Synchronous Optical Network - SONET

Here's another entry for what I've been calling the legacy Public Switched Telephone Network (PSTN) series. In my last legacy post we covered the European or “E” carrier system. Today, let's look at SONET.

In the United States T-1 carriers have been replaced in many locations with Synchronous Optical Network (SONET) systems. Internationally, the SONET equivalent is called Synchronous Digital Hierarchy (SDH). Both SONET and SDH systems consist of rings of fiber capable of carrying very high bit rates over long distances. Copper has been replaced by fiber to inter-connect most Central Offices (CO’s) in the United States at bit rates ranging from the SONET base rate of 51.84 Mbps up to 39,813,120 Gbps. 

The base SONET standard bit rate is 51.84 Mbps and is referred to as Optical Carrier  (OC) -1 or Synchronous Transport Level  (STS) -1. SONET uses a synchronous structure for framing that allows multiplexed pieces down to individual DS-0 channels to be pulled off a SONET signal without having to demultiplex the entire SONET signal. We can look at a table of SONET bit rates.

[The OC-3072 (160 Gbps) rate level is next in the sequence but has not yet been standardized.]

The OC-1 base is used for all higher level SONET specifications. For example, a SONET specification of OC-48 can be calculated by taking the OC-1 base rate of 672 DS-0 channels and multiplying it by the OC-48 suffix of 48.

We can do the same calculation for the OC-192 specification.

It is common to run SONET rings CO to CO with all SONET connected CO’s having SONET multiplexers that can demultiplex all the way down to an individual DS-0 channel level without having to demultiplex the entire SONET frame. 

In my next legacy post I'll take a look at how SONET is used for packet-oriented data transmission (e.g. Ethernet).