## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.4 F2g--2000-2005 (Wideband Era)  
**EPON--IEEE 802.3 ah:** The tremendous increase of Ethernet traffic due to the wide use of low-cost Ethernet LANs necessitated the deployment of the Ethernet-based PON (EPON) (2004) to convey Ethernet traffic with all the Ethernet features being fully supported [15]. This is because the Ethernet protocol was supplemented by QoS techniques (such as traffic prioritization at the MAC sublayer and virtual LANs) whereby voice, data and video are supported in a reliable way. EPON was rapidly adopted in Asia (first in Japan) and later in Europe. EPONs have the ability to carry variable-length Ethernet frames (packets) up to1518 bytes, as opposed to the fixed-length packets of BPONs, while they can also carry ATM cells. In EPONs, the reliability of the Ethernet is combined with the advantages of ATM technology in terms of service quality, and therefore EPONs appear as the predominant access networks over BPONs. _Splitting ratio and Maximum reach:_ The passive device is connected to the OLT with a single-mode optical fiber over a maximum distance of 20 km. The standard splitting ratio is 1:16 or 1:32 (when Reed-Solomon-based Forward Error Control is used) [6], while 1:64 or even 1:128 is possible to support many users over short distances. _DS/US Rates and Wavelengths:_ Both the downstream and upstream transmission is based on the Ethernet protocol IEEE 802.3, and the rate is 1.25 Gb/s (1 Gb/s data rate, plus 0.25 Gb/s for line encoding 8B/10B). The wavelength ranges of 1480-1500 nm and 1260-1360 nm are used in the downlink and the uplink, respectively [12]. _QoS:_ Packets transmitted by the OLT in the downlink reach all users, but they are only received by users who match the logical link identifier (LLID) [10]. The latter is a field that precedes the header of the Ethernet frame and is defined for each user at the time of \"registration\" on the PON. Variable length packets are placed in fixed duration frames of 2 us (typical duration). In the upstream direction, data frames from one user only reach the OLT, thanks to the directional properties of the passive device (optical combiner). That is, in the uplink the behavior of EPONs is similar to that of point-to-point architecture except that simultaneously transmitted frames by different users may collide. Therefore, an additional medium access control (MAC) mechanism is needed for collision avoidance and fair bandwidth allocation of the common link capacity (between the OLT and the passive device). All users should be synchronized and know which time slot is allowed to transmit. Each time slot has the ability to carry several Ethernet frames and can be of either fixed or variable size. When the time slot corresponding to a specific user arrives, data (Ethernet frames) are sent in burst. If there are no data to send, the user sends an empty frame [16]. The assignment of a timeslot to a user can either be static (using TDMA) or dynamic, using a dynamic bandwidth allocation/assignment (DBA) mechanism [17]. Using a DBA mechanism, the PON can support more users (due to efficient bandwidth management), while the users can enjoy enhanced services, requiring variable data rates that cannot be offered reasonably under static bandwidth allocation. The DBA is based on the queue length created by each user to store the data. The role of collecting information about the queue length and assigning timeslots to each user belongs to the OLT. Although this mechanism increases the transmission overhead, it leads to an efficient utilization of the common link capacity [18]. The multi-point control protocol (MPCP) is a MAC layer signaling protocol between the OLT and ONUs/ONTs for inter-ONU scheduling and service class support. MPCP consists of three functions whereby the PON manager/vendor can readily implement the desired DBA scheme [19]:  
* _Discovery Processing_: PON users are detected and registered.