SONET, SDH, and OCX

Modern networks usually require WAN connectivity at speeds higher that T1/E1 or T3/E3. Optical connections that come directly into data centers represent a valid alternative that can offer increased bandwidth. This is also called synchronous optical networking because a lot of digital signals can come over a single fiber into the customer’s facility. This means that you do not need to have separate connections to carry different services.

The digital signals transmitted over the fiber are multiplexed and the connection is called synchronous because the signals are using the same clock over the connection, regardless of the type of signal. There are two primary standards associated with synchronous optical networking that operate in similar fashion:

• SONET
• SDH

Synchronous Optical Networking (SONET) is one of the most common standards, and it was developed by the American National Standards Institute (ANSI). SONET is primarily used in the United States and Canada.

SONET is a circuit-based technology that delivers very high-speed service over optical networks using a ring topology. This topology type offers fault tolerance, redundancy, and the capability of being highly available. SONET functions using two Layer 2 technologies:

• ATM
• Packet over SONET (POS)

Synchronous Digital Hierarchy (SDH) is an international standard developed by the International Telecommunications Union (ITU). SDH is used everywhere around the world, except for the United States and Canada.

Although SONET and SDH are similar standards, they have different ways of calculating throughput and bandwidth values. They also use different terms to quantify bandwidth levels:

• SONET uses the Synchronous Transport Signal (STS)
• SDH uses the Synchronous Transport Module (STM)

The different OCx, SONET, and SDH bandwidth levels and the similarities between them are presented in Table 16.1 below:

Table 16.1 – OCx, SONET, and SDH

Some of the considerations that must be taken into account when new SONET connections are purchased include:

• Details about transport usage (whether the link will be used for data or voice transport)
• Details about the topology (linear or ring-based)
• Details about single points of failure in the transport
• Customer needs
• Costs
• Implementation scenarios (multiple providers, multiple paths, etc.)
• The type of oversubscription offered by the service provider

You should also know whether you are getting dedicated bandwidth or are sharing the bandwidth with other users. If you are getting services from two providers to achieve high availability and redundancy, they may have different SONET implementations and may follow different paths.

Service providers usually share the same physical fiber paths (e.g., along gas pipelines or public electrical paths), so even if dual service providers are used, because the physical fiber path is often the same, the risk of failure does not decrease. If something happens to the pipes that have the fiber links attached, all the providers that follow that specific path will suffer. The recommended scenario is having two service providers with different physical cabling paths.