Understanding the network.pdf - Back to Home

LEC-CO has interconnection trunk lines to LEC-Tandem offices and IXC-POPs.
Many people still refer to the LEC-CO as the Class 5 office.
• LEC-Tandem—The LEC-Tandem offices provide intraLATA (CO trunk line and
CLEC POP trunk line) connections. The interLATA trunk line connections are
used for routing inter-exchange call traffic between different ILEC-COs. The
CLEC POP trunk lines provide CLEC POP's access to the ILEC network. No
subscriber terminations are made in an LEC-Tandem office, they exist only
to provide intermediate distribution points between different COs.
• IXC-POP—These offices are the interexchange points between IXCs and ILEC
and CLEC. These facilities can be maintained by one or many different IXCs.
IXC-POPs are also used by network service providers (NSPs) for providing
customer attachment points to their data networks.
PSTN Basic Line Transport
The local loop transport circuit is the basis of all PSTN transport provisioning. The
minimum PSTN transport is a 4000Hz bandwidth data path, commonly called plain
old telephone service (POTS). POTS service is used for voice transmission between
the 300Hz and 3400Hz frequency ranges, which are the optimal ranges for
transmitting the human voice. It is also capable of data transmissions up to 64Kbps.
The actual transmission rate is limited to 52Kbps by the telephone company (TelCo)
for operational reasons. The transmission of digital data over POTS lines is provided
through the use of a codec (coder/decoder), which functions much like a modem. A
codec's job is to convert one signal format into another. A modem
(modulator/demodulator) takes the binary data path sent by the computer and
converts it into analog signals for transmission over the PSTN. The modem on the
receiving end converts the analog signals back into digital format.
All POTS circuits operate in full-duplex mode because they need to transmit and
receive on the same wire pair. Consequently, when your voice is being transmitted
over the line, you and the other party both hear your voice in the receiver. This echo
normally does not present a real problem for voice transmission, and because most
people are not speaking at the same time, they never even notice. With modem
transmissions, however, the echo from this full-duplex operation does present a
small problem. To overcome the echo, modems can spilt the circuit's bandwidth in
half, so that one modem transmits on the upper half of the passband and the other
transmits on the lower half. This approach can severely limit the amount of data that
can be transmitted at one time. So, for any modem that operates faster than
1200bps, a symbol-encoding scheme is used to represent data, and a technique
called Echo Cancellation (EC) is employed. Modems that employ EC save a copy of
their transmitted data and then use the copy to filter the data when it is sent back
over the line.