Channel Banks – A Technical Look

By Bill Ranney

There is a lot of talk in the industry today about digital telecommunications and channel banks. In previous articles, I have written about the application of T1 and how it could make money for you. In this article, I will describe technically how it all works.

A channel bank provides an important function in telecommunications. It is the cornerstone, or foundation, for all digital transmissions. A channel bank is a highly complex piece of telecommunications gear designed to change analog voice and data signals into the digital format. It is called a bank because it may contain enough processing power to convert a bank of up to 24 individual channels to a digital format and back again. These 24 channels make up a T1 circuit.

The process begins when an analog signal such as a voice conversation needs to be transmitted digitally. The analog voice transmission enters the channel bank where first it is filtered. These electronic filters limit the voice range between 200 Hz and 3400 Hz. Hz stands for Hertz, or cycles per second. To get a better feel for this frequency range, the 200 Hz is the very low or bass tones, and the 3,400 Hz signals are the very high pitched tones. Compare this to your high fidelity stereo system, which has a range of about 20 Hz to around 20,000 Hz or higher, or the human voice that can generate sounds that range from 100 Hz to over 8,000 Hz (opera singers). No wonder music on hold sounds so poor. Anyway, this limited range is a throwback to the old analog multiplexing days so the carrier (AT&T) could multiplex (aggregate) up to 10,800 analog voice conversations on a single pair of coax wires.

The analog voice signal is now passed to a device where the amplitude voltage is then measured, or sampled, 8000 times each second. This is known as Pulse Amplitude Modulation, or PAM sampling, and forms the basis for the digitization of the voice signal. In 1933 a scientist named Harry Nyquist theorized that 8000 samples per second was enough to accurately reproduce any wave form that has a 4000 Hz range. Specifically, Nyquist’s Law states that the sampling frequency must be greater than twice the bandwidth of the input signal to extract all the information in a continuous time varying wave form, such as a voice signal. As a comparison, digital compact discs, or CD’s, sample the music signal at 44,000 times per second.

Next, the processing unit compares the measured voltages to a specific range of voltages. It transforms the voltages in each range into a discreet 8 bit binary code word. This is called quantizing. For example, if the voice amplitude is between 1.5 to 1.6 volts, the code word might be 01101111. It can be reasoned from this that the digital representation is then an approximation of the analog signal, and some error or distortion is introduced at this stage. This error is known as quantization noise. Since the quantization uses 8 bits to represent the voltage range, there can be 256 distinct ranges that the signal can be broken into: 128 representing positive voltages and 128 representing negative voltages. This range is adequate for telephone conversations. For high quality audio music digitization, CDs use 16 bits, giving them over 65,000 discreet range levels.

The foregoing process is called Pulse Code Modulation or PCM. Today this process is all done in one electronic integrabit codeted circuit chip called a code c (which is short for coder-decoder). It can be seen that an 8 bit code word, generated 8000 times per second, results in a channel bandwidth of 64,000 bits per second. Why then, do you hear so much about 56k channels? The reason for this is, in their wisdom, the channel designers decided to rob bits from each channel for signaling purposes. In every 6th word the 8th bit was robbed for signaling. For voice, this does not affect the quality of the transmission whatsoever, but on a data transmission, bit robbing is disastrous. So, rather than give you 7 5/6th out of 8 bits, you get only 7 bits per channel. Thus you have 7 bits times 8000 cycles per second or 56,000 bps. Again, as are ference point, a CD has a channel speed greater than 700,000 bits per second.

All this information is then fed into a Line Interface Unit (LIU), which is usually an integral part of the channel bank. Depending on which manufacturer has made the equipment you are using, the LIU can be many different things. Some manufacturers have a specific LIU card while others have integrated this function with other functions on a single card. Generally, the LIU card places and removes the digitized signal into frames and onto the T1 circuit for transport to a remote site. A T1 frame consists of 24 eight bit time slots, with each voice circuit occupying a unique time slot.

To get channel banks to talk to each other across a T1 span, a signaling scheme must be used. As stated above, the signaling bits occupy the 8th bit in every 6th frame. These bits are called the A and B signaling bits. There are many different types of signaling schemes available for channel banks such as FXS, FXO; DPT, DPO; E & M; and, OCU/DP. These different signaling types manifest themselves in different types of channel cards. Most channel banks allow a mixture of all types of channel cards to be used in them.

Simple loop or ground start trunks require FXS (foreign exchange subscriber) signaling at the customer premises and FXO (foreign exchange office) signaling at the other end of the T1 circuit, either in the central office or at a remote site. FXS signaling also requires some sort of ringing generator be included in the channel bank to ring the loop start trunk sand telephones. A ringing generator usually provides 85 volts at 20 Hz to the circuit toward the telephones.

DPT (dial pulse terminating) and DPO (dial pulse originating) signaling are used when one needs to pass DTMF (dual tone multi-frequency) or dial pulse digits across a T1, say to a PBX or TAS switch for DID trunks. DTMF tones are the signaling tones you hear when you press the keypad of your telephone. The DPT card is used at the customer or end office site while the DPO card is used at the originating site. The customer switches signals to the DPT card by reversing the battery to it. This informs the DPT card that the digits can be accepted by the switch. The DPT card then sends the proper A and B bits to the DPO card and the digits are passed through.

E & M signaling is the simplest form of signaling available. It has two states, either off hook or on hook. This signaling is primarily used between PBX’s on tie trunks and central offices that are using in-band signaling. E & M also allows DTMF or dial pulse digits to be passed between equipment. If you order two -way DID trunks, what you are actually getting is E & M signaling with an expensive price tag. OCU/DP cards are used to transmit data across a T1 span.

All of these functions, incorporated with some alarm reporting, constitute what is known as a channel bank. Size can range from a large jewelry box piece of equipment to refrigerator sized. Prices of a channel bank depend on what functionality and signaling is required to support the end user. Complete systems can run from several thousand dollars on up. There are also refurbished channel banks available, many of which have come out of central office environments. Users can save quite a bit of money by using refurbished gear, which is generally very reliable. Channel banks can be intelligent, meaning that they can be accessed remotely via a modem dial-up connection for configuration and diagnostics, or non-intelligent. The intelligent ones cost more, of course. All types of data can be run through channel banks with the proper channel and interface cards.

As technology continues to change, the functionality of channel bank equipment will increase while the size will get smaller and the price will continue to drop. For connection to a T1 circuit, a channel bank can’t be beat for versatility.

Bill Ranney is president of Transnet Engineering, Inc., providing telecommunications equipment such as T1 channel banks, CSU/DSUs, voice and data multiplexers, routers and bridges, and ISDN terminal adapters to answering services and other businesses throughout the USA. Mr. Ranney can be reached in Boulder, CO at 303-413-0665.

[From Connection Magazine, July 1996]