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Recent technological advances and economies of scale have drawn increasing interest to optical-distribution networks with ATM PON. A functional overview of ATM–PON architecture is presented in
Figure 2. Functional Overview of ATM–PON Architecture
Figure 2 shows the ONT placed at the customer premises, which suggests FTTH/B architecture. The carrier's demarcation point would be the subscriber side of the ONT, typically in the form of a T1, Ethernet, integrated services digital network (ISDN), plain old telephone service (POTS), etc.
For FTTCab and FTTC architecture, an optical network unit (ONU), rather than an optical network termination (ONT), is used. It is placed in the outside plant and must be temperature-hardened and properly enclosed. The final drop to the network termination (NT) at the customer premises may be copper or fiber. The carrier demarcation point is the subscriber side of the NT in the form of a T1, Ethernet, ISDN, POTS, and etc.
Access to bandwidth on the PON may be obtained by several methods, including time division multiple access (TDMA), wave division multiple access (WDMA), code division multiple access (CDMA), and subcarrier multiple access (SCMA). TDMA in the upstream and TDM in the downstream were chosen by the Full-Service Access Network (FSAN) group and submitted to the International Telecommunications Union (ITU) for standardization, based on their simplicity and cost-effectiveness.
As shown in Figure 2, the network components supporting ATM PON consist of OLT, ONT, and a passive optical splitter. One fiber is passively split up to 64 times between multiple ONTs that share the capacity of one fiber. Passive splitting requires special actions for privacy and security, and a TDMA protocol is necessary in the upstream direction. The use of the optical splitter in the PON architecture allows users to share bandwidth, thus dividing the attendant costs. Costs are further reduced by a decrease in the number of opto-electronic devices needed at the OLT; one interface may be shared among many ONTs.
The ATM–PON system uses a double-star architecture. The first star is at the OLT, where the wide-area network interface to services is logically split and switched to the ATM–PON interface. The second star occurs at the splitter where information is passively split and delivered to each ONT. The OLT is typically located in the carrier's CO. The OLT is the interface point between the access system and service points within the carrier's network. When data content from the network reaches the OLT, it is actively switched to the passive splitter using TDM in the downstream. The OLT behaves like an ATM edge switch with ATM–PON interfaces on the subscriber side and ATM–synchronous optical network (SONET) interfaces on the network side.
The ONT will filter the incoming cells and recover only those that are addressed to it. Each ATM cell has a 28-bit addressing field associated with it called a virtual path identifier/virtual channel identifier (VPI/VCI). The OLT will first send a message to the ONT to provision it to accept cells with certain VPI/VCI values. The recovered ATM cells are then used to create the service interface required at the subscriber side of the ONT (see Figure 2).
Because TDMA is used in the upstream direction, each ONT is synchronized in time with every other ONT. The process by which this happens is called ranging the ONTs. Basically, the OLT must determine how far away in distance each ONT is so they can be assigned an optimal time slot in which to transmit without interfering with other ONTs. The OLT will then send grant messages via the physical layer operation, administration, and maintenance (PLOAM) cells to provision the TDMA slots that are assigned to that ONT. The ONT will then adapt the service interface to ATM and send it to the PON using the TDMA protocol.
Ethernet and T1s are two examples of what can be transported over the ATM–PON. As ATM–PON is service-independent, all legacy services and future services can be readily transported.
The basic frame format between the OLT and ONT for the symmetrical 155 Mbps rate is shown in Figure 3.
Figure 3. ATM–PON Frame Formats
The asymmetrical version of 622 Mbps/155 Mbps downstream/upstream is similar but beyond the scope of this document.
As can be seen in Figure 3 the downstream payload capacity is reduced to 149.97 Mbps because of the PLOAM cells. These cells are responsible for allocating bandwidth (via Grant cells), synchronization, error control, security, ranging, and maintenance.
In the upstream direction the capacity is reduced to 149.19 Mbps because there are 3 overhead bytes per ATM cell. In addition to the three overhead bytes per cell there are PLOAM cells in the upstream direction, the rate of which is defined by the OLT for each ONT, depending on the required functionality. The minimum PLOAM rate in the upstream direction is one PLOAM every 100 ms. This equates to approximately one PLOAM every 655 frames, which is negligible. Although the maximum PLOAM rate is undefined, it is also expected to be negligible. The 3 overhead bytes contain a minimum of 4 bits of guard time to provide enough distance in time to prevent collisions with cells from other ONTs. This field length is actually programmable by the OLT. The preamble field is used to acquire bit synchronization and amplitude recovery. The Delimiter field is used to indicate the start of an incoming cell.
Given that a single fiber is used for both the upstream and downstream paths, two wavelengths of light are used—1550 nm for the downstream and 1310 nm for the upstream. Although one wavelength can also be used, two provide better optical isolation between the laser transmitters and receivers and eliminate the need for expensive beam-splitting devices. Instead, low-cost planar light circuits (PLCs) can be used, which enable low-cost manufacturing techniques to be employed, somewhat similar to the production of silicon chips. ATM cells are directly converted to light and sent to the PON. Because of the broadcast nature of the PON, encryption techniques are employed to prevent security breaches. In the upstream direction, the ONT uses the TDMA protocol and again directly converts ATM cells to light for transport over the PON (see Figure 2).
A typical ATM–PON system can furnish up to 64 customer locations on a single, shared strand of fiber running at 155 Mbps. Most, however, will likely utilize 32 locations in the distribution and drop portion of the network in the near term. In the future, the ATM–PON specification does allow for up to 64 locations to be served.
