56 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 Table 1 available bandwidth among the users,whilst the upstream MAC EPON versus GPON layer is based on TDMA. Features EPON GPON GPON supports two layers of encapsulation where the Ethernet frame is encapsulated into a GPON Encapsulation Method (GEM) Standard IEEE nu-T frame which is encapsulated again into a GPON Transmission Transmission speed DS:1.2 Gbps DS:1.2/2.4 Gbps Convergence(GTC)frame.The GTC frame also includes pure ATM US:1.2 Gbps US:1.5/6.2/1.22.4Gbps cells and TDM traffic.The downstream frame is broadcast to every Split ratio 1:16 1:64 Line code 8B/10B NRZ ONU and the ONUs use the information in the Physical Control Protocol Ethernet ATM Block downstream (PCBd)field to extract its own data.In case Security Not guaranteed AES there is no data to be transmitted,the downstream frame will be Qos Not supported Supported transmitted continuously and utilized for time synchronization FEC Optional RS(255,239) Optional RS(255,239) (Ricciardi et al 2012).The upstream frame contains multiple transmission bursts arriving from the ONUs.Along with the pay- Downstream load,each of the upstream burst frames consists of the Physical Layer Overhead (PLOu),a bandwidth allocation interval which LLID Start Length LUD Start Length contains the Dynamic Bandwidth Report upstream (DBRu),and allocation identifiers (Alloc-IDs).When traffic reaches the OLT, ONU traffic is queued based on Classes of Service (CoS)with a 100 200 400 100 diverse QoS dependent on the type of the Traffic Containers (T- CONTs)that is specified in the Alloc-ID(Segarra et al,2013).GPON introduces five types of T-CONTs that provide QoS in the upstream direction.The T-CONT frame is used in GPON to establish a virtual connection between ONU and OLT as well as to manage fragment transmission. Upstream LUID 2 (ONU 1) (ONU 2) 1)T-CONT type 1 Supports fixed bandwidth that is sensitive to time.The jitter of (a)EPON frame structure. T-CONT type-1 is 0 which enhances the suitability it has for Constant Bit Rate (CBR)traffic. 2)T-CONT type 2 Frame Header (PBCd) Downstream UP BW map Payload This type supports Assured bandwidth where it has a higher delay than T-CONT 1.It is used with Committed Information Rate (CIR)traffic. Alloc-ID Start End Alloc-ID Start End 3)T-CONT type 3 Supports assured and non-assured bandwidths providing a 200 400 500 600 guaranteed minimum CIR and surplus Excess Information Rate (EIR).This type is appropriate for Variable Bit Rate(VBR)traffic that does not guarantee delay. 4)T-CONT type 4 Supports Best-Effort services such as Internet browsing.SMTP and FTP Upstream T-CONT 1 T-CONT 2 (ONU 1) (ONU 2) 5)T-CONT type5 This type is mix of all the above T-CONT types.It is appropriate (b)GPON frame structure. for general traffic flows(Begovic et al.,2011:Tanaka et al.,2010: Ricciardi et al,2012:Selmanovic and Skaljo,2010). Fig.4.Frame structure (a)EPON.(b)GPON. ONUs are located at different distances from the OLT as shown 2.2.Data link layer in Fig.5(a).When each ONU transmits its upstream traffic during the assigned time slot,there is a possibility that frames from dif- Fig.4(a)presents the EPON frame structure which uses the ferent ONUs collide at some point due to the difference in pro- native Ethernet frame to transmit traffic.The downstream MAC pagation delay.This scenario is illustrated in Fig.5(b).In order to layer has the same operation as a standard Gigabit Ethernet MAC guarantee that the upstream transmissions do not collide,a ran- (GbE MAC).where the traffic is broadcast to all users.In the ging process is performed by the OLT during the activation and downstream frame,the preamble field contains a logical link registration of the ONUs.The ranging process is based on calcu- identifier(LLID)which is a unique identifier assigned by the OLT to lating a specific delay time for each ONU according to its distance each ONU.The ONUs identify received traffic by matching the LLID from the OLT to equalize its transmission delay with other ONUs. of the received frame with its own LLID and if there is a match This delay is called Equalization Delay(ED).Each ONU will store then it will accept the received frame,otherwise it is discarded. and apply its ED to all the upstream transmissions.The ED values For upstream traffic,the MAC layer has been modified by the IEEE are broadcast to other ONUs using Physical Layer Operations and to operate using a TDMA approach,where the OLT assigns a spe- Maintenance (PLOAM)messages and each ONU resumes its cific time slot to every ONU taking into account the distance transmission based on the ED.Fig.6 shows an ONU in a ranging state.While one ONU is active and sending traffic,transmissions between each ONU and the OLT(Chen.2012). from other ONUs must be suspended (Kramer,1999). Fig.4(b)shows the frame structure of GPON.The downstream Multipoint control protocol (MPCP)has been introduced to MAC layer operates in the same manner as a GFP-framed SONET.It facilitate dynamic bandwidth allocation process.This is executed supports a frame of 125 us long that uses TDM to divide the at the MAC layer(Chochliouros,2009).For EPON,MPCP can be run2.2. Data link layer Fig. 4(a) presents the EPON frame structure which uses the native Ethernet frame to transmit traffic. The downstream MAC layer has the same operation as a standard Gigabit Ethernet MAC (GbE MAC), where the traffic is broadcast to all users. In the downstream frame, the preamble field contains a logical link identifier (LLID) which is a unique identifier assigned by the OLT to each ONU. The ONUs identify received traffic by matching the LLID of the received frame with its own LLID and if there is a match then it will accept the received frame, otherwise it is discarded. For upstream traffic, the MAC layer has been modified by the IEEE to operate using a TDMA approach, where the OLT assigns a spe￾cific time slot to every ONU taking into account the distance between each ONU and the OLT (Chen, 2012). Fig. 4(b) shows the frame structure of GPON. The downstream MAC layer operates in the same manner as a GFP-framed SONET. It supports a frame of 125 ms long that uses TDM to divide the available bandwidth among the users, whilst the upstream MAC layer is based on TDMA. GPON supports two layers of encapsulation where the Ethernet frame is encapsulated into a GPON Encapsulation Method (GEM) frame which is encapsulated again into a GPON Transmission Convergence (GTC) frame. The GTC frame also includes pure ATM cells and TDM traffic. The downstream frame is broadcast to every ONU and the ONUs use the information in the Physical Control Block downstream (PCBd) field to extract its own data. In case there is no data to be transmitted, the downstream frame will be transmitted continuously and utilized for time synchronization (Ricciardi et al., 2012). The upstream frame contains multiple transmission bursts arriving from the ONUs. Along with the pay￾load, each of the upstream burst frames consists of the Physical Layer Overhead (PLOu), a bandwidth allocation interval which contains the Dynamic Bandwidth Report upstream (DBRu), and allocation identifiers (Alloc-IDs). When traffic reaches the OLT, ONU traffic is queued based on Classes of Service (CoS) with a diverse QoS dependent on the type of the Traffic Containers (T￾CONTs) that is specified in the Alloc-ID (Segarra et al., 2013). GPON introduces five types of T-CONTs that provide QoS in the upstream direction. The T-CONT frame is used in GPON to establish a virtual connection between ONU and OLT as well as to manage fragment transmission. 1) T-CONT type 1 Supports fixed bandwidth that is sensitive to time. The jitter of T-CONT type-1 is 0 which enhances the suitability it has for Constant Bit Rate (CBR) traffic. 2) T-CONT type 2 This type supports Assured bandwidth where it has a higher delay than T-CONT 1. It is used with Committed Information Rate (CIR) traffic. 3) T-CONT type 3 Supports assured and non-assured bandwidths providing a guaranteed minimum CIR and surplus Excess Information Rate (EIR). This type is appropriate for Variable Bit Rate (VBR) traffic that does not guarantee delay. 4) T-CONT type 4 Supports Best-Effort services such as Internet browsing, SMTP and FTP. 5) T-CONT type5 This type is mix of all the above T-CONT types. It is appropriate for general traffic flows (Begovic et al., 2011; Tanaka et al., 2010; Ricciardi et al., 2012; Selmanovic and Skaljo, 2010). ONUs are located at different distances from the OLT as shown in Fig. 5(a). When each ONU transmits its upstream traffic during the assigned time slot, there is a possibility that frames from dif￾ferent ONUs collide at some point due to the difference in pro￾pagation delay. This scenario is illustrated in Fig. 5(b). In order to guarantee that the upstream transmissions do not collide, a ran￾ging process is performed by the OLT during the activation and registration of the ONUs. The ranging process is based on calcu￾lating a specific delay time for each ONU according to its distance from the OLT to equalize its transmission delay with other ONUs. This delay is called Equalization Delay (ED). Each ONU will store and apply its ED to all the upstream transmissions. The ED values are broadcast to other ONUs using Physical Layer Operations and Maintenance (PLOAM) messages and each ONU resumes its transmission based on the ED. Fig. 6 shows an ONU in a ranging state. While one ONU is active and sending traffic, transmissions from other ONUs must be suspended (Kramer, 1999). Multipoint control protocol (MPCP) has been introduced to facilitate dynamic bandwidth allocation process. This is executed at the MAC layer (Chochliouros, 2009). For EPON, MPCP can be run Table 1 EPON versus GPON. Features EPON GPON Standard IEEE ITU-T Transmission speed DS: 1.2 Gbps DS: 1.2/2.4 Gbps US: 1.2 Gbps US: 1.5/6.2/1.2/2.4 Gbps Split ratio 1:16 1:64 Line code 8B/10B NRZ Protocol Ethernet ATM Security Not guaranteed AES QoS Not supported Supported FEC Optional RS (255,239) Optional RS (255,239) Fig. 4. Frame structure (a) EPON, (b) GPON. 56 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74