Sonet

Definition•Short for Synchronous Optical Network, a standard for connecting fiber-optic transmission

systems.

•SONET defines interface standards at the physical layer of the OSI seven-layer model

•The standard defines a hierarchy of interface rates that allow data streams at different rates to

be multiplexed.

Definition• SONET establishes Optical Carrier levels from 51.8 Mbps to 9.95 Gbps.

• With the implementation of SONET, communication carriers throughout the world can

interconnect their existing digital carrier and fiber optic systems.

•At the base signalling level called "STS-1," SONET supports 51.84 Mbps. The next level of SONET

signalling, STS-3, supports triple the bandwidth, or 155.52 Mbps.

Optical Carriers The STS level is the rate at which SONET can multiplex multiple sources of data to a single fiber optic line. OC specifications are measured in multiples of 3, with a base of 1.

STS Level OC Specification Data Rate (Mbps)1 OC-1 51.843 OC-3 155.529 OC-9 466.5612 OC-12 622.0818 OC-18 933.1224 OC-24 1244.1636 OC-36 1866.2348 OC-48 2488.3296 OC-96 4976.64192 OC-192 9953.28

AdvantagesSingle-Step Multiplexing

In single-step multiplexing, lower-rate traffic is combined into higher-rate traffic without having to go

through stages.

Access to Low-level Signal Directly

Direct access to low-level signals allows the receiving system to know exactly where an STS-1 is

located within a STS-n and extract it for use at the local site.

AdvantagesSynchronous timing to eliminate bit stuffing

SONET is synchronous. This requires a constant timing source for both the receiving and

transmitting of traffic. The advantage of this common timing source is that it is possible to have

byte interleaving rather than bit interleaving at these higher speeds.

Allow transmission of data at higher speeds

DisadvantagesFixed circuits

SONET/SDH provisions point-to-point circuits between ring nodes. Each circuit is allocated a fixed

amount of bandwidth that is wasted when not used. That fixed allocation puts a limit on the

maximum burst traffic data transfer rate between end points. This is a disadvantage for data traffic.

DisadvantagesWaste of bandwidth for meshing

To create a logical mesh over a SONET ring is not only difficult, but also results in an inefficient

use of ring bandwidth. As the amount of data traffic that stays within metro networks is

increasing, a fully meshed network that is easy to deploy, maintain and upgrade is becoming an

important requirement.

DisadvantagesMultieast traffic

On a SONET ring, multicast traffic requires each source to allocate a separate circuit for each

destination. A separate copy of the packet is sent to each destination. The result is multiple

copies of multicast packets traveling around the ring, wasting bandwidth.