Traffic Grooming WAVELENGTH ROUTING NETWORKS

THE ITU-T G.709 STANDARD – THE DIGITAL WRAPPER 213 1 2 3 6 7 8 11 12 13 4 5 9 10 OXC 1 OXC 2 Figure 9.7 Path protection. that uses links {1, 6, 11} and its protection lightpath that uses links {3, 8, 13} do not use the same SRLG. That is, they are SRLG-disjoint. The concept of SRLG can also be used in the 1:N shared protection scheme. For instance, in Figure 9.7, the two working lightpaths {1, 6, 11} and {2, 7, 12} from OXC 1 to OXC 2 are SRLG-disjoint. Therefore, it makes sense that they both use the same SRLG-disjoint protection lightpath {3, 8, 13}. This is because a single failure of a physical resource along the path of either working lightpaths excluding the originating and termi- nating OXCs will not cause both working lightpaths to fail at the same time. That is, in this case, the protection lightpath will only be used by one of the two working lightpaths. In protection schemes, the backup protections routes are pre-planned and the neces- sary resources e.g. wavelengths, fibers, and bandwidth within an OXC are allocated in advance. During the normal operation of the network, these resources are either kept idle, or they are used to transmit low priority traffic which can be preempted any time a failure occurs. This guarantees a fast recovery from a failure at the expense of inefficient resource utilization. An alternative strategy, known as dynamic restoration, is to calculate a protection path and allocate resources for recovery at the moment when a network fail- ure occurs. This approach has a more efficient resource utilization but the recovery time is longer than in the case of a protection scheme. Dynamic restoration is a promising new approach that is being further studied.

9.3 THE ITU-T G.709 STANDARD – THE DIGITAL WRAPPER

Information on a lightpath is typically transmitted using SONETSDH framing. Also, Ethernet frames can be transmitted over an optical network. In the future, it is expected that information will be transmitted over the optical network using the new ITU-T G.709 standard, otherwise known as the digital wrapper. This standard defines the network node interfaces between two optical network operators, or between subnetworks of vendors within the same network of an operator. The following are some of the features of the G.709 standard: • Types of traffic : The standard permits the transmission of different types of traffic, such as IP packets and gigabit Ethernet frames using the generic framing procedure GFP, ATM cells, and SONETSDH synchronous data. • Bit-rate granularity : G.709 provides for three bit-rate granularities: 2.488 Gbps, 9.95 Gbps, and 39.81 Gbps. This granularity is coarser than that of SONETSDH, but is appropriate for terabit networks, since it avoids the large number of low bit-rate paths that would have to be used with SONETSDH. 214 WAVELENGTH ROUTING OPTICAL NETWORKS • Connection monitoring : G.709 also provides for connection monitoring capabilities that go beyond those of SONETSDH. Specifically, unlike SONETSDH, it is possible to monitor a connection on an end-to-end basis over several carriers, as well as over a single carrier. • Forward error correction FEC : As transmission rates increase to 10 Gbps and beyond, the physical parameters of the optical fiber play a significant role in the degradation of the transmitted optical signal. FEC can be used to detect and correct bit errors caused by physical impairments in the transmission links. FEC enables transmission at higher rates without degraded performance. It is useful for under-water transoceanic cables and intra-continental long-haul links. In ITU-T, an optical network is referred to as the optical transport network OTN. It consists of three layers: the optical channel Och, the optical multiplex section OMS, and the optical transmission section OTS. See Figure 9.8. The optical channel is an optical connection between two users, and it takes up an entire lightpath. Optical channels are mul- tiplexed and transmitted as a single signal over a fiber. The section between a multiplexer and a demultiplexer over which the multiplexed signal is transported is referred to as the optical multiplex section. Finally, the transport between two access points over which the multiplexed signal is transmitted is referred to as the optical transmission section. Each of the OTN layers is associated with a frame structure and appropriate overheads. Below, we examine the payload and overhead fields of the optical channel frame.

9.3.1 The Optical Channel Och Frame

The user data is transmitted in frames which contain several different types of overhead, the user payload, and the forward error correction FEC, as shown in Figure 9.9. The optical channel overheads are shown in Figure 9.10. The client’s payload is encapsulated with the Och payload unit OPU overhead which includes information related to the type of traffic submitted by the user. The resulting OPU is then encapsulated with the Och Och OMS OTS OTS OTS Figure 9.8 The OTN layer structure. Och overhead Och payload FEC Figure 9.9 The optical channel Och frame. THE ITU-T G.709 STANDARD – THE DIGITAL WRAPPER 215 Client payload Client payload OPU OH OPU ODU FEC OPU ODU OTU ODU OH OTU OH Figure 9.10 The optical channel Och overheads. data unit ODU overhead, which provides information for tandem connection monitoring, and end-to-end path supervision. Finally, the resulting ODU is encapsulated with the Och transport unit OTU overhead, which includes information for the monitoring of the signal on a section. The ODU is also encapsulated with the FEC. As in the SONETSDH frame, the OTU frame is arranged in a matrix consisting of four rows of 4080 bytes each see Figure 9.11. The data is transmitted serially row by row starting from the left of the first row. Recall that a SONETSDH frame is transmitted every 125 µsec. Higher transmission rates in SONETSDH are achieved by increasing the size of the SONETSDH frame. Unlike SONETSDH, the OTU frame size does not change as the transmission speed increases. Higher transmission rates are achieved by simply increasing the transmission rate of the OTU frame. This is a main departure from the traditional 125 µsec concept, that has been used in communication networks. Three transmission rates have been defined for the transmission of OTU frames: 2.488 Gbps, 9.95 Gbps, and 39.81 Gbps. The time to transmit an OTU frame is 48.971 µsecs when the transmission rate is 2.488 Gbps, 12.191 µsecs when the transmission rate is 9.95 Gbps, and 3.035 µsecs when the transmission rate is 39.81 Gbps.

9.3.2 Overhead Types

The OPU overhead The OPU overhead fields are located in rows 1 to 4 and columns 15 and 16. They provide information related to the client signal; that is, the data transmitted by the user. This overhead is created at the point where the client signal is originated, and it is used at 1 7 8 14 15 16 17 3824 3825. . .4080 Row Column byte 1 FAS OTU OH ODU OH Client payload O P U O H 2 3 4 FEC Figure 9.11 The format of the OTU frame.