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G1: This is known as the path status byte; it carries status and diagnostic signals.
F2: This byte is reserved for use by the network operator.
H4: This byte is known as the multi-frame indicator and is used to identify payloads carried within the same frame.
Z3 and Z4: These are reserved for future use.
Z5: This byte is used for tandem monitoring. A tandem is a telephone switch that is used in the backbone telephone network. It switches traffic between other telephone switches and it does not serve subscribers directly.
2.3.3 The STS-1 Payload
As we have seen above, the STS-1 SPE carries a payload of user data and the path overhead. The payload has been defined to carry multiple subrate data streams - that is, streams that are transmitted at rates below that of STS-1 (such as DS1, DS2 and E1 signals). Such a subrate stream is known as a virtual tributary. The payload can also carry an entire DS3 signal. In addition to PDH-type signals, the STS-1 payload has been defined to carry ATM cells and IP packets encapsulated in PPP frames. In this section, we examine the different STS-1 payloads that can be constructed.
a) Virtual tributaries
The STS-1 payload is divided into seven groups, known as virtual tributary groups (VTG). Each VTG consists of 108 bytes, which are contained in 12 columns. (Assume that each column always has nine rows.) The seven VTGs take up a total of 12 x 7 = 84 columns of the SPE. One of the remaining three columns is used to carry the POH, and the other two are reserved for future use (that is, they go unused). Each VTG can carry a number of virtual tributaries. The following virtual tributaries have been defined:
VT1.5: This virtual tributary carries one DS1 signal, which in itself carries 24 DS0 signals, at 64 Kbps each. VT1.5 is contained in three columns; that is, it takes up 27 bytes. Four VT1.5s can be transported in a single VTG.
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
VT2: This virtual tributary carries an E1 signal of 2.048 Mbps, which consists of thirty-two 8-bit time slots, of which thirty are used for voice and the remaining two are used for synchronization and control. VT2 is contained in four columns; that is, it takes up 36 bytes. Three VT2s can be carried in a single VTG.
VT3: This virtual tributary transports the unchannelized version of the DS1 signal, where the time slot boundaries are ignored by the sending and receiving equipment. All 192 bits are used to transport data, followed by the 193rd framing bit. It is also possible to use the entire frame (including the framing bit) in an unchannelized manner. The unchannelized version is known as the concatenated channel; it is indicated by the capital C (as opposed to the lowercase c used in SONET). VT3 is contained in six columns; that is, it takes up 54 bytes. This means that a VTG can carry two VT3s.
VT6: This virtual tributary transports a DS2 signal, which carries 96 voice channels. VT6 is contained in twelve columns; that is, it takes up 108 bytes. A VTG can carry exactly one VT6.
The STS-1 payload can only carry one type of virtual tributary. That is, the seven VTGs can only carry VT1.5s, VT2s, VT3s, or VT6s. This means that the STS-1 payload can carry a total of twenty-eight DS1s, twenty-one E1s, fourteen DS1Cs, or seven DS2s. More sophisticated payload construction permits mixing different types of virtual tributaries. For instance, the payload can carry two VTGs (each with one VT6 tributary) and five VTGs (each with four VT1.5s).
b) Asynchronous DS3
The DS3 signal multiplexes 672 voice channels and has a transmission rate of 44.736 Mbps. In the unchannelized format, it is used to carry a continuous bit stream. Both the channelized and unchannelized DS3 are carried in STS-1, and the DS3 signal occupies the SPEs entire payload.
c) ATM cells
ATM cells are directly mapped into the STS-1 SPE so that the ATM bytes coincide with the SONET bytes. The total number of bytes available for user data in the STS-1 frame is: 87 x 9 = 783. Of these 783bytes, 9bytes are used for the path overhead, leaving 774bytes for user data. An ATM cell consists of 53bytes; thus, 774/53 = 14.6ATM cells can be stored in an SPE. In other words, the SPE cannot contain an integer number of ATM cells. As a result, an ATM cell might straddle two successive SPEs, with part of it in one SPE, and the rest of it in the next SPE. The ATM cell can be cut off at any byte, whether it is in the header or the payload. ATM cells can also skip past the path overhead bytes.
An example of how ATM cells are mapped in the STS-1 SPE is shown in Figure 2.9. For presentation purposes, we only show one SPE that straddles over two STS-1 frames. The SPE begins on column 10, which means that the path overhead bytes occupy column 10. Assume that the first cell begins immediately after the first path overhead byte on column 11. (The cells are shaded and are not drawn to proportion.) ATM cell 1 occupies row 1, columns 11 to 63. ATM cell 2 occupies the remaining bytes (from row 1, column 64 to row 2, column 27). The path overhead byte on the second row is skipped when the cell is mapped into the SPE. ATM cell 3 is mapped on row 2, columns 28 to 80.