The DOCSIS MAC Frame Format

282 ACCESS NETWORKS otherwise, cells transmitted from different ONUs might collide. A medium access protocol permits users to transmit in the upstream direction without collisions. The mechanism used for the downstream and upstream transmission is described below. An example of downstreamupstream transmission is given in Figure 11.19. The OLT transmits three cells: one for ONU A, one for ONU B, and one for ONU C. In Figure 11.19, a cell is represented by a square, with the name of the destination ONU written inside. The optical signal carrying these cells is split into three, and each ONU receives the same optical signal with all three ATM cells, of which it reads only the one destined for it. In the upstream direction, each ONU transmits one cell, and thanks to the medium access mechanism, the cells arrive at the OLT one after the other without any collisions. In this example, collisions can only occur on the link between the splitter, indicated by the circle, and the OLT. The link between an ONU and the splitter is not shared by other ONUs. Each cell transmitted by an ONU is propagated to the splitter with no possibility of colliding with cells from other ONUs. If all three of the ONUs transmit a cell at the same time and assuming that their distance from the splitter is the same, the cells will arrive at the splitter at the same time and will collide. The splitter will combine the three signals into a single signal, resulting in garbled information. As can be deduced from the above discussion, the splitter has two functions. On the downstream direction it splits the signal, and in the upstream direction it com- bines the incoming signals into a single signal. Thus, it works as a splitter and as a combiner at the same time. The downstream and upstream signals are transmitted on different wavelengths, and thus it is possible for both transmissions to take place at the same time. The optical line terminator OLT consists of an ATM switch, ATM interfaces to the backbone network, and ODN interfaces on the user side see Figure 11.20. Each ODN interface serves a different APON, and there are as many APONs as ODN interfaces. For instance, in the example given in Figure 11.18, there are N ODN interfaces and N different APONs, and in the example given in Figure 11.19 there is a single APON. APON was standardized by ITU-T in 1998 in recommendation G.983.1. APON has been defined by the full service access networks FSAN initiative as the common opti- cal transport technology. FSAN is an initiative from telecommunication operators and manufacturers formed in 1995 to develop a consensus on the system required in the local access network to deliver a full set of telecommunications services both narrowband and broadband. OLT C C ONU A ONU B ONU C B B A C B A C B B A C C B A A A Figure 11.19 An example of downstreamupstream transmission. THE ATM PASSIVE OPTICAL NETWORK 283 To the home To the network ATM switch ODN interfaces . . . ATM interfaces . . . Figure 11.20 The optical line terminator OLT. O L T ONT ONU Fiber FTTH FTTBC Fiber CopperVDSL ONU FTTCab Fiber CopperVDSL ONT ONT Figure 11.21 The G.983.1 network architecture. The G.983.1 network architecture is shown in Figure 11.21 Depending on the location of the ONU, we have the following three possible configurations: • Fiber to the home FTTH : The ONU is in the home. • Fiber to the basementcurb FTTBC : The ONU is in a building or a curb. Distribution to the home is done over copper using ADSL or VDSL. • Fiber to the cabinet FTTCab : The ONU is in a cabinet, and distribution to the home is done over copper via ADSL or VDSL. The FTTBC and FTTCab are treated the same by the G.983.1 network architecture. An ONU terminates the optical access network and provides user-side interfaces over copper using ADSLVDSL. An optical network terminator ONT is the device used at the customer site. For simplicity we shall refer to the ONTs as ONUs. The APON architecture uses different wavelengths for the downstream and upstream transmissions. A wavelength is an optical signal channel carried across an optical fiber see Section 8.3.1. A transmitter emits a laser at a specific wavelength which is used as the carrier of the optical signal. The laser is modulated by the digital signal to produce an optical signal which is guided through the optical fiber to an optical receiver. The APON architecture uses two wavelengths for downstream transmission and one wave- length for upstream transmission. Specifically, for downstream transmission a wavelength in 1490 nm is used for transmitting ATM traffic and a second one in 1559 nm is used for video distribution. For the upstream transmission, it uses a wavelength in 1310 nm which 284 ACCESS NETWORKS is shared by all of the ONUs. The G.983.1 standard also permits the use of additional unidirectional fibers, operating at 1310 nm. Two transmission options can be used: symmetric and asymmetric. In the symmetric option, both the upstream and downstream transmission rate for ATM traffic is 155.52 Mbps. In the asymmetric option, the downstream and upstream transmission rate for ATM traffic is 622.08 Mbps and 155.52 Mbps, respectively. The maximum fiber distance from an ONU to an OLT is 20 km, the minimum supported number of splits for a passive splitter is 16 or 32, and the minimum number of ONUs supported in an APON is 64. These specifications will probably change as the technology evolves. APON can provide high-speed access for Internet traffic, voice over ATM, voice over IP and video services. APON can be deployed in new neighborhoods and municipalities. In a new neighborhood, the fiber can be laid at the same time as the infrastructure. Municipalities have an interest in providing high-speed connectivity to their residents, and they can easily deploy APONs by passing the fiber through existing underground conduits that lead close to the homes. Also, power companies are a potential provider since they can deploy the fiber using the existing poles that support the electrical cables

11.3.1 Frame Structures for Downstream and Upstream Transmission

The downstream transmission for both 155.52 Mbps and 622.08 Mbps consists of a con- tinuous stream of time slots. Each time slot consists of 53 bytes, and it contains either an ATM cell or a 53-byte physical layer OAM PLOAM cell. PLOAM cells are only used every 28th time slot. Their function is explained in the following section. Groups of time slots are organized into frames. The frame structure for 155.52 Mbps and 622.08 Mbps transmission rates is shown in Figure 11.22. For the 155.52-Mbps transmission rate, the frame consists of 56 time slots, two of which are PLOAM cells and the remaining 54 are ATM cells. As can be seen, the first time slot of the frame carries a PLOAM cell and the remaining 27 time slots contain ATM cells. This group of 28 time slots repeats once more within the frame. That is, the 29th time slot is a PLOAM cell, and the remaining 27 time slots carry ATM cells. The frame for 622.08-Mbps transmission rate consists of 244 time slots, of which eight are PLOAM cells, and the remaining 216 are ATM cells. The first time slot of a frame contains a PLOAM cell and the next 27 time slots contain ATM cells. This group of 28 time slots repeats seven more times within the frame. The frame structure for the upstream transmission is shown in Figure 11.23. The upstream frame consists of 53 time slots. Each slot consists of 56 bytes, of which the first 3 bytes are used for overheads and the remaining 53 bytes carry either an ATM cell a Frame structure for 155.52 Mbps—56 time slots . . . PLOAM Cell 1 Cell 27 PLOAM Cell 28 Cell 54 . . . b Frame structure for 622.08 Mbps—224 time slots PLOAM Cell 1 Cell 27 PLOAM Cell 189 Cell 216 . . . . . . . . . Figure 11.22 Frame structures for downstream transmission.