Ethernet or GPON: Which technology is best suited for Information Transportation Systems (ITS) of the 21st Century?

 In this article, we are going to learn how the next generation of Digital Information Transportation Systems will be deployed across the enterprise and service provider network.

Information Transportation Systems (ITS):

Information Transportation Systems can be broken into three different networks, such as Enterprise networking, Service Provider Access Network and Transmission Network. ITS is comprised not only of active components but also of passive components. In the below figure, you can get some overview of different types of wired and wireless communication network solutions available at Rural and Urban Area.

Previously every service such as able TV, data and voice networks had independent networks and different operators or service providers. Until today most of South Asia’s network was covered by those individual operators. In countries like India, CATV operators or telephone operators are providing Internet service through co-axial or PSTN networks. Maybe a person from a developed world wouldn’t believe that in most of the rural places of India, the average network speed is in between 128- 256 Kbps.

In some rural areas of the United States, people are also using VSAT technology to get triple play (Voice, Data & Video) services. These services are not very cost effective and they are unreliable and often have too much down time due to severe weather conditions (snowing or rain or dust cyclone). Many of India’s rural areas are running only 2G networks, which is not sufficient to have a reliable data transaction. These things are playing major obstacles to most of rural India’s development. Proper ICT planning for digitization of Information Transportation Systems must be implemented. Otherwise, business can’t be developed in rural areas.

In the early 2000s, DSL and VDSL were the popular choices for providing broadband connectivity to businesses. But it had limitations regarding bandwidth (up to 52 Mbit/sec downstream and 16 Mbit/sec upstream), which was quite good for data & voice traffic. But the demand of streaming video is making the network more bandwidth hungry. Presently our legacy copper network doesn’t have the capability to provide the optimal solution for the required bandwidth in decades to come.

Fibre-based solutions have become cheaper (almost same price of 2 core SM Fibre with CAT 6A cable) every day. Previously most service providers chose to deploy some sort of fibre optics solutions (in backbone) to provide broadband connectivity to their end user. In the below diagram, I used the famous Fibre to the curve topology, which is mainly used in multi-dwelling building environments, where fibre is running up to the cabinet inside/outside of the building, and later it uses the existing CAT 6 cable wiring for connecting individual customers to the DSL Modem. This solution is called last mile Ethernet technology.

These hybrid solutions (Fibre & Copper) are quite good when you want to use the existing copper cabling solutions of the building, and this is very useful for old buildings where preparing new block wiring is very tedious and time consuming. But there are several drawbacks with this technology, as it works in a single point of failure. If the active components (access layer switch) get powered off or fail then all subscribers in the building will lose connectivity. Service providers must send a network technician to boot up the active components and troubleshoot the switch. This will increase the operation cost (OPEX), which is not good for running a smooth business.

There are other options shown in the above diagram. Instead of placing active components in the curve, service providers can use only fibre-based Passive Optical Network where every customer will have their own Optical Network Terminal (ONT) on their premises. It will be connected directly to the Optical Line Terminal (OLT) located at the central office/local exchange of the service provider network. This solution is very popular and we will discuss more about this in a later part of this article.

Converged Information Transportation Systems:

In the below diagram, you will see today’s converged Information Transportation Systems (ITS) architecture using PON & Ethernet Technology for transporting data from a user located in Enterprise network via Access & Transmission Networks of Service Provider to the Internet cloud.

In today’s enterprise network, most of the horizontal cabling still runs over copper (CAT 6A) cable, but the backbone network runs mostly over multi-mode fibre (up to 400 meter). In an access network, single mode fibre (distance from ONT to OLT) can reach up to 60 KM). For transmission networks, it uses Multi Service Transport Platform (MSTP) to connect different types of network clouds (CATV, TDM & Internet). Australian ISP Telstra chooses Ericsson for its next generation optical network to deploy all IP based transportation networks. This will improve Telstra’s Optical technology and will increase bandwidth capacity and also lower latency which is of growing importance as more and more operations move to the cloud. It is impossible to discuss in one article everything about transmission network, so the rest of this article will focus on Enterprise Networking and Access Network of Service Provider.

Access Network of Service Provider:

In today’s next generation broadband network, the access network of the service provider’s domain is very important and has a major role to play. Search engine giant Google is also investing heavily in Fiber optics technology to provide 1 Gbps connectivity to subscribers in Kansas City.

Gigabit Passive Optical Network (GPON):

Questions may come to your mind. What is Google using to provide 1 Gbps connectivity? How is it different from other PON based solutions?

Many people think that Google is using active point-to-point Ethernet FTTH, but in reality they are also using shared Passive Optical based networking technology (GPON) like other service providers such as Verizon. GPON provides operators a more cost-effective approach for delivering Gigabit services to the customer. Active/P2P Ethernet is superior in terms of delivering dedicated, symmetric bandwidth, but the reality is that very few end users are going to utilize the dedicated gigabit Ethernet port, especially on a “24/7/365″ basis. It is important to remember that Google is not the only service provider who is using the GPON based solutions for providing FTTH services, or even the first. There are many Internet service providers working with Alcatel, Huawei, Commscope and Cisco for providing GPON-based FTTH solutions to its end users.

From the above diagram, you can understand that every customer is equipped with separate Optical Network Units (ONU/ONT). Depending upon the requirement of its subscribers (Businesses & Home), service providers choose from different types of ONT (Alcatel, Cisco, Huawei, etc.) as Customer Preemies Equipment. ONT converts optical signals to electrical signals and have different types of ports available such as Gigabit Ethernet port, Fast Ethernet port, T1/E1 port, CATV port, etc. Another side of ONT is connected to the nearest splitter (located either in a building or roadside manhole) by using 2 core Single Mode Fiber, and the splitter is also connected to the nearest central office’s Optical Line Terminal (OLT). Every subscriber is using the shared bandwidth and is driven by Time Division Multiplexing (TDM) scheme of 2.488 Gbps as Downstream and 1.244 Gbps as Upstream. Every OLT-GPON Port can support up to 64 subscribers, and it can also support 8 different VLAN per ONT Physical port. Every Optical Line Terminal can support up to 3,600 subscribers (based on maximum 64 subscriber per GPON Port). There is a new 10G-GPON technology available, which is a continuation in the evolution of GPON technologies, increasing the downstream bandwidth four times and can reach up to 20 KM to 60 KM and split from 64 to 128.

Enterprise Networking:

According to tech website techopedia, an enterprise network is an enterprise’s communications backbone that helps to connect computers and related devices across departments and workgroup networks, facilitating insight and data accessibility. This definition is for data communication only, but remember present enterprise networks must provide other services such as voice, video, teleconference, building automation, and security and surveillance systems in a single networking platform. In the below diagram, I have given an example of different types of enterprise networking by industry verticals based on networking equipment manufacturer such as Cisco, Huawei, etc.

This type of definition for enterprise networking is quite good for business development managers of companies like Cisco or Alcatel to manage their channel partner and clients. As an ICT architect, I prefer to see enterprise networking in slightly different ways. For me, enterprise networking consists of two parts. Number one is about local area network (this is called Inside Plant Network) and the second one is about wide area/metropolitan area networks (this is called Outside Plant Network).

I hope you understand that all types of buildings are a simple LAN Network and have different types of services depending upon its business requirements. In a campus environment, we need to connect all the buildings via Fiber Optics backbone (either in GPON or Ethernet based).

Ethernet vs GPON in Enterprise Networking or in Campus Environment:

Companies like Tellabs pushing for Fiber to Desktop solutions have developed the smallest ONT (100 miniseries), which you can mount in the back of a desktop.

Alcatel-Lucent the pioneer of GPON technology is vocal about the possibility of its usage in enterprise networking. And GPON-based vendors even claim that GPON is greener and more cost effective in respect of Ethernet networking. On the other hand, Cisco being the leader of enterprise networking is claiming that their switching equipment is much greener than that of Motorola’s GPON-based solutions. There are several papers that have been published on both the technologies (about which is best), and I am not going to argue any of these. I would request my reader to go through them with the below links and reference list for more understanding about security, reliability, design, cost, power consumption for both the technologies.

As an ICT Architect, I see both these technologies as having equal importantance to businesses. In businesses like hotels and hospitals, we can use GPON-based solutions, but they will not be able to provide the same scalability of Ethernet networking. Ethernet technology has the capability to provide power over Ethernet and it gives significant advantages over GPON-based solutions. However, there are some Optical Network Terminals available (ex. Huawei MA 5650 Series) which can provide Power via cat 6 cable. This device is good for installing in a house or apartment where it can serve as Access Layer Switch also, but there is no advantage of using this in businesses (at least from a power consumption standpoint).

From the above design, you can understand that unlike Ethernet (which is three layered technology) GPON is only a two-layered (only active & distribution Layer) technology. GPON-based solutions are very good for residential projects, where individual apartments can be connected and controlled through their respective ONT. Service providers can provide value added services to the customer, and this will have significant impact in IoT enabled businesses of the 21st century. A customer can get real time updates about his/her home from a service provider’s cloud-based solution. This will be a great opportunity for service providers to generate businesses from some sort of Home Automation.

Case Study: Shopping Mall:

Last year, I met a customer (shopping mall owner) who wants a separate independent network for an individual shop owner. In Qatar, by law, IP-CCTV & Security systems must be connected and controlled by separate Ethernet based networks. And it must not connect to the main network. For me, this is no brainer, but businesses must comply with this law, otherwise they will not get approval from the Ministry of Interior.

They told me that they don’t want to connect other building engineering services (HVAC, Fire, AV, Power etc.) to the building’s IP network. They simply want to provide one telephone and data point inside every shop with their own network. But unfortunately other contractors already proposed Ethernet-based centralized network solutions. As per design (find below), every shop will have double UTP ports, and CAT 6 cable will run horizontally to connect to the nearest access layer switch located at Telecom Room (IDF). And every IDFs will connect to the Main Distribution Frame (Data Centre Core Switch) by using redundant multimode fibre uplinks. And there they need to install Firewalls also as a combination of Internet Gateway and 1st level of defense. For controlling the IP Phones they proposed IP-based Private Branch Exchange- Cisco Call Manager or Avaya IP Office.

The above solution is good but expensive in terms of capital expenditure but also it will cost mall operators significantly to run day-to-day operation. Instead of using Ethernet-based solutions, I proposed GPON-based solutions, where each shop will be equipped with separate ONTs (check the below diagram).

Mall owners only need to provide telecommunication block wiring using single mode fiber to each shop. Service providers will install their ONT inside the shops, and inside the main telecommunication room they will also install their splitter inside a Rack/cabinet. This solution was very liked by the mall owner because it reduces the initial investment (CAPEX) and also the cost of running the network for daily operation (OPEX).

Individual shop owners also have more flexibility to use their own systems such as IP telephone or digital telephone. And they can use IP-based point of sale devices or the old analogue based POS device. They can run their own wireless routers for connecting their mobile user. They will be charged directly by the service provider as per their usage, unlike Ethernet-based solutions where the bill would have come directly to the mall operator.

Conclusion:

As a vendor independent ICT Professional, I would love to say both GPON and Ethernet based technologies are an opportunity to serve humanity in better ways. I believe developing countries like India must invest heavily in fiber-based Information Transportation Systems to be a smart technology driven nation. Internet connectivity should be a basic human right, and it should be accessible to all areas of the USA, India, Africa and every corner of the world. This will not be completed in one day or even in decades to come, but we the ICT professional must design (vendor independently) the network in efficient ways to reduce the initial capital investment (CAPEX) as well as the daily maintenance cost (OPEX).

National Optical Fibre Network (NOFN)

Government of India has approved setting up of National Optical Fiber Network (NOFN) to provide connectivity to all the 2,50,000 Gram Panchayats(GPs) in the country. This would ensure broadband connectivity with adequate bandwidth. This is to be achieved utilizing the existing optical fiber and extending it to the Gram Panchayats.

At present OFC (Optical Fibre Cable) connectivity is available in all State Capitals, Districts, HQs and upto the Block Level. There is a plan to connect all the 2,50,000 Gram panchayats in the country. This will be done by utilizing existing fibres of PSUs (BSNL, Railtel and Power Grid) and laying incremental fibre to connect to Gram Panchayats wherever necessary. Dark fibre network thus created will be lit by appropriate technology thus creating sufficient bandwidth at the Gram Panchayats. This will be called the National Optical Fibre Network (NOFN). Thus connectivity gap between Gram Panchayats and Blocks will be filled.

Non-discriminatory access to the NOFN will be provided to all the Service Providers. These service providers like Telecom Service Providers(TSPs), ISPs, Cable TV operators and Content providers can launch various services in rural areas. Various categories of applications like e-health, e-education and e-governance etc. can be provided by these operators. The NOFN project is estimated to cost about Rs. 20,000 Cr. It is proposed to be completed in 2 years’ time. The project will be funded by the Universal Service Obligation Fund (USOF).

The company has been granted National Long Distance Operating (NLDO) license by DOT to w.e.f. 01.04.2013.

NOFN Concept Diagram

Technology

In NOFN the technology called GPON (Gigabit Passive Optical Network Technology) will be utilised.

A passive optical network (PON) is a network architecture that brings fiber cabling and signals to the home using a point-to-multipoint scheme that enables a single optical fiber to serve multiple premises. Encryption maintains data security in this shared environment. The architecture uses passive (unpowered) optical splitters, reducing the cost of equipment compared to point-to-point architectures.

The GPON (gigabit passive optical network) standard differs from other PON standards in that it achieves higher bandwidth and higher efficiency using larger, variable-length packets. GPON offers efficient packaging of user traffic, with frame segmentation allowing higher quality of service (QoS) for delay-sensitive voice and video communications traffic.

The main components of GPON technology are OLT, ONT/ONU, Splitters, OF cables etc.

ITU standard G-984 series as well as TEC spec GR no.PON-01/02 Apr 2008 define GPON technology

Pilot Projects

BBNL has embarked upon pilot projects in three blocks covering 58 Gram Panchayats in three different states. These blocks are Arian in Ajmer district (Rajasthan), Parvada in Visakhapatnam (Andhra Pradesh) and Panisagar in North Tripura district (Tripura).

The objectives of conducting a pilot project are as follows:

• The learning on technology choice and network architecture.
• Experiences gained in addressing ground realities in rural domain.
• NOFN NOC (Network Operation Center) related issues being developed by C-DoT- its integration and its testing at pilot locations.
• Experience gained by participation of TSPs, ISPs and application providers in utilizing bandwidth created by NOFN with respect to deliverables committed by BBNL.
• Synergisation by DIT/DoT to work together for pilots and plan the template for pilot testing of G2C services.
• Integration of NOFN pilots with existing networks from Blocks upwards. Also address the interfacing of NOFN with access operators at GPs.
• Synthesis of learning from the pilots and cross learning amongst 3- CPSUs in execution strategy.

The target date for completing Pilot Projects was 15/10/2012, and the same has been achieved with OFC laid out to all the Gram Panchayats in the Pilot Blocks and Electronic Equipment (OLT and ONT) having been tested for offering services.

Rollout of national optic fibre network delayed yet again

India's ambitious Rs 21,000-crore national broadband venture is set for another delay. The communications ministry is likely to miss the revised rollout targets of the venture with Bharat Sanchar Nigam , Power Grid Corporation of India and RailTel failing to award nearly Rs 6,000 crore of cable laying & trenching (C&T) contracts for the first phase.

Cable laying and trenching is the most expensive piece of the countrywide broadband rollout, popularly known as the national optic fibre network (NOFN) venture that will take superfast internet to the hinterlands.

Staterun BSNL, PowerGrid and RailTel have been mandated to handle cable laying and trenching responsibilities in the 70:15:15 ratio, and the final value of the contracts will be over Rs 11,000 crore.

The NOFN project has already suffered a two-year time overrun, following which the telecom department recently sought the approval of the Cabinet Committee on Infrastructure (CCI) to implement the pan-India broadband rollout in three phases, starting with high-speed broadband links in 1 lakh gram panchayats (GPs) at an investment of over Rs 11,000 crore by March 2014.

"We've informed telecom minister Kapil Sibal at a recent NOFN review meeting that it will be impossible to extend broadband connectivity to 1 lakh GPs by March 31, 2014, since BSNL, RailTel and PowerGrid have not finalised the C&T tenders," a top telecom department official told ET.

"Even if they manage to award the contracts by the month as promised, we will only be able to meet a tenth of the broadband coverage target in the first phase," he added. A senior executive of Bharat Broadband Network Ltd (BBNL), which is the executing agency of the NOFN venture, complained that "umpteen reminders had been sent to BSNL, which will award 70% of the C&T contracts to multiple vendors, but to no avail". A BSNL director, in turn, blamed BBNL for the delays.

He claimed that "awarding of C&T contracts had been held up in absence of key clearances from BBNL," adding that all relevant tenders would be finalised this month. DoT is concerned that further delays in executing the crucial NOFN project could derail the government's pan-India broadband penetration target of 175 million and 600 million subscribers by 2017 and 2020, respectively.

At present, India has barely 15 million broadband customers. The DoT is also upset that the contracts haven't been finalised despite the Telecom Commission recommending a payout of Rs 2,700 crore as administrative fees to BSNL, PowerGrid and Rail-Tel to hasten NOFN rollout. The TC is the highest decision-making body in the communications ministry.

The government wants to fast track the national broadband network rollout as it will be the backbone of a rural broadband ecosystem. The communications and rural development ministries have recently agreed to co-finance the delivery of government-to-citizen e-services by leveraging the NOFN backbone. The gamut of such community based broadband services, ranging from ehealth , e-education , e-governance to ecommerce across the Rs 2.5 lakh GPs will be delivered by BBNL.

Understanding GPON Architecture and Traffic Flows

This article discusses traffic flows in GPON. As discussed in GPON Fundamentals article, the OLT is the first aggregation point in GPON access network. The OLT terminates the GPON Transmission Convergence (GTC) layer on the user side and forwards Ethernet frames to Ethernet layer on the network side. Figure 1 shows the termination points for ONU/ONT scenario.

The U reference point represents the customer-facing interface of the ONU/ONT. It is possible that the U reference point can be within the ONU/ONT device when ONT and RG devices are combined into a single device.

The R/S reference point represents the OLT facing ONT interface. The S/R reference point represents the Optical Distribution Network (ODN) connecting GPON interface on the OLT. The S/R and R/S interfaces contain all the protocol elements necessary to allow communication between OLT and one or more ONTs over ODN.

The V reference point represents the network-facing interface of the OLT.


Relationship between T-CONT and GEM Ports

Definitions:

T-CONT: A traffic bearing object within an ONU/ONT that represents a group of logical connections, and is treated as a single entity for the purpose of upstream bandwidth assignment on the PON. In the upstream direction, it is used to bear the service traffic. Each T-CONT corresponds to a service traffic of one bandwidth type. Each bandwidth type has its own QoS feature.

ALLOC_ID: Each T-CONT is identified by the ALLOC_ID uniquely. The ALLOC_ID ranges from 0 to 4095. It is allocated by OLT i.e. a T-CONT can only be used by one ONU/ONT per PON interface on the OLT.

GEM Port: A GPON Encapsulation Method (GEM) port is a virtual port for performing GEM encapsulation for transmitting frames between the OLT and the ONU/ONT. Each different traffic-class (TC) per UNI is assigned a different GEM Port. Each T-CONT consists of one or more GEM Ports. Each GEM port bears one kind of service traffic i.e. a T-CONT type.

GEM Port ID: Each GEM Port is identified by a port ID uniquely. The Port ID ranges from 0 to 4095. It is allocated by the OLT i.e a GEM port can only be used by a single ONU/ONT per PON interface on the OLT.


Figure 2 shows the relationship between T-CONT and GEM Ports.

Between the ONT and OLT is the ODN, and Ethernet frames are carried over it through the use of GEM Channels. GPON has GEM channels as part of its GTC layer. The GEM channels carry variable-length Ethernet frames. GEM channels are identified by GEM Port IDs. This identifier is assigned by OLT upon creation of a new channel and is only valid during the entire life-cycle of the channel. Each GPON interface for a given ONT can have several GEM Ports. A GEM Port ID is unique per GPON interface and represents a specific traffic or group of flows between the OLT and the ONT.

There are 2 types of GEM Channels:

• Downstream-only GEM Channels - These channels are used to transmit downstream broadcast/multicast traffic from OLT to all ONTs. The ONTs identify traffic meant for them based on GEM Port ID.
• Bi-directional GEM Channels - These channels are used for upstream and downstream traffic between the OLT and the ONT. The frames are transmitted from the OLT into the GPON interface and are forwarded only on the U interface of the ONT on which that GEM Port has been assigned.

GEM Ports are used to differentiate among traffic classes (TCs). A U interface may have several GEM Ports associated with it that support different TCs. Thus, within a GPON interface, each GEM Port carries one or more traffic flows associated with a specific TC.

On U interface, traffic is classified into VLANs with various Ethernet priorities based on: Physical Port, VLAN ID, 802.1p bits, &/or DSCP. Once the traffic has been assigned a VLAN and COS (802.1p) values, these two values are used to select an upstream GEM Port so that QoS can be applied to the flows carried by the GEM Port. A GEM Port always belongs to a single T-CONT. In downstream direction, the ONT forwards the traffic received by GEM Ports to appropriate U interface.


1:1 VLAN

In a 1:1 VLAN architecture, the ONT maps each 1:1 VLAN into a unique U interface. There are 2 variations on tag assignment at V interface in upstream direction - the traffic at V interface could be double-tagged or single-tagged.

• For double-tagged VLANs at V, the ONT can either assign a C-VLAN ID or translate a C-VLAN ID. The OLT adds the S-VLAN ID. (Subscriber 1 in Figure 3)
• For double-tagged VLANs at V, the ONT can assign S-C VLAN IDs to incoming traffic, and the OLT passes through the traffic. (Subscriber 2 in Figure 3)
• For single-tagged VLAN at V, the ONT adds the S-VLAN ID or translates an incoming tag to S-VLAN ID, and the OLT passes through the traffic. (Subscriber 3 in Figure 3)

In the downstream direction, the OLT removes the outer tag or passes through the traffic to proper GEM port based on the tag value and priority bits. The ONT removes the tags and forwards frames from the GEM port to its associated U interface.

N:1 VLAN

For N:1 VLAN model, the ONT always adds the S-VLAN ID or translate an incoming tag to S-VLAN ID for upstream traffic. The OLT will pass-through any upstream traffic with S-VLAN ID on them. In the downstream direction, the OLT will pass-through traffic with S-VLAN ID to ONT by determining GEM Port based on MAC address and priority bits. If the GEM Port cannot be determined, then the frame is flooded using the unidirectional GEM Port associated with the S-VLAN ID. The ONT will remove the tag and forward frames from the GEM Port to appropriate U interface. For N:1 model, traffic is always single-tagged at V interface.

QoS and Traffic Management

As seen from figure 1, the GPON link connects the OLT and ONTs to transport Ethernet services. Please note that GPON can also encapsulate ATM and TDM (E1, E3) services. The GTC Adaptation sublayer maps Ethernet frames into GPON GEM frames. A QoS mechanism is required in GEM to support Ethernet QoS (i.e. 802.1p bits). In order to provide QoS, two mechanisms are employed-

• Classification of traffic into traffic classes
• Forwarding the traffic classes into GEM Ports and T-CONTs configured to emulate Ethernet QoS service

Upstream Traffic Management

Figure 5 shows a sample model of upstream traffic management. It shows 4 T-CONTs per PON interface where each T-CONT represents a specific traffic class (TC). The Classifier receives traffic from U interface and maps to queues as per configuration using associated GEM Ports. If a second UNI interface is present on the same ONT, it would also perform classification and map the traffic to TC(s). As mentioned above, a GEM Port must bear one or more T-CONTs. Other upstream traffic from other ONTs is mapped to other 4 T-CONTs according to the TC.

At the OLT, each TC is mapped into a separate queue. T-CONTs from various ONTs that share the same TC are mapped to the same queue, and a scheduler is used among the queues towards the network-facing port i.e. V interface.

Downstream Traffic Management
Figure 6 shows a sample model of downstream traffic management. In downstream direction, T-CONT is not used. Traffic received from the V interface at the OLT is assigned to queues according to the TCs. It is then transmitted in the downstream direction to the PON interface by using a scheduler. At the ONT, the traffic is classified again and placed into appropriate queues for each U interface. A scheduler is used to transmit frames to the U interface.

Basics of GPON

GPON stands for Gigabit Passive Optical Networks. GPON is defined by ITU-T recommendation series G.984.1 through G.984.6. GPON can transport not only Ethernet, but also ATM and TDM (PSTN, ISDN, E1 and E3) traffic. GPON network consists of mainly two active transmission equipments, namely- Optical Line Termination (OLT) and Optical Network Unit (ONU) or Optical Network Termination (ONT). GPON supports triple-play services, high-bandwidth, long reach (upto 20km), etc.

Figure 1 shows various FTTx network architectures.

A single fibre from the OLT runs to a passive Optical Splitter (passive means, it does not require any power to operate) which is located near the users' locations. The Optical Splitter merely divides the optical power into N separate paths to the users. The optical paths can vary between 2 to 128. From the Optical Splitter, a single mode (SM) fibre strand run to each user. This is shown in figure 2. GPON adopts two multiplexing mechanisms- a) in downstream direction (i.e. from OLT to users), data packets are transmitted in an broadcast manner, but encryption (AES) is used to prevent eavesdropping, b) in upstream direction (i.e. from users to OLT), data packets are transmitted in a TDMA manner.

The next section describes GPON key technologies.


ONU Identifier (ONU-ID)

ONU-ID is an 8-bit identifier that an OLT assigns to an ONU during ONU activation via PLOAM messages. The ONU-ID is unique across the PON and remains until the ONU is powered off or deactivated by the OLT.

Allocation Identifier (ALLOC_ID)

ALLOC_ID is a 12-bit number that the OLT assigns to an ONU to identify a traffic-bearing entity that is a recipient of upstream bandwidth allocations within that ONU. This traffic-bearing entity is also called T-CONT.

Each ONU is assigned a default ALLOC_ID which is equal to that ONU's ONU-ID, and may be assigned additional ALLOC_IDs as per OLT's discretion.

Transmission Containers (T-CONT)

A Transmission Container (T-CONT) is an ONU object representing a group of logical connections that appear as a single entity for the purpose of upstream bandwidth assignment on the PON. For a given ONU, the number of supported T-CONTs is fixed. The ONU autonomously creates all the supported T-CONT instances during ONU activation. The OLT discovers the number of T-CONT instances supported by a given ONU.

To activate a T-CONT instance to carry upstream user traffic, the OLT has to establish a mapping between T-CONT instance and an ALLOC_ID, which has been previously assigned to the ONU via PLOAM messages. Any ALLOC_ID assigned to the ONU, including the default ALLOC_ID, can be associated with single user traffic T-CONT.

There are 5 types of T-CONTs which can be allocated to the user-

1. Type 1: This T-CONT is of fixed bandwidth type and mainly used for services sensitive to delay and high priority like VOIP.
2. Type 2 and Type 3: Both T-CONT are of guaranteed bandwidth types and mainly used for video services and data services of higher priorities.
3. Type 4: This T-CONT is of best-effort type and mainly used for data services such as Internet and services of low priority which do not require high bandwidth.
4. Type 5: This T-CONT is of mixed type, involving all bandwidth types and bearing all services.

Dynamic Bandwidth Allocation (DBA)

The OLT is responsible for allocating upstream bandwidth to the ONUs. Because the access network is shared, ONU upstream transmissions could collide if they were transmitted at random times. ONUs can be located at varying distances from the OLT, and hence the transmission delay from each ONU is unique. The OLT measures delay and sets a register in each ONU via PLOAM (Physical Layer Operations, Administration and Maintenance) messages to equalize its delay with respect to all other ONUs on the access network. This is called Ranging.

Once the delay of all ONUs have been set, the OLT transmits grants to individual ONUs. A grant is permission to use a defined interval of time for upstream transmission. The grant map is dynamically re-calculated every few milliseconds. The map allocates bandwidth to all ONUs such that each ONU receives timely bandwidth for its needs.

DBA is a methodology that allows quick adoption of users' bandwidth allocation based on current traffic requirements and it is especially good for dealing with bursty upstream traffic. GPON uses TDMA for managing upstream access by ONUs, and at any one point in time, TDMA provides unshared timeslots (upstream bandwidth over time) to each ONU for upstream transmission.

DBA allows upstream timeslots to shrink and grow based on the distribution of upstream traffic loads. DBA functions on T-CONTs, which are upstream timeslots, and each is identified by a particular ALLOC_ID. An ONU must have atleast one T-CONT, but most have several T-CONTs, each with its own priority or traffic class, and each corresponds to a particular upstream timeslot on the PON. Without DBA support on the OLT, upstream bandwidth is statically assigned to T-CONTs, which cannot be shared, and can be changed only through a management system.

There are two forms of DBA - Status Reporting DBA (SR-DBA) and Non-Status Reporting DBA (NSR-DBA).

In NSR-DBA, an OLT constantly allocates a small amount of extra bandwidth to each ONU. If the ONU has no traffic to send, it transmits idle frames. If the OLT observes that an ONU is not sending idle frames, it increases the bandwidth allocation to that ONU. Once that ONU starts sending idle frames, the OLT reduces its allocation accordingly. NSR-DBA has the advantage that the ONUs need not be aware of DBA, however, its disadvantage is that there is no way for the OLT to know how to allocate bandwidth to several ONUs in the most efficient way.

SR-DBA involves explicit T-CONT buffer status provided by the ONUs when OLT polls them. In this method, the OLT solicits T-CONT buffer status, and the ONUs respond with a separate report for each assigned T-CONT. The report contains the data currently waiting in T-CONTs in the specified time slots. OLT receives the status (DBA) report, re-calculates bandwidth allocation (BW Map) through DBA algorithm and sends new BW Map to the ONUs in-band with downstream traffic. The ONU receives the BW Map from OLT and sends the data in the specified time slots. When an ONU has no information to send, upon receiving a grant from the OLT, it sends an idle cell upstream to indicate that its buffer is empty. This informs the OLT that the grants for that T-CONT can be assigned to other T-CONTs. If an ONU has a long queue waiting in its buffer, the OLT can assign multiple T-CONTs to that ONT.


GPON Transmission Convergence (TC) Layer

ITU-T recommendation G.984.3 describes GPON TC layer which is equivalent to Data Link layer of OSI model. It specifies GPON frame format, the media access control protocol, OAM processes and information encryption method. Figure 3 shows the GTC frame structures for downstream and upstream directions. The downstream GTC frame consists of the physical control block downstream (PCBd) and the GTC payload section. The upstream GTS frame contains multiple transmission bursts. Each upstream bursts consists of the upstream physical layer overhead (PLOu) section and one or more bandwidth allocation intervals associated with a specific ALLOC_ID.

The downstream GTC frame provides the common time reference for the PON and common control signaling for the upstream.

Downstream GPON Frame Format

A downstream GTC frame has a duration of 125us and is 38880 bytes long, which corresponds to downstream data rate of 2.48832 Gbps. Figure 4 shows a detailed downstream GTS frame format.

The OLT sends the PCBd in the broadcast manner, and every ONU receives the entire PCBd. The ONUs then act upon the relevant information contained therein. The Psync field indicates beginning of the frame to the ONUs. The Ident field contains an 8-KHz Superframe Counter field which is employed by the encryption system, and may also be used to provide low rate synchronous reference signals. The PLOAMd field handles functions such as OAM-related alarms or threshold-crossing alerts. BIP field is Bit Interleaved Parity used to estimate bit error rate. The downstream Payload Length indicator (Plend) gives the length of the upstream bandwidth (US BW) map. The Plend is sent twice for redundancy. Each entry in the Upstream Bandwidth (US BW) map field represents a single bandwidth allocation to a particular T-CONT. The number of entries is given in the Plend field.

The Allocation ID (ALLOC_ID) field indicates the recipient of the bandwidth allocation i.e. a particular T-CONT. The lowest 254 allocation ID values are used to address the ONU directly. During the ranging process, the first ALLOC_ID given to the ONU must be in this range. This ALLOC-ID is known as the default Allocation ID. This ALLOC_ID is same as ONU-ID number used in PLOAM messages. If further ALLOC_ID values are required for that ONU, they should be taken from those above 255. ALLOC_ID 254 is the ONU Activation ALLOC_ID- used to discover unknown ONUs. The Flag field allows the upstream transmission of physical layer overhead blocks for a designated ONU. The Slot Start and Stop field indicates the beginning and ending of upstream transmission window. The CRC field provides error detection and correction on bandwidth allocation field.

The GTC payload field contains a series of GEM (GPON Encapsulation Method) frames. The downstream GEM frame stream is filtered at the ONU based upon the 12-bit Port ID field contained in the header of each GEM frame. Each ONU is configured to recognize which Port-IDs belong to it. The Port-ID uniquely identifies a GEM Frame.


Upstream GPON Frame Format

The Upstream GTS frame duration is also 125us and is 19440 Bytes long, which gives an upstream data rate of 1.24416 Gbps. Each upstream frame contains a number of transmission bursts coming from one or more ONUs. Each upstream transmission burst contains an upstream physical layer overhead (PLOu) section and one or more bandwidth allocation intervals associated with individual ALLOC-IDs. The BW map dictates the arrangement of the bursts within the frame and the allocation intervals within each burst. Each allocation interval is controlled by a specific allocation structure of the BW map. Figure 5 shows upstream GTC frame format.

The physical layer overhead (PLOu) at the start of the ONU upstream burst contains the preamble which ensures proper physical layer operation of the burst-mode upstream link. The PLOu field contains the ONU-ID field which indicates the unique ONU-ID of the ONU that is sending this transmission. The upstream physical layer OAM (PLOAMu) field is responsible for management functions like ranging, activation of an ONT, and alarm notifications. The upstream power leveling sequence (PLSu) field contains information about the laser power levels at the ONUs as seen by the OLT. The dynamic bandwidth report (DBRu) field informs the queue length of each T-CONT at the ONT.


Mapping of GEM Frames into GTC Payload

GEM traffic is carried over the GTC protocol in transparent fashion. In the downstream direction, GEM frames are transmitted from the OLT to the ONUs using the GTC frame payload section. The OLT may allocate as much duration as it needs in the downstream, upto and including all of the downstream frame. The ONU filters the incoming frames based on Port-ID. In the upstream direction, frames are transmitted from ONU to OLT using the configured GEM allocation time. The ONU buffers GEM frames as they arrive, and then sends them in bursts when allocated time to do so by the OLT. The OLT receives the frames and multiplexes them with the frames from other ONUs.

Ethernet over GEM
The Ethernet frames are carried directly in the GEM frame payload. The preamble and SFD bytes are discarded prior to GEM encapsulation. Each Ethernet is mapped to a single or multiple (by fragmenting) GEM frames.