Difference from PDH

Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, using atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either asynchronous transfer mode (ATM) or so-called packet over SONET/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data. The basic format of an SDH signal allows it to carry many different services in its virtual container (VC) because it is bandwidth-flexible.

Structure of SONET/SDH signals
SONET and SDH often use different terms to describe identical features or functions. This can cause confusion and exaggerate their differences. With a few exceptions, SDH can be thought of as a superset of SONET.

[edit] Protocol overview
The protocol is an extremely heavily multiplexed structure, with the header interleaved between the data in a complex way. This is intended to permit the encapsulated data to have its own frame rate and to be able to float around relative to the SDH/SONET frame structure and rate. This interleaving permits a very low latency for the encapsulated data. Data passing through equipment can be delayed by at most 32 microseconds, compared to a frame rate of 125 microseconds and many competing protocols buffer the data for at least one frame or packet before sending it on. Extra padding is allowed for the multiplexed data to move within the overall framing because it being on a different clock than the frame rate. The decision to allow this at most of the levels of the multiplexing structure makes the protocol complex, but gives high all-around performance. SONET is the standard defined by the ANSI T1 for synchronous operation used in North America.

[edit] The basic unit of transmission
The basic unit of framing in SDH is a STM-1 (synchronous transport module level - 1), which operates at 155.52 Mbps. SONET refers to this basic unit as an STS-3c (synchronous transport signal - 3, concatenated), but its high-level functionality, frame size, and bit-rate are the same as STM-1.

SONET offers an additional basic unit of transmission, the STS-1 (synchronous transport signal - 1), operating at 51.84 Mbps - exactly one third of an STM-1/STS-3c. That is, in SONET the associated OC-3 signal will be composed of three STS-1s (or, more recently in packet transport, the OC-3 signal will carry a single concatenated STS-3c.) Some manufacturers also support the SDH equivalent: STM-0.

[edit] Framing
In packet-oriented data transmission such as Ethernet, a packet frame usually consists of a header and a payload. The header is transmitted first, followed by the payload (and possibly a trailer, such as a CRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and instead of being transmitted before the payload, is interleaved with it during transmission. Part of the overhead is transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted. In the case of an STS-1, the frame is 810 octets in size while the STM-1/STS-3c frame is 2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload. This is repeated nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster than STS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times over until 2,430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH, this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for STS-1; and 270 columns and 9 rows for STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.

The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. Their standards are extremely similar in implementation making it easy to interoperate between SDH and SONET at particular bandwidths.

It is worth noting that the choice of a 125-microsecond interval is not an arbitrary one. If one octet is extracted from the bitstream every 125 microseconds, this produces a data rate of 8 bits per 125 microseconds - or 64 kbps, the basic digital signaling rate for telecommunication systems world wide. This allows an extremely useful technique to be used in synchronous optical networking. The low data-rate channels or streams of data can be extracted from high data-rate streams by simply extracting octets at regular time intervals—there is no need to understand or decode the entire frame. This is not possible in PDH networking. It shows that a relatively simple device is all that is needed to extract a datastream from an SDH-framed connection and insert it into a SONET-framed connection and vice versa.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.