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As shown in the example given in Figure 11.18, the optical distribution network consists of optical fibers connected in the form of a tree. The signal transmitted from the OLT is passively split between multiple fibers, each leading to a different ONU. Passive splitters (i.e., without electronic components), indicated by circles in Figure 11.18, are used to split the signal. These are made by twisting and heating optical fibers until the power output is evenly distributed. When a signal is split, there is always power loss, which means that there is a limit on how many times it can be split.
An APON is a point-to-multipoint broadcast system in the downstream direction (i.e., from the OLT to the ONUs), and a multipoint-to-point shared medium in the upstream direction (i.e., from the ONUs to the OLT). The OLT transmits ATM cells which are received by all of the ONUs. The transmitted cells are scrambled using a churning key so that an ONU cannot read the cells destined to another ONU. Each ONU selects only the cells destined for it. In the upstream direction, only one ONU can transmit at a time;
Figure 11.18 An example of an APON.
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 downstream/upstream 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 combines 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 optical 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.
Figure 11.19 An example of downstream/upstream transmission.
THE ATM PASSIVE OPTICAL NETWORK
Figure 11.20 The optical line terminator (OLT).
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 basement/curb (FTTB/C): 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 FTTB/C and FTTCab are treated the same by the G.983.1 network architecture. An ONU terminates the optical access network and provides user-side interface(s) over copper using ADSL/VDSL. 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 wavelength 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