The Bellcore-driven North American standards defined a Synchronous Optical Network (SONET), while the CCITT/ITU developed a closely related international Synchronous
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ATM & MPLS Theory & Application: Foundations of Multi-Service NetworkingBit Rate (Mbps) Digital
Multiplexing Level
Number of
Voice Channels North America Europe Japan
0 1 0.064 0.064 0.064
1 24 1.544 1.544
30 2.048
48 3.152
2 96 6.312 6.312
120 8.448 7.876
3 480 34.368 32.064
672 44.376
1344 91.053
1440 97.728
4 1920 139.268
4032 274.176
5760 397.200
5 7680 565.148
Table 6-2. Summary of International Plesiochronous Digital Hierarchies
PDH: rates of higher speeds are defined, and direct multiplexing is possible without in-termediate multiplexing stages. Direct multiplexing employs pointers in the TDM over-head that directly identify the position of the payload. Furthermore, the fiber optic transmission signal transfers a very accurate clock rate along the transmission paths all the way to end systems, synchronizing the entire transmission network to a single, highly accurate clock frequency source.
Another key advance of SONET and SDH was the definition of a layered architecture (illustrated in Figure 6-7) that defines three levels of transmission spans. This model allowed transmission system manufacturers to develop interoperable products with compatible functions. The SONET/SDH framing structure defines overhead operating at each of these levels to estimate error rates, communicate alarm conditions, and provide maintenance support. Devices at the same SONET/SDH level communicate this over-head information as indicated by the arrows in Figure 6-7. The path layer covers end-to-end transmission, where ATM switches or MPLS label-switching routers (LSRs) operate as indicated in the figure. This text refers to a transmission path using this definition from SONET/SDH. Next comes the maintenance span, or line layer, which comprises a series of regenerators (or repeaters). An example of a line-layer device is a SONET/SDH cross-connect. The section regenerator operates between repeaters. Finally, the photonic layer involves sending binary data via optical pulses generated by lasers or light emitting diodes (LEDs).
SONET standards designate signal formats assynchronous transfer signals (STSs); they are represented at N times the basic STS-1 (51.84 Mbps) building block rate by the term STS-N. SONET designates signals at speeds less than the STS-1 rate as virtual tributaries (VTs). The optical characteristics of the signal that carries SONET payloads is called the optical carrier (OC-N).An STS-N signal can be carried on any OC-M, as long as M≥N. The standard SONET STS and VT rates are summarized in the text that follows.
The CCITT/ITU developed a similar synchronous multiplex hierarchy with the same advantages using a basic building block called the synchronous transfer module (STM-1)
Figure 6-7. Example of SONET/SDH architecture layers
with a rate of 155.52 Mbps, which is exactly equal to SONET’s STS-3 rate to promote interoperability between the different standards. The SDH standards also define a set of lower-speed signals, called virtual containers (VCs). Therefore, a direct mapping between the SONET STS-3N rates and the CCITT/ITU STM-N rates exists. An STM-1 frame is equivalent to an STS-3c frame in structure. The pointer processing and overhead byte defi-nitions differ between SONET and SDH, so direct interconnection is currently not possible.
Different vendor transmission equipment does not interwork easily even within SONET and SDH implementations, since proprietary management systems often utilize part of the protocol overhead for maintenance and operational functionality. Table 6-3 shows the SONET speed hierarchy by OC level and STS level as it aligns with the international SDH STM levels and the bit rates of each.
Table 6-4 illustrates a similar mapping to that of Table 6-3, comparing the mapping of the North American and CCITT/ITU PDH rates to the corresponding SONET virtual tributary (VT) and SDH virtual container (VC) rates and terminology. Note that the com-mon 1.5, 2, 6, and 44 Mbps rates can be mapped consistently by using AU-3-based map-ping, as shown later in Figure 6-11. SDH provides some alternative multiplexing paths for PDH signals, whereas SONET provides one way for each. SDH supports all PDH sig-nals except for DS1C (3.152 Mbps). Also note that SONET does not support the popular International E3 rate. The other common rates are 155 and 622 Mbps and above. The rates indicated in the table include the actual payload plus multiplexing overhead, including path overhead. The table includes ATM-carried payload rates for commonly used ATM mappings over SONET/SDH for comparison purposes.
SONET and SDH standards evolved the state of the art in restoration through the con-cept of ring switching [Goralski 00]. A ring-switching architecture could protect either a line or path segment and make traffic flow in either one direction or both directions around the ring. SONET line switching operates on an entire OC-N bundle, while path switching can operate on a tributary. A unidirectional ring means that normal routing of
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ATM & MPLS Theory & Application: Foundations of Multi-Service NetworkingSONET Level SDH Level Bit Rate (Mbps)
Table 6-3. SONET STS-N/OC-N and SDH STM-M Speed Hierarchy
both directions of working traffic flows in the same direction around the ring. Con-versely, in a bidirectional ring, normal routing of the different directions of working traffic flow in the opposite direction around the ring. This results in the following ring-switching restoration schemes:
▼ Bidirectional line-switched ring (BLSR)
■ Bidirectional path-switched ring (BPSR)
■ Unidirectional line-switched ring (ULSR)
▲ Unidirectional path-switched ring (UPSR)
Any of these schemes can operate using either two or four fibers. This results in a po-tential for eight combinations; however, commonly fielded configurations are two-fiber UPSR, two-fiber BLSR, and four-fiber BLSR. A number of tradeoffs exist in deciding which ring technology is best. The directions of traffic flow in unidirectional rings have asymmetric delays, while bidirectional rings have symmetric delay. Two-fiber rings are often deployed in metropolitan areas, while four-fiber rings are more efficient and can protect against some scenarios involving multiple failures in regional deployments.
North American
VT2.0 2.048 (E1) 2.240 VC12 2.048 (E1) 2.304
VT3.0 3.152 3.392 N/A N/A
STS-3c 139.264 (OC-3) 150.336 VC4 139.264 (E4) 150.336
ATM on STS-1 49.536 50.112
Table 6-4. SONET/SDH Digital Hierarchy Payload and Overhead Rates
Two-fiber rings are less expensive to deploy initially but are less efficient than four-fiber rings when fully loaded.
The large-scale deployment of SONET/SDH rings by carriers and ISPs is important to performance, since the switchover time from the occurrence of a failure to a completely restored circuit line or path segment is 50 ms or less. The fact that SONET/SDH rings re-store traffic in quite literally the blink of an eye has significant consequences. It means that a click might be heard on a voice call, a digitally transmitted video might lose a few frames, or packet transfer would be momentarily disrupted. We will see later that ATM and MPLS technology is striving to meet the impressive restoration time performance of SONET/SDH rings. It is interesting to note that dense wavelength division multiplexing (DWDM) ring implementations also leverage the basic concepts of these SONET/SDH ring protection schemes.