Introduction to G.703
Introduction to G.703
G.703, an ITU-T standard that outlines how to interface digital high-speed circuits, has become the basis of all telecommunications networks. But it didn’t come about overnight. Alexander Graham Bell invented the telephone in 1876, but it would take another 50 plus years before Bell engineer Henry Nyquist calculated digital voice transmission.
Nyquist, who had spent the 1920s working on telegraphy speeds, determined in 1933 that to digitise speech, the analogue speech pattern had to be sampled 8000 times a second. Theory, however, didn’t immediately translate into practical application. That’s because telephony has evolved over a much longer time period than data communications.
Originally, voice channels were multiplexed together by frequency-division multiplexors, which allocated a bandwidth of 3.4 kHz for each voice channel, and a guard tone between each of the mux’s channels minimised crosstalk or interference. This was the first analogue form of multiplexing, but the quality of the voice signal wasn’t superb.
Then came digitisation. With it, 8 bits of telephone voice are sampled at 8000 times a second. Its formula looks like this: 8 x 8000 = 64000 or 64 kbps. This digitising method is called Pulse Code Modulation (PCM) and is defined in detail by the G.711 standard. Companding, a process in which a signal’s amplitude range is compressed before transmission then expanded when received, minimises the number of bits that the PCM must sample in the analogue voice signal before it’s encoded, or “quantised,” in digital form. Companding is available in two encoding varieties: A-law, which is used in Europe, and Mew-law (or µ-Law), common in North America and Japan.
G.703’s physical interface.
In the U.K., BT®, the initial telephony provider, built up its G.703 infrastructure using 75-ohm BNC connectors. Most PTTs in the U.K. have followed the same standard and type of interface closely. That’s because when the marketplace was opened up to competitive forces, these providers were allowed to use BT’s existing infrastructure. European PTTs, however, have adopted the 120-ohm interface with RJ-45 connectors.
There are also two types of logical presentation: unstructured and structured. Here’s how they differ:
Line encoding with G.703.
All these line encoding techniques are three-level encoding schemes. In contrast to most data communication protocols in which only two-level schemes typically represent a mark “1” and a space “0,” the three-level system allows an extra change of state, (i.e., a clock to be included). This scheme is used to balance out voltages across wires and, more importantly, encode a clock along with the data structure. The signal is a 1-volt peak-to-peak signal.
Framed vs. unframed services.
The type of service provided by a PTT is usually an unframed one. And although it’s possible to order an end-to-end framed service in the rest of Europe, you can’t order one in the U.K.
If you require several devices to be connected at each end, you’ll have to use multiplexors that support G.704 framing. This is the only way multiplexors can divide the bandwidth into blocks that can be allocated to individual end-user equipment. If the equipment doesn’t support G.704 framing, it can’t divide the bandwidth in a way that makes it suitable for support of multiple devices. But if you only need to support a single end-user device, then support for G.704 framing isn't required, and setup is much easier. In fact, the setup only involves choosing a clock source, whether you want to generate the clock internally (in Master Mode) or receive it across across the network externally (Slave Mode).
PBX signalling techniques
CCS uses Timeslot 16 to carry a protocol (a defined set of messages or instructions common to connecting devices) between the PBXs. Within that protocol, messages are exchanged relating to information—such a handset lifted, number dialled, ringing tone, engaged tone—for each of the 30 voice channels.
Here are the CCS protocols:
G.704 Timeslot Allocation
It has three synchronisation bits, a Cyclic Redundancy Check (CRC-4) and then national or international bits (depending on the use of the circuit). Some manufacturers use these bits for sending remote management information across the link. Following Timeslot 0 are another 31 timeslots, each of which can be used for user data (voice, data or video). The only other special timeslot is Timeslot 16, which when used between PBXs, carries signalling information such as handset lifted, number dialled, ringing tone and other functions. Although this information can be carried in any timeslot (other than Timeslot 0), it has traditionally used Timeslot 16. Common Channel Signalling (CCS) and Channel Associated Signalling (CAS) and are the two main types of PBX/exchange signalling.
CAS uses bits within Timeslot 16 of the 32 timeslots (0–31) to represent the status of each of the 30 voice channels (see the diagram above). There are 8 bits present in Timeslot 16 (as is the case with the other 31 timeslots), and each complete E1/T1 frame carries information relating to 2 timeslots. That is, within the 8 bits, the first 4 bits represent the first timeslot, and the last 4 bits represent the second. The first frame represents timeslots (voice calls) 1 and 17, the next frame would represent timeslots 2 and 18, and so on up to 15 and 31. This means that 16 frames, together called a “super frame” (or “multiframe”), are needed to provide the information on all channels.