CSU/DSU's

For additional details, click "Here" (special thanks to General Datacom)

CSU’s and DSU’s (Channel Service Units and Data Service Units) are digital hardware devices that interface a customer’s telephone system to the LEC’s. They perform the same basic function as modems - except they are completely digital, and run at higher speeds. The only CSU/DSU that would use any type of analog signal would be one that supports asynch dial-up for backup in case of failure.

Previously, the phone company always owned the CSU and the customer always owned the DSU, so they were always separate units. Now, CSU’s and DSU’s are often combined into one unit ( CSU/DSU ). Their basic function is to adapt one kind of digital signal into another kind - i.e. the customer’s digital signaling format into the CO’s digital signaling format. The CSU always connects directly to the telco line, and the DSU sits between the CSU and the customer’s equipment.

NOTE: the combined CSU/DSU (also called DSU/CSU) box has become so common, that many products marketed as "DSU’s", are actually CSU/DSU’s.

The CSU, looks like a basic modem, and has several functions. It provides correct termination, performs line conditioning, coding, and equalization, and supports loopback tests. Before deregulation in the early 1980’s, CSU’s were always leased to the customers and owned by the telco’s, and the power to run them was supplied by the telephone line well. Now users generally buy and install their own CSU’s and need to supply their own power, since not all telephone lines supply power anymore.

Every switched digital circuit must have a CSU. A CSU cannot deal with data rates below 64 kbps, and so a 56 kbps connection requires a 56k DSU on both ends.

1) termination

2) line conditioning

3) loopback - allows the telco to test the customer’s.

The main function of the DSU is to convert the CPE digital data to the phone company’s format and vice-versa (such as: 24 channels of raw data <---> T1 ). The DSU sits between the CSU and the CPE (router, multiplexer, terminal server, etc.). Basically there are 2 flavors of DSU’s :

1) In-Band signaling - regular old AMI encoded signaling, 56k per channel, which is mostly used for voice, or switched 56. AMI signals usually use D4 framing (also called SF, or Super Frame). AMI encoding was originally developed for switched voice circuits - it uses "bit robbing" (robs one bit from each byte for switch signaling), which requires 8k of overhead for every 64k. So for each 64k, there is only 56k of data. - perfect for one voice circuit, or switched 56 data. AMI is always in channels of 56k. Higher data rates, where 64k or multiples of 64k use B8ZS (clearline).

2) Out-of-Band signaling - clearline - B8ZS (Binary 8-zero substitution) - a standard, T1 DSU using B8ZS takes a serial input stream of 24 DS0’s at 1.536 Mbps (24 x 64k ). The DSU inserts a "framing bit" every 193rd bit. In order to continue to pass all , for a total of 8k of framing bits for every