VOICE-OVER PACKETS IN A PACKET NETWORK
Direction of transmission
A typical data frame.
is set up by a signaling routine. The distant subscriber has a telephone address represented
by a distinct telephone number consisting of 7 to 12 digits. The digit sequence of the
number sets up a circuit route and connectivity for conversation. The circuit is maintained
in place for the duration of the conversation, and it is terminated and taken down when
one or the other party hangs up ("goes on hook"). The address sequence of dialed digits
is sent just once, at the initiation of the connectivity. This whole process of setting up
a circuit, holding the connectivity in place, and then taking down the circuit is called
Signaling on data circuits is approached quite differently. There is the permanent virtual
circuit (PVC), which has all the trappings similar to a voice circuit. The similarities stop
here. Data transmission consists of frames or packets of data. A frame (or data packet)
is made up of a header and payload. In some cases a portion of the "header" may be
appended at the end or on the tail of the data frame (or packet). But every data frame (or
packet) has a header consisting of a destination address (or addresses) and the originator's
address. It nearly always will also contain some control information. This may be a word
(or byte/octet) count of the payload, a CRC sequence for error detection and/or correction,
message priority, or some other type of control sequence or sequences.
Digital circuits on the PSTN have either 24-octet frames in the case of T1 (DS1) or
32-octet frames in the case of E1. Each 8-bit octet represents a voice circuit. Such a
circuit may be set up using an Initial Address Message of CCITT Signaling System No.
7 or a sequence of DTMF tones where each frequency pair represents a digit in the range
of 0 through 9. Once a circuit is set up, no more address messages or DTMF tones are
required until the circuit is taken down.
This is not the case on a data circuit. Such a circuit also uses frames, but each and
every frame has a standard header. A typical data frame is illustrated in Figure 15.2. The
frame structure and how various octets of the header (and tail) are utilized are governed
by a protocol. Various data protocols were discussed in Chapters 1113.
We can clearly see that there are two differing philosophies here, one for data commu-
nication and the other for digital voice. Digital voice is sometimes called "circuit-switched
voice." A majority in the telecommunication community saw how advantageous it would
be if we could marry the two and make them one. That is one singular approach for
both voice and data. Meanwhile, data hobbyists were trying to transmit voice using data
packets. The Internet protocol (IP) became the data protocol of choice, but there were
DRAWBACKS AND CHALLENGES FOR TRANSMITTING VOICE
ON DATA PACKETS
We have come to measure "quality" of packetized voice service by the equivalent service
offered by the switched digital network (i.e., the PSTN), sometimes called G.711 (1)