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.
32 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP
case above for ATM cells. IP packets can also be encapsulated in PPP frames instead of HDLC. The PPP frames are delineated with the HDLC flag 01111110. As is the case
with ATM, a frame can straddle over two adjacent SPEs. The interframe fill 7E is used to maintain a continuous bit stream when there are no IP packets to transmit. An example of
mapping either HDLC or PPP frames in the SPE payload is shown in Figure 2.10 note that the frames are not drawn to proportion.
2.4 THE SONET STS-3 FRAME STRUCTURE
The SONET STS-3 frame consists of 2430 bytes and is transmitted 8000 times per second i.e. once every 125 µsec. This gives a total data rate of 8000 × 2430 × 8 bitssec i.e.
155.520 Mbps. The frame is displayed in a matrix form consisting of nine rows and 270 columns, wherein each matrix cell corresponds to a byte.
The channelized STS-3 frame is constructed by multiplexing byte-wise three chan- nelized STS-1 frames. That is, bytes 1, 4, 7, . . . 268 of the STS-3 frame contains bytes
1, 2, 3, . . . 90 of the first STS-1 frame. Likewise, bytes 2, 5, 8, . . . 269 of the STS-3 frame contains bytes 1, 2, 3, . . . 90 of the second STS-1 frame, and bytes 3, 6, 9, . . . 270 of the
STS-3 frame contains bytes 1, 2, 3, . . . 90 of the third STS-1 frame. This byte-wise multi- plexing causes the columns of the three STS-1 frames to be interleaved in the STS-3 frame
see Figure 2.11. That is, columns 1, 4, 7, . . . 268 contain the first STS-1 frame; columns 2, 5, 8, . . . 269 contain the second STS-1 frame; and columns 3, 6, 9, . . . 270 contain the
third STS-1 frame. As a result of this multiplexing, the first nine columns of the STS-3 frame contain the overhead part, and the remaining columns contain the payload part.
Error checking and some overhead bytes are for the entire STS-3 frame, and are only meaningful in the overhead bytes of the first STS-1 frame the corresponding overhead
bytes in the other STS-1 frames are ignored.
The STS-3c OC-3c was designed originally for the transport of ATM cells. The STS- 3c frame consists of nine columns of overhead and 261 columns of payload. ATM cells
or IP packets are packed into a single SPE in the same manner as described above in Section 2.3.3.
Higher levels of channelized STS-N are constructed in the same manner as STS-3 by multiplexing N STS-1s.
Overhead section
1 2 3
4 5
6 7
8 9 10 11 12
270
. . . 1
st STS-1
2 nd
STS-1 3
rd STS-1
1 st
STS-1 2
nd STS-1
3 rd
STS-1 1
st STS-1
2 nd
STS-1 3
rd STS-1
1 st
STS-1 2
nd STS-1
3 rd
STS-1 1
st STS-1
2 nd
STS-1 3
rd STS-1
Payload Section
Figure 2.11 The channelized STS-3 frame.
SONETSDH DEVICES 33
2.5 SONETSDH DEVICES
Several different types of SONETSDH devices exist. The SONETSDH terminal multi- plexer TM
device multiplexes a number of low-speed signals into a higher-speed signal. It also works in the opposite direction as a demultiplexer. That is, it demultiplexes a high-
speed signal into a number of lower-speed signals. For instance, a SONET TM device multiplexes a number of DSn signals into a single OC-N signal see Figure 2.12a. It can
also demultiplex an OC-N signal into a number of DSn signals. It consists of a controller, low-speed interfaces for DSn, an OC-N interface, and a time slot interchanger TSI,
which is used to map the incoming time slots of the DSn signals into the STS-N SPE.
The SONETSDH adddrop multiplexer ADM is a more complex version of the TM device. As shown in Figure 2.12b, a SONET ADM receives an OC-N signal from
which it can demultiplex and terminate any number of DSn andor OC-M signals, where M N. At the same time, it can add new DSn and OC-M signals into the OC-N signal.
Specifically, the incoming OC-N signal is first converted into the electrical domain, and then the payload is extracted from each incoming frame. Recall that the payload consists
of a fixed number of bytes, or time slots. These time slots carry different virtual tributaries, such as DSn and OC-M signals, some of which are dropped i.e., terminated. That is,
the information is first extracted from the appropriate time slots that carry these virtual tributaries, and then it is transmitted to local users through the ADM’s low-speed DSn
and OC-M interfaces. An OC-M interface is typically connected to a TM device. This termination process frees up a number of time slots in the frame, which, along with other
unused time slots, can be used to carry traffic that it is locally generated. That is, DSn and OC-M signals received from its low-speed DSn and OC-M interfaces can be added into
the payload of the frame using these unused time slots. The final payload is transmitted out at the same SONET level as the incoming OC-N signal.
SONET or SDH ADM devices are typically interconnected to form a SONET or an SDH ring. SONETSDH rings are self-healing; that is, they can automatically recover
from link failures. Self-healing rings consist of two or four fibers, and are discussed in the following section.
In Figure 2.13, we show a SONET ring interconnecting four ADM devices. For pre- sentation purposes, we assume that these four ADM devices are connected by a single
fiber and that the direction of transmission is clockwise. Each ADM device serves a num- ber of local TM devices and other users. User A is connected to TM 1, which in turn is
connected to ADM 1. User A has established a connection to user B, who is attached to ADM 3 via TM 2. In Figure 2.13, this connection is signified by the dotted line. Let us
assume that user A transmits a DS1 signal. This is multiplexed with other DS1 signals in
DSn DSn
OC-N TM
DSn OC-M OC-N
OC-N ADM
a Terminal multiplexer b Add-drop multiplexer
....
Figure 2.12 The SONET TM and ADM.
