The Section, Line, and Path Overheads

30 SONETSDH 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 SPE’s 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 × 9 = 783. Of these 783 bytes, 9 bytes are used for the path overhead, leaving 774 bytes for user data. An ATM cell consists of 53 bytes; thus, 77453 = 14.6 ATM 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. THE SONET STS-1 FRAME STRUCTURE 31 10 Cell 1 Cell 2 Cell 2 Cell 3 Cell 14 Cell 15 Cell 15 90 4 1 9 2 8 3 POH Figure 2.9 Mapping ATM cells in the STS-1 SPE. ATM cell 14 is mapped on row 8, columns 15 to 67. ATM cell 15 is mapped from row 8, column 67 to row 9, column 31. As we can see, it skips the ninth path overhead byte, and it runs into the next SPE. ATM users do not always transmit continuously. During the time that ATM cells are not generated, idle cells are inserted so as to maintain the continuous bit stream expected from SONET. This is done in the transmission convergence TC sublayer of the ATM physical layer. These idle cells can be identified uniquely since their header is marked as: VPI = 0, VCI = 0, PTI = 0, and CLP = 0. ATM cells are similarly mapped within the SPE of an STS-3c, STS-12c, etc. The basic rate that has been defined for the transport of ATM cells is the STS-3c. d Packet over SONET PoS IP packets can be directly carried over SONET links. This scheme is known as packet over SONET PoS , and is used to interconnect IP routers. IP packets are first encapsulated in HDLC; the resulting frames are mapped, row by row, into the SPE payload, as in the 10 90 4 1 9 2 8 3 POH 7E 7E 7E 7E 7E 7E Figure 2.10 Packet over SONET PoS.