Monday, December 25, 2006

What Is SONET And What Does It Do?

SONET is Synchronous Optical NETworking. SONET equipment generally uses one wavelength, or lambda, to carry an OC level (see below), which can be divided into time slots for individual circuits. SONET is generally used at the carrier level to build diverse networks to carry the Internet backbone, point-to-point leased lines, and pretty much anything else with a SONET interface (ATM & frame relay switches, voice switches, digital cross connects, other multiplexers). SONET in Europe and Asia is known as SDH (Synchronous Digital Heirarchy). Asia's SDH differs from Europe's in some respects.

SONET OC levels:
OC1 - 52mb/s
OC3 - 155mb/s
OC12 - 622mb/s
OC48 - 2.5gb/s
OC192 - 9.6gb/s
OC768 - 40gb/s

You can divide OC circuits into what are called STS channels, or tributaries. Generally each OC level has a corresponding STS level, and higher bandwidth optical equipment can carry more than one STS channel, such as a combination of any of the following:

VT-1.5 = T-1 (1.44mb/s)
VT-2 = 2mb/s
OC-1 = STS-1 (usually used for DS3/T3)
OC-3 = STS-3c or STS3 (3 STS1 channels)
OC-12 = STS-12c or STS12
OC-48 = STS-48c or STS48
OC-192 = STS-192c or STS192
OC-768 = STS-768c or STS768 (have not encountered these systems yet)

SDH equivalents:
STS-3c = STM-1
STS-12c = STM-4
STS-48c = STM-16
STS-192c = STM-64

The "c" in STS3c or OC3c stands for concatenated, meaning that the entire 155mb/s is dedicated to one channel (one payload), unlike an STS3, which would be 3 STS1 channels (3 payloads). For instance, on an OC12 ring, you could have 9 STS1 channels and 1 STS3c channel, or 3 STS3c channels and 3 STS1 channels. Once the channels are demultiplexed, they are split into tributaries, the lower-bandwidth interfaces used for connecting to other networking equipment.

Each OC level can be a ring interface to a UPSR (Unidirectional Path Switched Ring) or BLSR (Bi-directional Line Switched Ring). On these rings, generally one line goes east, and another west. If one side fails, individual STS channels (UPSR) or the entire line (BLSR) can be switched to the other path or span, depending on the technology.

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Optical Carrier Bandwidth

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Blogger 土魚瓜瓜 said...

thank you for this simple and easy to understand teaching.

3:13 AM  

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