SONET and SDH

Date: 2015-10-07; view: 406.

Several mentions have been made of SONET as being both the forerunner and the North American equivalent of SDH. The SONET standards were designed around the need for efficient transport of the existing North American PDH rates, the most important of which are the 1.5Mbit/s and 45Mbit/s rates. This leads toa standard where a 45Mbit/s signal is loaded into the equivalent of a VC3, which, in turn, is loaded into the SONET counterpart ofan STM, known as a Synchronous Transport Signal - I (STS-1), which runs at 51,84Mbit/s i.e. precisely one third of the STM-1 rate. As with SDH, there is also a concatenation mechanism for dealing with customer signals which do not easily lit inside any of the already defined VCpayload areas. This mechanism is particularly useful for carrying rates such as 140Mbit/s, where the concatenation results in a SONET NNI signal known as an STS-3c, which has a bit rate that is identical toan STM-1, and an SOH and payload area ofexactly the same size as well.

Other terminology within SONET is also different, in that SONET refers to Virtual Tributaries (VTs) rather than VTs, and the equival­ent of the VC3,referred to above, is known as a Synchronous Payload Envelope (SPE). It also makes a distinction between the STS logical signal and its optical manifestation at an NNI, in which case it is referred to as an Optical Carrier (OC). North American transmission systems are usually referred to by their transmissionrates in OCs e.g. OC3 or OC48.