30 degree increments for a total of twelve symbols.

Then, it uses amplitude shift keying as shown on 4 of those symbols to get 16 symbols, so that 4 bits can be sent in each symbol. Bell 209 (V.29) modems use a transmission rate of 2400 baud, for a total of 9600 bps.

This method is known as 16-QAM beacuse it provides 16 signal levels. There are standards for QAM up to 4096-QAM, which would provide 11 bits per baud, but they use a slightly different approach.

Trellis Coding is QAM, but it uses two signals added together and it leaves some signals unused. A constellation is shown below:

This constellation has 36 different signal levels defined. Rotation around the origin represents different phase angles, and at each angle, different points represent different amplitudes. By creating two waves, one a sine and one a cosine, it is possible to double the number of states, as each wave has both a phase angle and an amplitude.

The problem is that as the number of points in the constellation increases, it is more likely that a small perturbation of the signal will result in an error at the receiver. So Trellis Coding leaves some points unused. Normally, it tries to separate legal signals with some signals that are not legal, so a small signal degradation will result in an illegal signal being received, rather than a legal one. In this manner, it is possible to add error checking to the basic protocol.

Some of the modem standards that use Trellis Coding are:

Standard	QAM Method	Bit Rate
V.32	32-QAM Trellis	9600 bps
V.32bis	64-QAM Trellis	14,400 bps
V.33	128-QAM Trellis	14,400 bps
V.34	4096-QAM	28,800 bps

56kbps Modems

56 kbps modems don't really exist, per se. The maximum bit rate on a telephone line is somewhere in the neighborhood of 33,600 bps. However, specifications such as V.90 take advantage of the fact that on the downstream side, it is possible to go faster since it isn't necessary to perform the PCM conversion on digital lines. This allows an upstream rate of 33.6 kbps and a downstream rate of 56 kbps.