Specifications

192 IBM Voice Over Frame Relay Perform Guide
Regarding the speech path, there is 2-wire and 4-wire E&M. 2-Wire E&M uses a
single pair of wires for the speech path. 4-wire E&M improves the speech quality
by using an additional pair of wires, one pair in each direction of speech.
Separating the two directions of speech onto individual wire pairs facilitates
amplification or regeneration, which is usually required for longer circuits. It also
eliminates the need for hybrid circuits, which are the principle source of echo.
The interface derives its name from the fact that it employs two signaling leads,
one called the "E" or Ear lead and the other called the "M" or Mouth lead. From
the switch (PBX or CO) point of view, the E lead represents the receive path or
the speaker of the telephone, while the M lead represents the transmit path or the
microphone. The E&M leads are connected to a carrier system (usually T1 or E1
today, but analog in the past) which is co-located in the same room as the switch.
(Think of the carrier system as a DSU.) Thus, the E&M wires are very short.
However, the carrier system provides the means to extend the E&M circuit to
great distances. It does this by providing long-distance transmission of the voice
and also converting the E&M signals to a form that can be transported along with
the speech. For example, T1 and E1 carriers transport E and M states as digital
bit streams.
C.6.2 Digital Interfaces
Many telecom equipment have digital interfaces today. These are high-speed
interfaces used for transporting voice traffic. T1and T3 are the standard interfaces
in North America. T1 interface operates at 1.544 Mbps and supports 24 voice
channels. T3 operates at 44.736 Mbps and is usually comprised of 28 T1s
muliplexed together, which is equivalent to 672 voice channels.
Europe supports the E1and E3 standards. E1 operates at 2.048 Mbps and
supports 30 voice channels. E3 operates at 34.368 Mbps and is usually
comprised of 16 E1s multiplexed together, which is equivalent to 512 voice
channels.
T1 and E1 use standard G.703 interfaces, which may be 120 Ohms 4-wire
balanced or 75 Ohms coaxial unbalanced interfaces.
Japan supports the J1 and J2 standards. J1 operates at the same speed as T1.
J2 operates at 6 Mbps and is equivalent to 4 J1s or 96 voice channels.