In digital transmission, "clocking" is one of the most important considerations. Clocking means using a series of repetitive pulses to keep the bit rate of data constant and to indicate where the ones and zeroes are located in a data stream.
Asynchronous multiplexing uses multiple stages. Signals such as asynchronous DS1s are multiplexed, extra bits are added (bit-stuffing) to account for the variations of each individual stream, and are combined with other bits (framing bits) to form a DS2 stream. Bit-stuffing is used again to multiplex up to DS3. DS3s are multiplexed up to higher rates in the same manner. At the higher asynchronous rate, they cannot be accessed without demultiplexing.
In a synchronous system, such as SONET, the average frequency of all clocks in the system will be the same (synchronous) or nearly the same (plesiochronous). Every clock can be traced back to a highly stable reference supply. Thus, the STS-1 rate remains at a nominal 51.84 Mb/s, allowing many synchronous STS-1 signals to be stacked together when multiplexed without any bit-stuffing. Thus, the STS-1s are easily accessed at a higher STS-N rate.
SONET's optical interface is broken up into four conceptual layers. Like the ISO and TCP network model, data is passed up and down between these layers. From the bottom up, the layers are:
The basic building block of SONET is the STS-1 frame. The lenth of this frame is defined by the number of bits it is possible to transmit in 125us at the clock rate of the network link.
A standard STS-1 frame consists of 810 bytes, conceptually arranged a two-dimensional matrix. Each column of the matrix is 9 bytes high, and each row of the matrix is 90 bytes wide. The first for columns of the frame contain overhead information from the top three layers of the SONET protocol. The remaining columns are used for the data payload bytes.
Since SONET is synchronous, no bits are wasted on preample for clock synchronization. However, slight differences in clock rate can be accounted for by stuffing/removing bits in the payload section of the frame to slightly adjust the length of the frame.
A SONET frame is sent every 125us regardless of whether or not there is any data to carry. Since data can appear at any time, the data may begin in the middle of a SONET frame. Each from contains a pointer to show where the data starts, and whether or not it is to be concatinated with other frames.
The "two-dimensional" frame structure allows for easy identification and location of the payload within the frame, and easy separation of the overhead bytes from the payload bytes.