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 Mbps. The next level of SONET signalling, STS-3, supports triple the bandwidth, or 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 LevelOC SpecificationData Rate (Mbps) 1OC OC OC OC OC OC OC OC OC OC

Advantages Single-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.

Advantages Synchronous 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

Disadvantages Fixed 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.

Disadvantages Waste 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.

Disadvantages Multieast 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.