SONET Information
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This is a VERY simplified diagram of how a SONET ring works:

|            |
|            |
|    OC-3    |
|            |
| __________ |               7 Cards
|/ || || || \|              _________~        4 Ports
 \_||_||_||_/              /_________~       _________~
   || || ||   STS (DS-3)  /__________~      /_________~   T-1 Circuit
   || || |L--------------<_________________/__________________________
   || || L---------------<___________~     \__________~

This is how we jump onto a SONET ring. From the top, the SONET ring is an OS-3, which breaks down into 3 "STS" circuits, which are equal to a DS-3. Each STS then interfaces to seven cards, which have four ports each. Each port on each card is equal to one T-1 circuit, which gives the total of 28 T-1 circuits per STS (DS-3). You may be asked which channel on the SONET ring you are to use, or which Timeslot to use. These terms are synonomous, and refer to the STS/card/port identification. Therefore, channel assignments would have the following naming convention:

  • Channel 111 = STS 1, Card 1, Port 1 (first channel on the OC-3)
  • Channel 374 = STS 3, Card 7, Port 4 (last channel on the OC-3)

Each OC-3 = (3 x DS-3) = (84 x T-1).


The following is from a conversation that I had with a BellSouth tech regarding an installation that I was working on.

What breaks the OC-3 down into STS's?

The OC-3 is a SONET level optical interface which in the case of Okuma is the speed/bandwidth of the ring. The OC-3 signal is transmitted around the ring on fiber facilities and terminates into a piece of equipment called a "node" at either a customer premises or our central offices. This node equipment essentially is a fiber multiplexer that takes in the OC-3 SONET signal and has the capability to deliver from the box interfaces for a DS3 signal or an STS signal that would be used to connect to another mux that has the capability of provisioning the DS1 circuits. Physically speaking, what you would see is a shelf (up to 3 on an OC-3) that accommodates actually 8 cards with 4 ports on each card. 7 of the cards are active and 1 is provided for redundancy or failover should one of the 7 fail. So, on an OC-3, you could provision up to 84 DS1s (3 shelves with 28 DS1s per shelf) or 3 DS3s or any combination up to a total of 3 DS3s worth of bandwidth. Basically speaking, there are 3 STSs on an OC-3 and those STSs can be used to provide DS3s (each one takes an entire STS) or DS1s (28 DS1s per STS).

What breaks the STS down into 7 cards?

As stated above, the STS feeds the mux that actually has the 8 cards (7 live and 1 backup) with each card providing 4 ports for DS1s.

At the end of the line (T-1 Port), is it just copper?

The DS1s are provided out of the mux and off of the 4 port card via some type of DSX panel. Normally, we provide the circuit to the customer via a standard interface, most likely an RJ48C which is referred to as a Smartjack. Some customers may to crossconnect with us at the DSX, but normally the smartjack is provided.

What kind of interface would the CSU/DSU attach to?

As stated above, the RJ48C smartjack provides for a modular 8 pin jack that would be used for connection to the CSU/DSU.



My name is Michael Oliver, and I can be contacted by email here.
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