Features and advantages of SDH/SONET

Today's carrier backbone networks are supported by synchronous optical network (SONET) and synchronous digital hierarchy (SDH) transmission technologies. SONET is the standard used in the United States and SDH is the standard used outside the United States.

SONET/SDH specification outlines the frame format, multiplexing method, and synchronization method between the equipment, as well as the specifying optical interface.

SONET/SDH will continue to play a key role in the next generation of networks for many carriers. In the core network, the carriers offer services such as telephone, dedicated leased lines, and Internet protocol (IP) data, which are continuously transmitted.

The individual data is not transmitted on separate lines; instead it is multiplexed into higher speeds and transmitted on SONET/SDH networks at up to 10 gigabits per second (Gbps). The following sections describe the need for SONET/SDH.

Synchronous Digital Transmission

Until the introduction of SONET in the mid-1980s, plesiochronous digital hierarchy (PDH) systems commonly used data multiplexing technology. The primary problem with PDH was that to extract low-speed traffic, all traffic that was multiplexed to higher speeds had to be de-multiplexed into lower speeds. With PDH, the equipment had to support multiplexing and de-multiplexing the signal, adding cost and complexity to the network.

SONET was introduced as a synchronous transmission system that could directly extract low-speed signals from multiplexed high-speed traffic. Based on the ANSI standard, the CCITT approved the international standard known as SDH based on the SONET technology.

Mid-Span Meet

The adoption and acceptance of SONET allowed carriers to be able to choose equipment from different vendors instead of using only a single vendor with a proprietary optical format. The ability to mix equipment from different vendors in one system is called the "Mid-Span Meet".

Speed

SONET and SDH give carriers much more bandwidth to carry voice and data traffic than PDH technology. The base rate for SONET is 51 Mbps. Synchronous transport signal (STS-n) refers to the SONET signal in the electrical domain, and optical carrier (OC-n) refers to the SONET signal in the optical domain. The base rate for SDH is 155 Mbps. Synchronous transport module (STM-n) refers to the SDH signal level in both the electrical and optical domains. See Table 1.

Reliability

Carriers require an extremely reliable network and cannot afford downtime. Therefore, most SONET/SDH networks have a ring structure, which adds high reliability to the overall transmission network. Even if the optical fiber is cut, the transmission path is backed-up and restored within 50 ms. Figure 1 shows an example SONET ring.

Figure 1. SONET Rings

A SONET/SDH transmission network is composed of several pieces of equipment, including:

1. Terminal multiplexer (TM)
2. Add-drop multiplexer (ADM)
3. Repeater
4. Digital cross-connect system (DCS)