## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs  
The first PON configurations are based on time-division multiplexing (TDM). In TDM-PONs [10], the passive device operates as splitter in the downstream direction, while it operates as a combiner in the upstream direction. TDM-PONs use one transceiver at the OLT for all ONUs/ONTs, which need synchronization with the operating frequency of the OLT. Each ONU/ONT possesses its own transceiver. New users can readily be added to TDM-PONs. Figure 6 illustrates a typical TDM-PON where the PON operation in the uplink is distinguished from that in the downlink [11]. The PON comprises an OLT and (as an example) three ONUs/ONTs, while different wavelengths are used per traffic flow direction. The ONUs/ONTs buffer the data of their users to be transmitted all together (burst) to the OLT at the assigned time slots in the uplink, as shown in the upper part of Figure 6. Due to the different distances between the OLT and the ONUs/ONTs, the OLT optical receiver must be adapted to different received powers and align the different received phases of the bursts (signals). To accomplish them, the transmission overhead from the ONUs/ONTs increases and thus the throughput decreases. On the other hand, the transmission from the OLT down to the ONUs/ONTs is much easier. As it is shown in the lower part of Figure 6, the same frame arrives to all ONUs/ONTs but each ONU/ONT selectively receives its own data based on an address (ONU/ONT id). The following TDM-PON configurations have been standardized by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) [12]:  
Figure 6: The uplink and downlink frame of a TDM-PON. **APON/BPON--ITU-T Rec. G.983.x:** APON (ATM-PON) is the first PON which was standardized (1998) based on ATM (asynchronous transfer mode) technology because the telecommunications community considered ATM to be the leading technology in backbone networks [10, 13, 14]. ATM has the advantage of providing QoS guarantees for voice and data transmission services (either broadband or narrowband services). BPON (1999) is an enhanced version of APON, adding dynamic bandwidth allocation. With the advent of APON/BPON, the use of SDH/SONET systems or the legacy access services (e.g., DS1, DS2, E1, E2) could not be justified economically. _Splitting ratio and Maximum reach:_ In the passive device, the splitting ratio is 1:32 or 1:64. Progress in the field of photon technology allows for the use of passive devices where the splitting ratio reaches 1:128 or even 1:256. Erbium-doped fiber amplifiers (EDFAs) are used to amplify the signals. The passive device is connected to the OLT with a single-mode optical fiber over a maximum distance of 20 km. As each user is at a different distance from the OLT, ONUs should be carefully synchronized to avoid conflicts. _DS/US Rates and wavelengths:_ In the downstream (DS), the transmission rate was standardized at 155.52 Mb/s or 622.08 Mb/s, making use of a wavelength in the range of 1480-1500 nm; in the upstream (US), the standard transmission rate was 155.52 Mb/s, using the wavelength range of 1260-1360 nm [12]. Moreover, a symmetrical rate between uplink and downlink exists at 622.08 Mb/s. In the link layer, the frame format used in the downlink is different than that in the uplink in the case of different transmission rates. Specifically, in the 155.52 Mb/s downlink, the frame is composed by a concatenation of 56 ATM cells of 53 bytes (the 1 st and the 29 th ATM cells are PLOAM (Physical Layer Operation Administration and Management) cells. The latter mainly carry messages of transmission opportunities, which are granted by the OLT to the PON-users; that is, the ONU/ONT is informed of the start time and duration of the next uplink frame, which can be used for transmission within the time division multiple access system. In the 622.08 Mb/s downlink, the frame is formulated by concatenating four 155.52 Mb/s frames.