L

1. LEASED LINE BUILD-UP OVER
PUBLIC SWITCHTED TELEPHONE NETWORK (PSTN)
CITY A CITY B
TM TM
HM INTER-CITY CARIER HM
TAX TAX
PHOM NTM NTM
PHOM HOM
LEX LEX FOM LEX LEX
MDF at LEX
Building
Primary Cables
Street Pillar Boxes
Secondary Cables
Building DBs
E E E E
P P P P
B B B B
A A A A
Point-to-point Point-to-point
X X X X
Leased lines Leased lines
Channel Splitter Channel Splitter
Data Router V Data Router V
D D
P P
S S

2. The above schematic diagram shows
• how a PSTN network is laid out and
• how a point-to-point leased line is built on it.
• It also explains how the channel splitter at either end of the leased line splits the
bandwidth into some portion which runs a circuit switched network through a VDPS
which is basically an EPAX (not an EPABX used for PSTN) with 4W,E&M trunk cards.
This acts as the router for the circuit switched network in PVDTN.
• The remaining portion of the bandwidth is terminated on the data router sitting on the
company LAN. The data routers at each location are connected to each other through
the allocated bandwidth on this point-to-point leased line to create the private IP
network for all data communications.
This is how the PSTN works.
1. The telephones , fax machines, computers attached to dial-up modems are connected
to the DB in the building as also any EPABXs that may be in the building.
2. The DBs in the buildings in an area are connected to the area street pillar boxes
through secondary cables of 10, 20, 50,or 100 pair cables. The SPBs are connected
to the MDF in the local exchange (LEX) building.
3. All the telephones, fax machines, and computers connected to the PSTN are led to
the LEX from the MDF. The LEX routes the call to numbers within the same or other
exchanges in the city or across the country. The calls from the LEX are transported to
other exchanges within the same city through Higher order muxes (HOMs) connected
to them. They connect to other cities through the trunk automatic exchange (TAX).
4. The only way to enter the PSTN network is through the LEX.
The leased line build up over the PSTN network is as follows
1. The leased line paths are shown in red. From the channel splitter (in case of digital
leased lines) or the VDPS (in case of analogue leased lines) the cable pairs are
terminated on the building DB.
2. Dedicated cores in the secondary cables, and primary cables take the leased line to
the MDF in the LEX building.
3. From this instead of entering the LEX, they get connected to the Fisrt Order Mux
(FOM). Other leased lines also share this FOM with dedicated channels for each
leased line.
4. These are then led to a higher order mux from which they may be connected to other
HOMs in the city or to the Non-Tax HOM (NTM) in the TAX building.
5. These leased lines from the city are muxed along with the PSTN circuits into the inter-
city carrier. They are demuxed at the other end and sent on their respective paths.
6. There is no possibility of any PSTN trunk being mixed with any of the point-to-point
leased lines.
7. The point-to-point leased line has no public domain network access. A PSTN
telephone cannot connect to a PVDTN telephone and vice-versa.
8. Point-to-point leased lines may be thus built up between any two locations in
the country or the world over the universal PSTN network. International circuits
are called IPLC