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Protocol Basics, Internet Protocol (IP) Optical fiber is a demanding mistress. Its vast capacity needs to be accessed by a protocol that can support high data rates of up to 10 Gbit/s per wavelength or, in the future, 40 Gbit/s. There also needs to be inherent reliability and preferably an extensive range of network monitoring and management functions. It is also desirable to be able to easily access smaller portions of the large data streams without any complicated demultiplexing techniques. The protocol used in modern networks to satisfy these cravings is Synchronous Digital Hierarchy (SDH) or the almost identical Synchronous Optical NETwork (Sonet) which is primarily used in the U.S., Canada, and Japan.

SDH is placed at the bottom of the protocol stack in the physical layer, along with the very fiber upon which it transmits its signal. Any IP traffic that is destined to be transmitted across a fiber-based SDH network will be framed by a Layer 2 protocol before being ready to take its orders from the SDH equipment.

The fundamental principle of the SDH protocol is time-division multiplexing (TDM), which works in contrast to a system such as Ethernet where data is sent in sporadic bursts. TDM ensures that there is a constant stream of data travelling through the network and taking advantage of the fiber bandwidth available. Lower bit-rate streams of information are combined, or multiplexed, up into higher bit-rate streams at the native bit rate of the SDH system. The fundamental SDH frame is known as STM1 (synchronous transport module); its Sonet counterpart is OC3 (optical container). Each provides a bit rate of 155 Mbit/s with a total frame size of around 20 kbits.

The SDH frame has a two-dimensional structure, as shown in the diagram. Each frame is physically transmitted through the fiber row by row, each row from left to right. An STM1 frame is formed by nine rows and 270 columns of bytes of information. The first nine columns form what is known as the "section overhead" or SOH, which provides a comprehensive range of facilities such as error monitoring, network management, and automatic switching between fiber links should one be unavailable (known as "protection switching"). The remainder of the frame is termed a "virtual container" or VC, and in the case of an STM1 is known as a VC4. This contains the data – except for the first column, which is the "path overhead" or POH, whose function it is to monitor the quality of the link and indicate the type of data payload it is carrying.

Contained within a VC4 may be multiple VC12 containers of 2-Mbit/s capacity, or VC3 containers, each giving 34 Mbit/s. These allow a greater level of "granularity," as the SDH equipment can access each of these individual streams for routing to the specific customer. The STM1 frame can also be scaled up, which is done by factors of four. An STM4 (or OC12) frame gives a bit rate of 620 Mbit/s and will still have 9 rows but 1080 (4 x 270) columns of bytes, with the overhead sections also being scaled accordingly.

Setting up a circuit of SDH frames between two locations is not as easily done as when sending out data on an Ethernet cable in a LAN (local area network). In fact, it requires human intervention to set up circuits via network management software and may take several minutes to perform. In such high-capacity links, however, the speed of configuration is not necessarily a top priority.

In a practical application, an SDH "line system" will have a multiplexer that takes its inputs from a variety of sources in different Layer-2 data formats. These are aggregated up to form frames at the line rate of the system, for example up to STM64 for a 10-Gbit/s bit-rate system. These frames are then transmitted out onto optical fiber links, and there is the possibility for multiple SDH multiplexers to each give out one wavelength of a WDM system. Along the length of the system there can be regenerators that will convert the optical signals to electrical, clean them up, and then re-transmit them out in optical format again. At the end of the system will be an SDH demultiplexer that now accesses the individual data streams from the STM64 frames as required.

There may also be an SDH add/drop multiplexer (ADM) with the ability to remove and insert lower bit-rate streams from the signal; or, alternatively, a digital crossconnect may be present with the ability to switch individual VC4’s between different fiber links. All such switching and add/drop functionality is performed purely in the electrical domain, as the optical SDH signals will always be converted to electrical signals upon arrival. It is worth noting that interworking between SDH and Sonet systems is possible at matched bit rates (e.g., STM4 and OC12), although a slight modification to the overheads is required, as they are structured a little differently.

Key Points

• Synchronous Optical NETwork (SONET) in the U.S., Canada, and Japan; Synchronous Digital Hierarchy (SDH) elsewhere
• Time-division multiplexing (TDM) transmits a constant stream of information
• Fundamental SDH frame is STM1 (synchronous transport module); Sonet version is OC3 (optical container) – each providing 155 Mbit/s
• STM4 provides four times the STM1 capacity; STM16 is a further fourfold increase, etc.
• Two-dimensional frame structure with overheads for functions such as error monitoring, network management, and protection switching
• Virtual container carries the data (a single VC4 in an STM1 frame)
• Container may carry smaller streams as low as 2-Mbit/s VC12s that can be accessed directly at demultiplexers
• SDH circuits set up through network management software