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The growing demands for high speed connectivity to keep pace with bandwidth intensive applications and services have spawned the idea of developing PONs with capabilities beyond those of copper and wireless-based technologies in access network. In this article, an approach for the design of an energy efficient bandwidth allocation mechanism for the shared upstream communication link in the Fiber to the Home (FTTH) access network is presented and evaluated using Mixed Integer Linear Programming (MILP) model. In the MILP model, two objective functions for minimization of power consumption and minimization of blocking were evaluated. The results have shown that with the objective of power minimization approach, Optical Network Terminals (ONTs) are efficiently grouped to the minimum number of active networking Optical Line Terminal (OLT) switches, traffic is groomed, ports are efficiently utilized, and hence total power consumption is minimized. Results have shown that with energy efficient bandwidth allocation approach consideration, energy savings can reach up to 80% for different examined traffic loads following uniform distribution.

The unprecedented growth in the number of internet users along with the emergence of a plethora new bandwidth-hungry applications and services have led to significant research efforts over the past two decades in order to set new requirements for future high-speed networks [

PONs have shown great performance compared to copper and wireless access network. PON has overcome many limitations and shortcomings appeared in conventional access network technologies [

Over the last decade, many studies have been demonstrated by the FTTH European Council to predict the upstream and downstream of traffic growth. One latest study has reported that the 2020 demand estimation for downlink and uplink traffic growth is 8 Gbyte and 3 Gbyte per day respectively [

Different studies with different methodologies and approaches appeared in previous work to improve the design of PONs network and mainly tackled two issues; efficient dynamic bandwidth allocation and power consumption. Wavelength grouping for efficient utilization of resources in PON networks has been studied thoroughly in [

In this paper, Section II shall present a review on GPON FTTH conventional network design covering concepts, technologies, and optical cable distribution network design. Problem statement, network design and the optimization approach used to solve the problem are discussed in Section III. Section IV describes the optimization mathematical model implemented with emphasis on minimization of power consumption. Section V demonstrates obtained results with technical discussion. In Section VI, the paper is concluded.

State of art GPON networks depicted in

OLT switch as depicted in

on each GPON Port [

One OLT chassis as the depicted in

ONTs units are installed in each of the subscribers’ premises and one ONT as shown in

The network cable distribution design is based on distributed splitting, where several different configurations of cascaded passive optical splitters are planned in every route to meet a pre-designed split ratio from the exchange office to end users. The cable distribution network starts from the exchange office and terminates at the end users’ premises.

Four categories of fiber cables are considered in the design; Primary Optical feeder cable, Secondary Optical feeder cable, distribution Optical cable and Drop Optical cable.

route where 24 strands of the primary feeder optical cable are spliced with a 24-core secondary feeder fiber cable. A total of four of 24-core optical cables are jointed at 4 different intersection points to pass through the various main blocks of the sub-areas.

At the Block intersection, one fiber from each of the 24 fibers of the secondary optical fiber cable is spliced with 1 × 4 passive optical splitter. The four outputs of the splitter are spliced with 4 cores of a distribution cable. Each of the fibers of the distribution cable is spliced with 1 × 8 passive splitter located near 8 subscribers. Drop of cables is laid from each of the 8 outputs of the splitter to connect to an ONT device in the subscriber home. The output of the 1:8 splitters, a drop cable of 1 fiber strand in case of single family premise or 2 strands in case of multistoried building will be laid up to the building.

In conventional FTTx PON designs as depicted in

The solver program used is CPLEX [

In this section, two objective functions are evaluated in the MILP optimization model; minimization of power consumption and minimization of blocking. The

objective function with the minimization of the total power consumption aims to minimize total active power consuming components needed to service all queued demands through means of dynamic bandwidth assignment and energy efficient grouping in the upstream direction from ONUs to ISPs via the minimum possible number of OLT ports and switches. Minimization of the total power consumption shall not only result in minimization of total number of access modules (AM) ports and OLT switches but also will result in maximization of throughput in the upstream direction through efficient utilization and assignment of uplink resources.

The second objective function with the minimization of total blocked demands aims to minimize the total number of rejected requests and hence maximize the total number of served demands.

The parameters and variables used in the model are defined in

Before introducing the constraints and quantities used in the model, evaluated objective functions are defined as.

∑ i ∈ T γ i ( Φ + ∇ ) + Γ ∑ i ∈ P ∑ j ∈ A ( ( ∑ s ∈ P ∑ d ∈ G λ i j s d ) / C ) + φ ∑ i ∈ O β i (1)

The objective function with the minimization of the total power consumption aims to minimize total active power consumption from components such as total number of active OLT switches, controller cards, total number of OLT cards, total number of AM ports, and total number of active ONT units.

∑ s ∈ P ∑ d ∈ G Λ s d (2)

The objective function with the minimization of total blocked demands aims to minimize the total number of rejected requests to maximize the total number

N | Set of all vector nodes. |
---|---|

N_{x} | Set of neighbor nodes of node x, where ( x ∈ N ). |

T | Set of OLTs. |

P | Set of PONs representing a splitter/coupler connecting group of homes. |

G | Set of Gateways at central office connecting OLTs with ISPs. |

O | Set of ONT Units connecting homes. |

A | Set of access ports. |

C | Wavelength capacity in the upstream flow from a PON to an OLT port. |

λ i j | Data rate demand between every pair of source and destination ( i , j ) . |

φ | Denotes power consumption of an ONU. |

Φ | Denotes power consumption of an OLT chassis. |

∇ | Denotes power consumption of OLT’s controller card (Local Manager). |

Γ | Denotes power consumption of an OLT AM port. |

M | Large Multiplication constant, M = 1000 . |

Κ | Small Multiplication constant, K = 10 . |

( s , d ) | Denotes source and destination vector nodes. |

( x , n ) | Denotes end points of a physical link. |

λ x n s d | Portion of traffic demand ( s , d ) traversing physical link ( x , n ) . |
---|---|

γ i | Binary variable equals 1 to indicate OLT switch is active, otherwise equal 0. |

Λ s d | Binary variable equals 1 if demand L s d is not served, otherwise equals 0. |

ζ i j | Binary variable equals 1 to indicate if OLT port is active, otherwise equals 0. |

β i | Binary variable equals 1 to indicate if ONT is active, otherwise equals 0. |

ψ i | Number of activated ports in OLT switch i. |

of served demands.

These objective functions are subject to the below constraints.

∑ n ∈ N x x ≠ n λ x n s d – ∑ n ∈ N x x ≠ n λ n x s d = { λ s d ( 1 − Λ s d ) x = s − λ s d ( 1 − Λ s d ) x = d 0 otherwise ∀ s ∈ P , ∀ d ∈ G and ∀ x ∈ N (3)

Constraint (3) is the flow conservation constraint. It ensures that the total traffic going into a node is equal to the total traffic leaving it for all nodes except the source and destination of a demand.

∑ s ∈ P ∑ d ∈ G λ i j s d ≤ C ∀ i ∈ P and ∀ j ∈ A (4)

Constraint (4) ensures that the upstream traffic shall not exceed the capacity of the uplink wavelength between PONs and OLT ports.

( ∑ s ∈ P ∑ d ∈ G λ i j s d ) / C ≤ 1 ∀ i ∈ P and ∀ j ∈ A (5)

Constraint (5) ensures that the utilization of the uplink wavelengths between PONs and OLT ports are not exceeded.

∑ s ∈ P ∑ d ∈ G λ s d ≤ ∑ x ∈ T ∑ s ∈ P ∑ d ∈ G ∑ n ∈ N x λ n x s d (6)

Constraint (6) ensures that the total number of queued demands at the ONTs is equal to the total number of served demands by the OLTs. This constraint is enforced to prevent blocking of any demand to occur.

∑ s ∈ P ∑ d ∈ G ∑ n ∈ N x λ n x s d ≥ γ x (7)

M γ x ≥ ∑ s ∈ P ∑ d ∈ G ∑ n ∈ N x λ n x s d (8)

T o t _ N u m _ O L T s = ∑ x ∈ T γ x (9)

∀ x ∈ T

Constraints (7) and (8) ensures that assigned OLT switches to serve ONTs are switched on, while Equation (9) counts the total number of active OLT switches.

( K ( ∑ s ∈ P ∑ d ∈ G λ x j s d ) / C ) ≥ ζ x j (10)

( K ( ∑ s ∈ P ∑ d ∈ G λ x j s d ) / C ) ≤ M ζ x j ∀ x ∈ P & ∀ j ∈ T (11)

ψ x = ∑ x ∈ P ζ x j ∀ j ∈ T (12)

ψ x ≤ χ ∀ x ∈ T (13)

Constraints (10) and (11) ensures used AM ports in OLT Switches are switched on, and constraint (13) ensures total number of active AM ports calculated in Equation (12) does not exceed total number of AM ports ( χ ) in OLT switch.

In this work, two objective functions are compared. First objective aims to minimize total power consumption through efficient grouping of ONT units to minimum possible number of OLT switches and AM ports. This allows the switch off of not used or idle switches and ports within the network. The second objective function is the objective with minimization of blocking with a constraint to satisfy all demand. There is no emphasis of the second objective model on the mechanism of how and when the resources are assigned. Hence, the resources are assigned in random manner.

The OLT switch used in the model is the Cisco ME4600 [

The results in Figures 6-10 have shown that as the rate or the size of average

demands increases, the number of activated AM ports and OLT switches increases. Hence the power consumed by the network is increasing as the size of demand is increasing. This result is common for the two objective functions. However, the percentage of increase is higher with the minimization of blocking objective as the assignment of resources with that approach is random. Random assignment is inefficient as links and hardware resources are underutilized. This can be clearly observed from

On the other hand, minimization of blocking objective approach is inefficient as demands are randomly assigned to OLTs. This method provisions all ONTs with resource to communicate with ISP through OLT switches, however links and hardware resources are underutilized especially at low rates.

For a comparison on the efficiency of the utilization of resources between the two objectives, number of active OLT switches evaluated with average rates between 300 and 900 Mb/s for the minimization of power consumption is 1 with average utilization ranges between 25% and 90%. While the objective of minimization of blocking has shown that the number of activated OLT switches for the same average range of rates is 4 with 10% to 50% of utilization of resources.

The rapid growth in bandwidth-hungry applications and services has set new requirements for high speed infrastructure in access network to overcome many limitations appeared in current implemented technologies. PONs in access network in the last decade were found as a premium solution to resolve many challenges appeared with the conventional wireless and copper-based designs. PONs have shown a proven performance in providing high per user access rate and reducing the overall network power consumption. In this work, a mathematical optimization approach was presented to reduce the power consumption through means of consolidation and efficient utilization of network and hardware resources. Different loads of traffic rate have been evaluated following uniform distribution. Hardware and network resources have shown efficient utilization and power savings results have reached 80% for the approach with minimization of power consumption objective when compared with conventional approaches in network designs such as the approach of minimization of blocking. The work presented in this article has been limited to mathematical models. However, for a future work, computer simulations to design heuristics to validate the results obtained from the MILP mathematical models shall be of great interest.

The author declares no conflicts of interest regarding the publication of this paper.

Hammadi, A.A. (2020) A Framework for an Energy-Efficient Bandwidth Allocation Approach through Dynamic ONTs Grouping in Flexible GPON Access Networks. Int. J. Communications, Network and System Sciences, 13, 1-14. https://doi.org/10.4236/ijcns.2020.131001