Architecture Of Access Network

Published Date: 02 Nov 2017

The access part of the network, a public switched network, in which each subscriber access nodes. Network access can be defined as the last link in the network to the customer premises and the first point of contact with the network infrastructure. [20]

In the access network, also known as the "first-mile network" linking the central service offices (CO), business and residential subscribers. Demand for bandwidth in the access network has grown rapidly in recent years [21].

3.1.1 Architecture for access network

Where,

H = Hub

RN = Remote Node

NIU = Network

Interface

Unit

Figure 3.1: Architecture of access network [22]

The Hub is usually part of a larger network, but can be seen as the source of data in the network interface units (une) and the purpose of the data network interface units. The network interface unit can be placed on the site can serve a number of a subscriber or subscribers.

The different combinations of services and network topologies are possible, the broadcasting service supports broadcast or switched network and dial-up service. The broadcasting network, the remote node (RN) transmissions received Hub for all network interfaces. The switched network, a node processes the data off of the shaft, and then sends the traffic to different flow units for each network interface [22].

3.1.2 Classification for access networks

The access network can be classified as distribution network and feeder network.

Table 3.1: Classification of access network [22]

Distribution

Network

Feeder Network

Shared bandwidth

Dedicated bandwidth

Broadcast

CATV (HFC), TPON

WPON

Switched

Telephony, DSL,WRPON

Distribution network can be further classified as broadcast network or switched network.

Broadcast networks are suitable for the provision of distribution services and has the advantage of the same network interface units, each subscriber. Broadcast networks can be cheaper than the networks.

Switched networks are more suitable for switched services, and greater security. Troubleshooting networks is usually easier because of the intelligence of the network instead of network interface units, the broadcast networks. The switching network, a network interface units can be easier than the transmission network.

Feeder Electronic network bandwidth is shared or dedicated bandwidth. The disadvantage of the electric network bandwidth is shared by the requirement that each network interface unit operation of the entire network bandwidth. Dedicated bandwidth and QoS guaranteed network interface units. [22]

3.2 OPTICAL ACCESS NETWORK

The optical access network to the access network is implemented using optical fiber. Optical Access has improved access to the network bandwidth up to several gigabits per second (Gbps). The optical access network operates, manages, maintains and distributes a unique public access provisions and the physical layer of a multi-wavelength optical fiber wide area network services. Optical Network enables operators to deploy an optical platform services metro, regional and long-haul backbone networks to maximize service flexibility, minimizing bandwidth costs, simplify network operations and consolidate network architectures [23].

3.2.1 Optical access network scenarios

The optical access network comprises the following scenarios:

1. FTTB Scenario

The scenario for business users to access, fiber to the business (FTTB) scenario is divided into business units (SBU) and business multi-tenant unit (MTU) of capacity. SBU is a relatively small number of these ports.

2. FTTC & FTTCab Scenario

The access to the sidewalk or deck over fiber, fiber to the curb and fiber to the cabinet (FTTC and FTTCab), we assumed that the small dwelling unit (MDU), which is a relatively large number of ports.

3. FTTH Scenario

The housing of the fiber, fiber to the home (FTTH) scenarios mainly the family unit (SFU), a relatively small number of ports.

3.2.2 Optical access network architecture

The digital optical networking concept defines a new network architecture that integrates service management strength, flexibility, and by the end of the digital transmission capacity WDM. A digital optical network offers an opportunity to significantly improve transport networks to simplify the architecture, consolidation transport and bandwidth management, which allows you to add more simple and flexible add / drop form and profitable capacity to increase the network "on-net "mark [23].

There are two standard optical linear and ring architectures, both of which can provide network protection and restoration services. SONET rings are the most commonly used architecture. Networks can be considered as a linear step will be to create a loop or ring. Many of SONET ring architecture the number of fibers in the direction of transmission, and the protection switching levels [24].

3.2.3 Why optical networking?

Size and Weight: Since the individual optical fibers are usually only 125 meters in diameter, a multiple fiber cable can be much smaller than the corresponding metal cables.

Bandwidth: Bandwidth of fiber optic cables that can be orders of magnitude larger than the wire rope. Enter without fiber systems can be easily upgraded with a higher data rate type systems. Program update achieved by changing the light source (laser LED), improved modulation technique for improving the receptor or wavelength division multiplexing.

Repeater spacing: The low-loss optical fiber cable, the distance between the repeaters can be significantly increased. Furthermore, loss of the optical fiber bandwidth is independent. So repeat distance increases the bandwidth of the system.

Electrical isolation:  Optical cable is electrically non-conductive, to eliminate electrical problems. Optical systems are immune to the surge, lightning induced currents, short circuits and ground loops. The fibers are not susceptible to electromagnetic interference from power lines, radio signals, adjacent cable systems, or other electromagnetic sources.

Crosstalk: Because there is no optical coupling from one fiber to another within a cable, fiber optic systems are free from crosstalk.

Environment: Properly designed fiber optic systems are relatively unaffected by adverse temperature and moisture conditions.

Frequency allocations: The fiber cable systems do not require the allocation of frequencies. In addition, there is no ground cable systems, multipath fading and interference problems common radio systems. [24]

3.2.4 Types of optical network

There are mainly two types of optical network

Active Optical Networks

Passive Optical Networks

Sometimes active fiber architectures running and / or the Active Star Ethernet passive architectures include passive optical network (PON). The selection of the architecture, there are many things to take into account, including the existing external plant network location, the cost of network deployment, subscriber density and return on investment (ROI) [25].

3.2.4.1 Active optical networks

This is an easy, point-to-point topology. It provides dedicated bandwidth per subscriber. Each signal leaving the central office (CO) is intended only for clients for whom it is intended. Incoming signals from the customers avoid colliding at the crossroads, because there is no buffering of the electrical equipment. Active optical networks, depending on your extended electric-powered equipment, such as a switch, router, or multiplexer [26].

Advantages of active optical networks:

Dedicated fiber

Maximum bandwidth

Maximum flexibility

Synchronous bandwidth

Operates at distances up to 80 km, regardless of the number of

subscribers

More economical in low density subscriber areas

Does not require complex preplanning for large scale areas

Supports up to 1 Gbps per customer

Weaknesses of active optical networks:

Requires many long fiber runs

Requires longer distance lasers

Does not share Optical Line Terminal (OLT)

Requires power for cabinets

Powered electronics distributed in the access network

Due to these weaknesses in active optical network, passive optical networks are the current choices [25].

3.3 PASSIVE OPTICAL NETWORKS (PONS)

Passive optical networks were formed in 1980. A PON is a point-to multi-point network architecture. The passive optical network optical network architecture, which can provide much higher bandwidth in the access network compared to copper-based networks [27].

The PON means there are no active components between the central office (CO) and customer premises. In other words, only the optical fiber and passive optical elements do not require electricity or active treatment [28]. Passive optical networks (PON) is an efficient point-to-multipoint solutions to meet the demand for increased capacity in the access part of the central offices of the communication infrastructure service (COS) and Web clients. . Uses a dynamic bandwidth allocation (DBA) algorithm, the Media Access Control (MAC) protocol ensures network efficiency. [27]

A PON is a point-to-multipoint (P2MP) optical network is active elements in the signal path "from the source to the destination. The only interior elements used in PON passive optical components such as optical fiber, splices and splitters. The access network based on a fiber PON only requires N + 1 km fiber transmitter and L [29].

Figure 3.2: Basic PON diagram [30]

3.3.1 Why PON?

Fiber data rates can be upgraded as technology improves.

Enormous information carrying capacity

Initially 155 Mbps

Then 622 Mbps

Then 1.25 Gbps

Now 2.5 Gbps

Shared infrastructure translates to lower cost per customer.

Providers can share the costs of fiber, installation and central office equipment among multiple customers since multiple users share a single strand.

Minimal number of optical transceivers

Feeder fiber and transceivers costs divided by N customers

Passive splitters translate to lower cost

Can be installed anywhere

No power needed

No active electronics in the access network

Best suited for triple play services

Upgrades or new services can be accomplished with equipment changes at the network ends and on a per customer basis

Immune to electromagnetic noise

Secure [29] [31].

3.3.2 PON architecture

Figure 3.3: Basic PON architecture [33]

From the above figure, PON terminologies can be defined as:

The central office (CO) head end is called an Optical Line Terminal (OLT)

Optical Network Units (ONUs) are the central premises devices. (Sometimes called Optical Network Terminals, ONTs)

The entire fiber tree (including feeder, splitters, distribution fibers) is an Optical Distribution Network (ODN)

All trees emanating from the sane OLT form an OAN

Downstream is from OLT to ONU as shown in figure

Upstream is the opposite i.e. from ONU to OLT [29]

A PON consists of an OLT in the central office of the service provider (CO), and a set of ONUs near the end users. A PON configuration reduces the fiber and central office equipment required for plants, such as point-to-point architectures. Send downlink signals share some of the assumptions of the fiber. Upstream signals are connected to a multiple access protocol.

The OLT provides the interface between the PON and the service provider's network. OLT in the Network Control Panel. This card is connected to the central office, crossing networks, video and data sent back home, but this is a special distributed feedback (DFB) laser calibration is always on. This control card works like a traffic signal to the remote loading complete data / video performance of upstream and downstream. Send downlink signals share some of the assumptions of the fiber. The OLT sends all information, all subsequent ONU. The data is intended to concentrated ONU ONU special and ignore the data. The agency also works with OLT upstream data traffic. Since the point-to-multipoint network topology without collision avoidance mechanism, the load can interfere with each other if not rejected if the upstream transmit data. OLT undertake this role and will report to the UN, and if you can not transmit upstream data. Uplink signals are connected to a multiple access protocol, typically Division Multiple Access (TDMA). Extinguish the "range" that timeslot assignments for the ONU upstream release [34]

ONT (Optical Network Terminal) is an expression of ITU-T, while the ONU (Optical Network Unit) is an IEEE term, however, the two terms have the same meaning. A device called ONT, also called an ONU, converts the optical signal to an electrical signal. Pour require power to operate, so some providers connect battery backup for power outages. ONU thin film filter technology to convert the optical and electrical signals. One of the UN is a device that terminates the PON and presents customer service points for the user. Some ONUs implement a separate subscriber unit to provide services, such as telephony, Ethernet data, or video. [26]

The PON employs a passive (no power required) asset allocation of the number of optical fibers in a fiber. This device is called an optical splitter, and vice versa, to combine multiple optical fibers only. This device is an optical coupler. In its simplest form, an optical fiber coupler consists of two fused together. Received signal power from either input port is divided between the two output ports. The power division ratio of the divider plate can be adjusted to a length of the molten area, and thus a constant parameter.

3.3.3 PON standards

ITU-T G.983

ATM Passive Optical Network (APON):

APON was launched in 1995 and standardized ITU-T G.983 and ITU. It was the first standard passive optical networks. It was the first technology was developed based on FTTH PON (Fiber To The Home) installed, as most of legacy ATM network infrastructure was built. This is mainly in business applications [32].

Broadband Passive Optical Network (BPON):

PON not only offers services based on ATM, but the video distribution services, leased lines and Ethernet access and express the bandwidth capacity of PON BPON systems have been introduced. APON a standard BPON architecture. BPON two main advantages, firstly, that the length of the third wave of video services and in the second case, a stable standard recycling ATM infrastructure. Added support for WDM, dynamic and higher upstream bandwidth allocation, and survival. Recommendations ITU G.983.1 BPON was standardized, G.983.2, G.983.3, G.983.4, and G.983.5 [32].

ITU-T G.983

Gigabit Passive Optical Network (GPON):

An evolution of BPON GPON standard. Full Services Access Network (FSAN) standardization work started in 2001 GPON network planning 1Gbps throughput. Supports higher for enhanced security, and choice of Layer 2 protocol (ATM, GEM, Ethernet). GPON using IP-based protocols for data transfer. GPON significant boost both aggregate bandwidth and bandwidth efficiency through the use of a large, variable-length packets. GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic, with frame segmentation to better service for delay-sensitive traffic such as voice and video communications. Generic Framing Procedure using GPON (GFP) protocol service supports both voice and data oriented. GPON architecture converged data and voice services speeds up to 2.5 Gbps. GPON standards split into four layers:

G.984.1: Requirements

G.984.2: Physical Layer

G.984.3: Transmission Convergence Layer

G.984.4: Management Layer

The big advantage of GPON over other systems that interface to all major services and Generic Frame Procedure (GFP)-enabled network packets belonging to different protocols can be transmitted to native format [32].

IEEE 802.3 ah

Ethernet Passive Optical Network (EPON):

This is an IEEE / EFM (Ethernet in the First Mile) Using Ethernet packet data. 802.3 ah now part of the IEEE 802.3 standard. Ethernet the dominant protocol for local area networks. EPON uses IP-based data transfer protocol. Adoption of Ethernet technology would lead to a single access network protocols simplifies network management on the client side. EFM EPONs introduced implement the topology point multipoint (P2MP) network in passive optical splitters. EPON is similar to the suppliers but APON Ethernet frames / packets instead of ATM cells [27], [32].

IEEE 802.3 ah

10 Gigabit Ethernet Passive Optical Network (10GEPON):

802.3 ah EPON supports. 10GEPON use a separate wavelength 10G and 1G downstream and remains a single wavelength both 1G and 10G upstream TDMA separation. It is also compatible with WDM-PON [27].

WDM Passive Optical Network (WPON):

It is an optical point-to-point connection. WDM add more bandwidth over long distances, spending more premium optical bandwidth for each user. Simplify network that supports point-to-point WDM-PON link is assigned a pair of bi-directional wavelength for each user connected to the PON. The multi-wavelength WDM-PON can be separated from the optical network unit (ONU) into multiple virtual Pons coexist on the same physical infrastructure. Alternatively, the wavelengths can be used together through the use of statistical multiplexing to ensure the optimal wavelength and reduces the delays experienced by the United Nations [27].

Figure 3.4: History of PONs [28]

Table 3.2: Summary of PON’s standards data rates [32]

Layer 1-2

ONUs per ONT

Upstream

Downstream

Mbps/ONT

APON

ATM

32

622 Mbps

1.2 Gbps

37.5 Mbps

EPON

Ethernet

32

1.25 Gbps

1.25 Gbps

78.1 Mbps

GPON

Gigabit Ethernet

32

64

155 Mbps

622 Mbps

1.25 Gbps

2.5 Gbps

1.25 Gbps

2.5 Gbps

19.5 Mbps

39.0 Mbps

The difference between four major PON technologies i.e. BPON, EPON, GPON, WDM PON is shown in the following given table

Table 3.3: Summary of PON standards [25]

A/BPON

EPON

(GEPON)

GPON

10GEPON

WDM PON

Standard

ITU G.983

IEEE 802ah

ITU G.984

IEEE 802av

ITU G.983

Data packet Cell Size

53 bytes

1518 bytes

53 to 1518 bytes

1518 bytes

Independent

Maximum Downstream Line Rate

622 Mbps

1.2 Gbps

2.4 Gbps

IP;2.4 Gbps,

Broadcast;5Gbps

On-demand;2.5

Gbps

1-10 Gbps per channel

Maximum Upstream Line Rate

155/622 Mbps

1.2 Gbps

1.2 Gbps

2.5 Gbps

1-10 Gbps per channel

Downstream wavelength

1490 and 1550 nm

1550 nm

1490 and 1550 nm

1550 nm

Individual wavelength channel

Upstream wavelength

1310 nm

1310 nm

1310 nm

1310 nm

Individual wavelength channel

Traffic Modes

ATM

Ethernet

ATM Ethernet or TDM

Ethernet

Protocol Independent

Voice

ATM

VoIP

TDM

VoIP

Independent

Video

1550 nm overlay

1550 nm overlay/IP

1550 nm overlay/IP

IP

1550 nm overlay/IP

Max PON Splits

32

32

64

128

16/100’s

Max distance

20 km

20 Km

60 Km

10 Km

20 Km

Average Bandwidth per user

20 Mbps

60 Mbps

40 Mbps

20 Mbps

Up to 10 Gbps

3.4 GIGABIT PASSIVE OPTICAL NETWORKS (GPON)

Currently one of the fastest GPON access technology. In fact, it's a rating BPON, but uses IP-based protocols for data transfer. GPON plays a key role in delivering high bandwidth to residential end-users and high-capacity network nodes. Supports higher security and increased the number of Layer 2 protocol (ATM, GEM, Ethernet) and a wide range of services (triple play services such as HDTV, data, video conferencing, business and residential service). The ITU-T GPON line presents the first description in 2003, and most of the work was completed in March 2005 FSAN. Typical performance parameters of the GPON upstream and downstream bit rates of 2.5Gbps and 1.25Gbps [26].

3.5 SCOPE

ITU-T Recommendation G.984 deals with the general characteristics of gigabit-capable passive optical network (GPON) systems in order to guide and motivate the physical layer and the transmission convergence layer specification. The overall features of the user network interface (UNI) and the service node points (NHIS), which are required by network operators. This recommendation is also compatible for x-series (ITU-T G.982 and ITU-T G.983 recommendation).

GPON systems are characterized by an optical line termination (OLT) system and an optical network unit (ONU) or optical network termination (ONT) in passive optical distribution network (ODN) interconnecting them. There is usually a one to many relationship between the OLT and ONU / ONT, respectively. [35].

3.6 GPON STANDARDS

ITU-T G.984.1

Parameter description of GPON network

Requirements of protection switch-over networking

ITU-T G.984.2

Specifications of ODN parameters

Specifications of 2.488Gbps downstream optical port

Specifications of 1.244Gbps upstream optical port

Overhead allocation at physical layer

ITU-T G.984.3

Specifications of TC layer in the GPON system

GTC multiplexing architecture and protocol stack

GTC frame

ONU registration and activation

DBA specifications

Alarms and performance

ITU-T G.984.4

OMCI message format

OMCI device management frame

OMCI working principle [36].

3.7 GPON ARCHITECTURE

3.7.1 Optical Distribution Network (ODN)

PON context, a tree in the optical access network, complemented by energy or wavelength splitters, filters or other optical devices passive optical distribution network. The optical signal is distributed to the central office via an optical distribution network (ODN). The end points of the network, devices called optical network terminals (ONT) process to convert the optical signal to an electrical signal. (The FTTP architectures, ONT are privately owned.) Signal usually travels electrically between the ONT and the end-user devices. The general scenario is illustrated in Figure 3.1. Optical distribution network of competing technologies [26].

Fig: 3.5 ODN [31]

3.7.2 Optical Line Terminal (OLT)

OLT is a tool which eliminates the common endpoint of an ODN, implements a PON protocol defined by ITU-T G.984 and adapts PON PDUs in the uplink communications service provider interface. The OLT provides functions and maintenance of closed ODN and the United Nations. [1] The OLT sends all information, all subsequent ONT. The data specific targets and ONT ONT ignore the directional information. The agency also works with OLT upstream data traffic. Since the network topology is point-to-multi-point collision avoidance mechanism. The ONT can interfere with each other if not rejected if the upstream transmit data. OLT undertake this role, and inform you when you can and can not transmit upstream data. [37]

    The diagram shows the general structure of an OLT. The PMD (physical media dependent) layer defines the optical transceiver duplex unit and the wavelength of the United Nations or OLT. The medium access control (MAC) layer is the right time to use the physical environment to fight the common thread between the United Nations to prevent another. The crossover for connecting the OLT and ONU switching between different PON system and spine. The service adaptation layer OLT provides the translation between formats spine signs and signals in PON. The interface of the OLT to the main network service called Network Interface (SNI)[38].

Figure 3.6: Generic structure of an OLT [38]

3.7.3 Optical Network Terminal (ONT)

The ONT is a device that terminates the PON and presents customer service points for the user. ONT is a term of the ITU-T, while the UN is an expression IEEE [5] The ONT provides all services directly to the customer .. This is equivalent to a twisted pair telephone service, RJ-45 Ethernet services and an F connector (coaxial) television services based on RF [37]. The United Nations provides the connection to the PON OLT MAC and the UN stage PMD. The service layer of the adoption of the United Nations provides the translation between the signal format required for connecting consumer devices and PON signal format. The interface to the UN that the client's network user name of network interface devices. The service mux / demux operation phase multiplexing different customer interfaces. Generally more available to the UN UNIS different types of services (data, voice). All UNI may support a different signal format and require the use of other private adoption. The figure shows the adaptation layer to the United Nations translation services required for the signal format for connecting consumer devices and PON signal format. The interface of a UN force in the client user interface called the network of networks. The service mux / demux operation phase multiplexing different interfaces. [38]

Figure 3.7: Generic structure of an ONU [38]

3.7.4 Optical splitter

The PON employs a passive (no power required) asset allocation of the number of fibers of a fiber optic signal and vice versa, to combine multiple optical signals in a fiber. This device is an optical splitter [39].

The purposes of power splitting include:

Sharing the cost and bandwidth of OLT among ONUs.

Reducing the fiber mileage in the field. Apart from the simple one-stage splitting strategy, as described in the figure. Splitters may be cascaded in the field as shown in the figure.

Figure 3.8: splitting strategies in GPON: (a) one-stage splitting, (b) multistage splitting

3.7.4.1 Split ratio

Basically, the biggest reason for the division of GPON, appealing to the operators. However, a greater sharing ratio is larger optical zone, which creates the need for higher power budget to support the physical distance. Distribution coefficients from 1:64 realistic physical layer, given current technology. However, in anticipation of the continued development of optical modules, the TC layer must take into account the distribution coefficients of up to 1:128.

3.7.4.2 Splitting loss

Power level at the coupler’s output versus power level at its input, measured in decibels. For an ideal 2 × 2 coupler with equal power splitting, this value is 3 dB.

3.7.4.3 Insertion loss

Power loss resulting from imperfections of the manufacturing process. Typically, this value ranges from 0.1 to 1 dB.

3.7.4.4 Directivity

Amount of input power leaked from one input port to another input port. Couplers are highly directional devices with the directivity parameter reaching –40 to –50 dB. [35]

3.8 FRAME STRUCTURE

The frame structure supported by GPON is discussed as under.

3.8.1 Layers in GPON

The frame structure in terms of layers is described below.

3.8.1.1 G-PON Physical Medium dependent layer (GPM)

EPA specifications for the G-PON physical medium dependent layer, which is equivalent to the first layer of the OSI reference model. The physical layer contains Converter Optical / Electrical and CDR (clock data recovery). The data transmitter of the physical layer of the second medium and vice versa.

EPA first determines the optical transmitter and receiver each transmission rate. G-PON in the class A / B / C, ITU-T G.982 as the difference between the OLT optical level and the United Nations, called the budget and the transmission power and reception sensitivity values ​​are given in each class. As the investigation of 2.4 Gbit / s upstream services have not been adequately continue to be a problem in the future. The optical transceiver to keep costs low, Forward Error Correction (FEC) can be used as an option. [40].

Table 3.4: GPM main specification [8]

Access

speed

Downstream: 1.24Gbps, 2.488Gbps

Upstream: 155Mbps, 622Mbps, 1.244Gbps, 2.488Gbps.

Power

budget

Class A(5-20 dB)

Class B(10-25 dB)

Class C(15-30 dB)

Burst overhead

Burst overhead is specified at each transmission speed. (De-regulated from B-PON)

FEC

FEC (forward error correction) is introduced to reduce an optical module cost, and aimed to ease transmitting power and receiving optical sensitivity of an optical module.

Power leveling

The ONU optical output can be adjusted in two steps to relieve APD (Automatic Power Distribution) tolerance of OLT.

3.8.1.2 G-PON Transmission Convergence layer (GTC)

The transmission convergence layer data equivalent to one layer to the second layer of the OSI reference model. GTC defines the data frame, sequence control between the OLT and the UN, as well as the role of encryption to prevent eavesdropping or masking. Since the G-PON is expected to receive all services efficiently, a new transmission method is called GEM (GPON encapsulation method) was adopted to include data services and TDM services. GEM provides a framework for control of a variable-length header bytes long. ATM service is transmitted from the original ATM cells. Finally, the frames are transferred to the ATM and GEM GTC TC cells (Figure 3.5.) Since the downstream traffic is forwarded to the PON system is needed to ensure data theft. [41]

Figure 3.9: frame structure of GPON [41]

3.8.2 Framing in GPON

3.8.2.1 GTC downstream framing

Each subsequent frame GTC 125 ms, which is the physical control block downstream (PCBD) and payload field in the figure below. The information contained PCBD control media access to the upstream (U.S.) bandwidth (BW) map. As shown in the figure, the OLT determines the start and end time of each T-CONT can be transmitted upstream data pointers on the map of the U.S. BW. The indicators are given in units of bytes. This allows you to control the upstream bandwidth of 64 kbps granularity. PCBD The header contains a fixed and a variable part. The fixed part contains the physical synchronization field and a field camp PLOAM year. These fields are protected by parity check bits 1 byte interleaving. The 4-byte physical broadcasts synchronization pattern indicates the beginning of a downstream frame. This year, 4-byte field specifies the use of FEC. The 13-inch PCBD PLOAM field used to communicate messages to the ONU physical layer OAM. UN PLOAM features include high and low, ranging from, power leveling, updating cryptographic keys, bug reports physical layer, etc. The variable part of the header contains copies PCBD downstream effective length (plena) descriptor, which specifies the length of the sheet upstream bandwidth and ATM partition the T-CONT. each ONU can set multiple T-counters. The U.S. BW map defines upstream bandwidth allocation to achieve entry. Each entry is 8-byte access to maps of the U.S. BW Alloc-ID contains a T-CONT, the start time and end time, the T-CONT forwarded to the upstream direction, and a 12-bit flag indicates that the distribution must be used. The remaining downlink frame downstream helpful. Since each frame down to 125 ms duration, the downstream frame length and different speeds 1.24416 2.48832 Gbps Gbps, which are 19,440 and 38,880 bytes, respectively. PCBD block the same for both speeds. PCBD block is the same for both speeds. [42]

Figure 3.10: Upstream and downstream framing [43]

PCBd

n

Payload

n

PCBd

n+1

Payload

n

Psync

4 bytes

Ident

4 bytes

PLOAMd

13 bytes

BIP

1 byte

Plend

4 bytes

Plend

4 bytes

US BW MAP

N*8 bytes

Figure 3.11: Downstream frame structure [42]

PSync - fixed pattern used by ONU to located start of GTC frame

Ident - MSB indicates if FEC is used, 30 LSBs are super frame counter

PLOAMd - carries OAM, ranging, alerts, activation messages, etc.

BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP

PLend (transmitted twice for robustness)

US BW map - array of Blen 8B structures granting BW to US flow. [42]

3.8.2.2 GTC upstream framing

GTC upstream of the upstream transmission frames of duration 125 ms virtual shown. Upstream virtual framework of various eruptions ONT. Each burst begins above the physical layer (Plou). The preamble begins Plou help the OLT burst mode receiver to synchronize with the transmitter in the ONT. The delimiter following the beginning of the preamble to the eruptions up. The length and format of the preamble and delimiter specified by using PLOAM messages after OLT. The display field (Search) Plou the real-time status reports to the UN OLT. [42]

T can be assigned to a UN number of counters. If one of the time slots assigned to the United Nations continuous T-Alloc-ID Clear counters many different Plou only be transmitted once. It is shown that ONT A. After PLOu field, all three fields are optional overload off:

1. Physical layer operation administration and management upstream (PLOAMu).

2. Power leveling sequence upstream (PLSu).

3. Dynamic bandwidth report upstream (DBRu).

The transmission of these fields dictated by the OLT through the flags on the map of the U.S. BW PCBD. At the request of the Olt, the field is 120 bytes PLSu sent to the UN to end energy measurement. The DBRu field are related to each T-CONT reporting traffic conditions upstream of the associated T-CONT. DBRu DBA field in the report can be used to indicate the length of the transmission line dynamic bandwidth allocation. G-PON supports the status report (SR), and status reports (NSR) DBA mechanisms. The NSR DBA OLT controls the upstream traffic, and the UN protocol required OLT in the queue status. The SR is a DBA tho UN reports traffic conditions expected in the buffer queue. [42]

Figure 3.12: GTC upstream framing [42]

3.8.2.3 GEM encapsulation

GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic, with frame segmentation to provide better quality of service (QoS) for delay-sensitive traffic such as voice and video communications. [9] In the downstream direction, GEM frames later section of the cargo area by GEM (fig.3.12).

Figure 3.13: GEM frame in upstream payload [42]

GEM frame encapsulation serves two functions:

Multiplexing of GEM ports.

Payload data fragmentation.

The format of the GEM encapsulation is shown in fig.3.13.

Payload length indicator (PLI) 12-bit

Port ID

12-bit

Payload type indicator

(PTI) 3-bit

Header error control

(HEC) 13-bit

GEM fragment

payload

L-byte

PTI code

Meaning

000

User data fragment, no congestion, not EOF

001

User data fragment, no congestion, EOF

010

User data fragment, congestion, not EOF

011

User data fragment, congestion, EOF

100

GEM OAM

101

Reserved

110

Reserved

111

Reserved

EOF = End of frame

FFS = For future study

Figure: 3.14 GEM Encapsulation format [42]

The GEM header contains a 12-bit long useful indicator (PLI), which determines the length of the GEM payload bytes. This allows the maximum load length to 4095 bytes. Any user can use the structure as 4095 bytes must be fragmented. The 3-bit type of charge indicator (PTI) indicates that the payload contains user data frames or GEM OAM frames. It also indicates that the frame of user data in the payload is the last fragment. The port ID 4096 12-bit port number allows for multiple traffic. The header error control (HEC) field BCH code is used to protect the integrity of the header signal. This field is also used to synchronize the GEM frame. [42] The advantage of the GEM fragmentation process is the opportunity to support high-priority traffic. 125-ms latency period corresponding to the frame. In order to turn off the user data rate in a jewelry set is also useful to rest all zero. If there is no user data to send, idle GEM frames are sent to fill empty spaces and maintaining synchronization of the receiver. [35]