How do you design and implement SDH frame synchronization and alignment?

1 Frame structure

SDH frames consist of two main components: the section overhead (SOH) and the synchronous payload envelope (SPE). The SOH contains information for managing the network, such as alarms, errors, and performance indicators. The SPE contains the actual data payload, which can be a mix of different services, such as voice, data, or video. The SPE also has its own overhead (POH) that carries information for mapping and demapping the payload. The SOH and the SPE are arranged in a fixed matrix of bytes, called the SDH frame.

2 Synchronization and alignment

To ensure that the transmitter and the receiver are synchronized and aligned, SDH uses two methods: frame alignment and pointer processing. Frame alignment is the process of identifying the start and the end of each SDH frame, using a special pattern of bytes in the SOH, called the frame alignment signal (FAS). Pointer processing is the process of adjusting the position of the SPE within the SDH frame, using a pair of bytes in the SOH, called the pointer. The pointer indicates the offset of the first byte of the SPE from the pointer location. This allows the transmitter and the receiver to cope with different clock rates and jitter in the network.

3 Error detection

To detect errors in the SDH frame structure, SDH uses various methods, such as parity checks, cyclic redundancy checks (CRCs), and bit interleaved parity (BIP). Parity checks are simple methods that use an extra bit to indicate whether the number of 1s in a byte is even or odd. CRCs are more complex methods that use a polynomial function to generate a checksum for a block of bytes. BIP is a method that calculates a parity bit for each column of bytes in the SDH frame, and inserts it in the SOH. These methods allow the receiver to detect if any bit has been corrupted during transmission.

4 Error correction

To correct errors in the SDH frame structure, SDH uses two main methods: forward error correction (FEC) and automatic protection switching (APS). FEC is a method that adds extra bits to the data payload, using a coding scheme, such as Reed-Solomon or Bose-Chaudhuri-Hocquenghem. These bits allow the receiver to recover the original data, even if some bits are corrupted. APS is a method that switches to a backup path, in case of a failure or a degradation in the primary path. APS uses signaling messages in the SOH to coordinate the switch between the transmitter and the receiver.

5 Benefits and challenges

SDH frame synchronization and alignment, and error detection and correction, provide several benefits for optical network performance and reliability. They enable efficient multiplexing and demapping of different data streams, without requiring complex conversion or adaptation. They also ensure high quality of service and availability, by reducing bit errors and minimizing service disruptions. However, they also pose some challenges for network design and implementation. They require careful planning and coordination of network parameters, such as clock sources, frame sizes, and protection schemes. They also increase the complexity and cost of network equipment and maintenance.