FRAME STRUCTURE
6.2 SOH and Sonet
6.2.1 Sonet Structure
The basic structure in Sonet is a frame of 810 bytes which is sent every 125 J,lsec. This allows a single byte within a frame to be part of a 64 Kbps digital voice channel. Since the minimum frame size is 810 bytes then the minimum speed at which Sonet will operate is 51.84 megabits per second.
81e bytes X 8eee frames/sec X 8 (bits)
=
51.84 megabits/sec40 See Appendix C, "Getting the Language into Synch" on page 291
I
\
i
9Rows
j
•• - - - - 90 Bytes - - - - -....
B B B 87 Bytes
Payload
tt
Section and Line 'Overhead"(3 bytes per line)
Figure 40. Sonet STS-1 Frame Structure. The diagramatic representation of the frame as a square is done for ease of understanding. The 810 bytes are transmitted row by row starting from the top left of the diagram. One frame is transmitted every 125
microseconds.
This basic frame is called the Synchronous Transport Signal level 1 (STS-1). It is conceptualised as containing 9 rows of 90 columns each as shown in Figure 40.
• The first three columns of every row are used for administration and control of the multiplexing system. They are called "overhead" in the standard but are very necessary for the system's operation.
• The frame is transmitted row by row, from the top left of the frame to the bottom right.
• Of course it is necessary to remember that the representation of the structure as a two-dimensional frame is just a conceptual way of
representing a repeating structure. In reality it is just a string of bits with a defined repeating pattern.
The physical frame structure above is similar to every other TOM structure used in the telecommunications industry. The big difference is in how the "payload"
is carried. The payload is a frame that "floats" within the physical frame structure. The payload envelope is illustrated in Figure 41 on page 120.
Chapter 6. High Speed Time Division Multiplexing Systems 119
4 87 bytes
i
9I
86 BytesRows
j
~path
Layer OverheadFigure 41. Sonet Synchronous Payload Envelope
Notice that the payload envelope fits exactly within a single Sonet frame.
The payload envelope is allowed to start anywhere within the physical Sonet frame and in that case will span two consecutive physical frames. The start of the payload is pointed to by the H1 and H2 bytes within the line overhead sections.
Very small differences in the clock rates of the frame and the payload can be accommodated by temporarily incrementing or decrementing the pOinter (an extra byte if needed is found by using one byte (H3) in the section header).
Nevertheless, big differences in clock frequencies cannot be accommodated by this method .
Frame 2
Frame 2
• 4 - - - - - 90 Bytes ---+~
Start of
l
H1 H2 H3 Synch Payload Envelope
,Ir 125
I
+-I
H1 H2 H3
125
~I I -,----,-I I - - - - I J
Micro sec
Micro sec
Figure 42. Synchronous Payload Envelope Floating in STS-l Frame. The SPE is pointed to by the HI and H2 bytes.
(
\,
6.2.2 SDH
Multiple STS-1 frames can be byte multiplexed together to form higher speed signals. When this is done they are called STS-2, STS-3 etc. where the numeral suffix indicates the number of STS-1 frames that are present (and therefore the line speed). For example STS-3 is 3 times an STS-1 or 155.52 megabits per second. This multiplexing uses the method illustrated in Figure 43.
An alternative method is to phase align the multiple STS frames and their payloads. This means that a larger payload envelope has been created. This is called "concatenation" and is indicated in the name of the signal. For example, when three STS-1s are concatenated such that the frames are phase aligned and there is a single large payload envelope it is called an STS-3c.
STM-l
AAAAAAAA-STM-l
BBBBBBBB-ABCDABCDABCDAB.
STM-l cccccccc- STM-4
STM-l
DDDDDDDD-Figure 43. STM-l to STM-4 Synchronous Multiplexing
In the rest of the world, Sonet is not immediately useful because the "E3" rate of 35 megabits does not efficiently fit into the 50 megabit Sonet signal. (The
comparable US signal, the T3 is roughly 45 megabits and fits nicely.)
The CCITT has defined a worldwide standard called the Synchronous Digital Hierarchy, which accommodates both Sonet and the European line speeds.
This was done by defining a basic frame that is exactly equivalent to (Sonet) STS-3c. This has a new name. It is Synchronous Transport Module level one or STM-1 and has a basic rate (minimum speed) of 155.52 megabits per second.
This is shown in Figure 44 .
... - - - 278 Bytes - - - -...
t
Section and Line Overhead
t
(9 Bytes per line)
Figure 44. SDH Basic Frame Format
Chapter 6. High Speed Time Division Multiplexing Systems 121
6.2.3 Tributaries
Faster line speeds are obtained in the same way as in Sonet - by byte
interleaving of multiple STM-1 frames. For this to take place (as in Sonet) the STM-1 frames must be 125 J1sec frame aligned. Four STM-1 frames may be multiplexed to form an STM-4 at 622.08 megabits per second. This (again like Sonet) may carry four separate payloads byte multiplexed together (see Figure 43 on page 121). Alternatively, the payloads may be concatenated (rather than interleaved) and the signal is then called STM-4c.
Within each payload, slower speed channels (called tributaries) may be carried.
Tributaries normally occupy a number of consecutive columns within a payload.
A US T1 payload (1.544 megabits/sec) occupies three columns, a European E1 payload (2.048 megabits/sec) occupies four columns. Notice that there is some wasted bandwidth here. A T1 really only requires 24 slots and three columns gives it 27. An E1 requires 32 slots and is given 36. This "wastage" is a very small price to pay for the enormous benefit to be achieved by being able to demultiplex a single tributary stream from within the multiplexed structure without having to demultiplex the whole stream.
The tributaries may be fixed within their virtual containers or they may float, similar to the way a virtual container floats within the physical frame. Pointers within the overhead are used to locate each virtual tributary stream.