CPE Basics

The following diagram depicts a typical access circuit for both data and voice.  The circuit involves the customer, the LEC, and the IXC.  This is an unusual diagram in that it shows the areas of responsibility, which come into play for the installation and any subsequent troubleshooting that may be required.

The components are listed below:

Customer side - typically begins at the V.35 side of the providers CSU/DSU.  With the exception of Managed Network Services, the V.35 cable and router are usually owned by the customer.

Network side - this is confusing, because the customer has their own "network", which is usually a LAN.  However, the "Network side" refers to the provider's network, not the customer's.  The Network side of the circuit is comprised of both LEC and IXC components, and there is a drop-off point between the two entities.  The LEC carries the initial access circuit into their Central Office, and hands the circuit off to a nearby IXC POP.

MPOP (Minimum Point of Penetration) - this is basically the LEC drop-off, or LEC demarc to the customer.  In the great majority of cases, the demarc is extended, and a biscuit is used.  Therefore the RJ48x jack on the biscuit is the MPOP.  Sometimes, though rarely, the CSU/DSU plugs directly into the Smart Jack (no LEC demarc extension, no biscuit), and for those cases the RJ48s of the Smart Jack is the MPOP.

Smart Jack - often misinterpreted as a wall jack - it is actually a box or a card (shelf style), about the same size as a CSU/DSU.  Is is typically hidden away in a wiring closet, and is installed by the LEC.  The Smart Jack circuitry is capable of supplying loopback, so long as the correct loopback code is sent to it from the Network side.  Both the IXC and the LEC have the loopback code - so either is capable of testing the circuit, up to the Smart Jack.  The customer side interface on the Smart Jack is an RJ48s, which has no hard loopback capability.

OCU (Office Channel Unit) Data Port -  Smart Jacks are T1's, OCU's are for 56/64k.  The OCU card is used within a channel bank, or stand-alone, for 56k and 64k digital circuits.

MPOP-to-Biscuit cable  -  this is an RJ48 cable that plugs into the Smart Jack, and is hard-wired into the back of the biscuit.

Biscuit - it is rare that the CSU/DSU cable plugs directly into the smart jack. Instead, the LEC demarc is typically extended to a small wall-mounted biscuit.   This biscuit is typically hard-wired on the backside, and the bottom of the biscuit has an RJ-48x jack, which supplies a hard loopback when the cable is disconnected, via a shorting bar.  The plastic color on the RJ48x biscuit jack is black  Occasionally, the RJ48c biscuit is used (no hard loopback capabilities, with grey plastic jack).

CSU/DSU (Channel Service Unit/Digital Service Unit) - this is usually supplied by IXC or LEC CPE department.  For DTS (Dedicated Transmission Services) - often called private lines - Kentrox Datasmart CSU/DSU's are used.  For Frame Relay, Visual Network ASE's are used, which record statistics and allow a server to run queries on those statistics.  For voice PBX's, a Kentrox CSU is typically installed.  For dial backup capabilities, an Adtran TSU is required.

SDP (Service Delivery Point) - the physical end point where the IXC is supplying the customer with service.  This is typically the V.35 jack on the back of the CSU/DSU.  However, if the customer owns their own CSU/DSU, or if the IXC manages the router, the SDP moves accordingly.

V.35 vs RS-232   -  the functionality of V.35 and RS232 transmit and receive signals are identical.  It is only the electrical specifications that differ.  RS232 uses 9-pin and 25-pin connectors, while V.35 uses Winchester connectors at the CSU/DSU (DCE), and DB25, DB60, and several other connectors at the DTE jack (typically a router module, for data).  For cases where multiple services are run on a single T1, and add/drop CSU/DSU is used, with a V.35 cable going to the customer's data equipment, and RS-232 DB15 going to the customer's PBX.