Analysis of Gigabit Passive Optical Network Technology in Access Network Applications

It is of vital importance to network operators to adopt the most appropriate solution to achieve the last few miles of access, and the high bandwidth, high efficiency and easy scalability of passive optical network (PON) technology make it very competitive. This paper analyzes the characteristics of PON technology in access network applications in detail.

Choosing the most appropriate solution to achieve the last few miles of network access is a challenge facing network service providers. Whether it is a wireless link or a copper or fiber link, there are inevitably many competing technologies to choose from, especially in the early stages of the implementation of many competing standards. Finding the most suitable solution is not easy, and a large number of interrelated factors must be considered, and this choice is usually high-risk because any decision involving large-scale investments will affect the company's operations for many years to come.

This issue is particularly significant for the ongoing development of high-speed PON in the Asian market, especially when it comes to Gigabit Passive Optical Network (GPON) systems. While other regions are considering the pros and cons of different fiber access methods, service providers in Asia are actively deploying and testing this technology as a solution to many of their problems.

Passive Optical Network Standards

The target market for access networks is usually densely populated areas in urban areas with large numbers of residential and commercial users in a few buildings. In addition, in many areas, the expansion of traditional PSTN circuits is limited, giving service providers ample "new market" opportunities to introduce new advanced multimedia and broadband services without being hindered by the need to support obsolete technologies.

PON is an ideal solution for such applications. Specifically, a PON consists of an optical line terminal (OLT) located at the central office and a group of associated optical network terminals (ONTs) located at the customer end, with an optical distribution network (ODN) consisting of optical fibers and passive optical splitters or connectors between them (see Figure 1).

In the PON topology, an OLT can have multiple PON modules, each of which drives a separate PON network through an inexpensive passive splitter and is connected to multiple ONTs by distribution fiber. Fiber and passive optical devices eliminate the need for active electronic devices and related maintenance in access network distribution equipment.

The transmission and processing of downstream and upstream data streams in PON are different. Downstream data is broadcast from the OLT to each ONT, and each ONT determines and processes the relevant data by matching the address in the data packet/data unit. Due to the shared nature of ODN, the processing of upstream data streams is more complicated. In order to prevent collisions, it is necessary to coordinate the transmission streams from each ONT to the OLT. Upstream data is transmitted according to the control mechanism in the OLT, using the time division multiple access (TDMA) protocol, which allocates dedicated transmission time slots to each ONT. These time slots are synchronized, so data streams from different ONTs will not collide.

Just like the early DSL with different optional technologies, service providers must be able to choose the most suitable PON solution. How to choose the most suitable PON technology?

In the mid-1990s, an organization composed of major network operators established the Full Service Access Network (FSAN) Alliance, whose purpose was to develop a common standard for PON equipment. This standard gradually developed into B-PON, using ATM as its transmission protocol. In addition, the IEEE also established the Ethernet First Mile Group (EFM) in 2001, focusing on the standardization of 1Gbps Ethernet dedicated symmetrical systems. The two organizations launched APON and EPON technical standards respectively.

During this period, the FSAN group began to standardize PONs that operate at rates above 1Gbps. More importantly, while covering basic transmission issues, this standard also provides support for multiple services in a highly scalable manner, as well as management, maintenance, and configuration functions. This work ultimately produced a gigabit-rate PON standard that supports the transmission of data in formats such as IP and TDM with extremely high efficiency while achieving high rates, as shown in Figure 2.

It is particularly important that the world's major service providers participated in the formulation of the system requirements for the main part of the GPON standard, which is reflected in the ITU G.984.1, the Gigabit Service Requirements (GSR) standard. GSR has now become the standard for all work in this field, ensuring the compatibility and interoperability of the network.

Key features of GPON technology

As defined by the GSR, the key characteristics of GPON include:

1. Full service support. Including voice (TDM, SONET/SDH), Ethernet (10/100BaseT), ATM, leased lines, etc.;

2. The physical coverage range is at least 20 kilometers, and the protocol supports a logical coverage distance of 60 kilometers;

3. Support various rate options using the same protocol, including symmetrical 622Mbps, symmetrical 1.25Gbps, 2.5Gbps downstream data flow and 1.25Gbps upstream data flow;

4. Powerful OAM&P functions, providing end-to-end service management;

5. Implement security protection of downstream data streams at the protocol layer through the PON multicast feature.

Specifically, these features provide network operators with three key advantages: Gigabit speeds, maximum performance and network efficiency, and exceptional flexibility and scalability:

1. Rate and flexibility. GPON offers a wider range of data rates than its competing technologies, up to 2.488Gbps downstream and 1.244Gbps upstream. It also uses a new transmission convergence layer (TC), which uses a GEM and frame-based protocol for service mapping, which is an optimized version of ITU-T G.7041 GFP. Because GEM is a common mechanism for transmitting different services in a simple and efficient manner, GEM is an important foundation for the GPON TC layer. In addition, the use of GEM obviously supports TDM services.

2. Efficiency. Broadband is a limited resource for operators, so it is necessary to achieve maximum network utilization efficiency and obtain the maximum benefits under limited bandwidth conditions. However, since different PON technologies have different characteristics, the overall cost of the solution must be considered comprehensively. The device costs of different PON technologies are similar. For example, the EPON system requires the use of related equipment such as VoIP, so these devices must be calculated in the cost. In addition, considering the cost per bit and the revenue that can be obtained from the infrastructure, this network that provides 100% efficiency at a rate of 1.25Gbps is obviously more attractive than a network with a rate of only 622Mbps and an efficiency of only 50%.

Other issues discussed in this article involve the actual efficiency of different PON protocols. Four factors must be considered here: line coding, PON TC or MAC layer efficiency, transmission protocol (ATM, Ethernet or GFP) efficiency and service adjustment efficiency. Combining these two factors gives us the available bandwidth defined as "revenue" in the network, as shown in Figure 3.

3. Scalability. The key issue that must be considered when adopting a solution for the access layer is how to carry an increasing number of protocols and technologies. We currently have to support many TDM and data services, as well as emerging applications such as storage area networks (SANs) and data video.

GPON provides a clear transition path to add these services to PON through GFP's adaptive approach, without disrupting existing equipment or changing the transport layer. This is in stark contrast to APON and EPON technologies, which require specific adjustments for each service, especially when dealing with emerging services.

Benefits of GPON to Operators

1. Economically provide Ethernet services and safeguard TDM revenue. GPON supports the introduction of new Ethernet services with higher bandwidth while providing traditional voice services at a much lower cost than before.

2. Users have higher bandwidth. GPON solutions can provide a single optical wavelength rate of up to 2.5Gbps on the access network, and a multi-wavelength rate of up to 20Gbps in a single optical fiber cable, providing economical T1/E1 and Ethernet connections. Competitive technologies usually have a rate of only 622Mbps or lower, making it difficult to sustain any new application.

3. Voice and data are fully integrated in a single fiber. Voice and data are transmitted in their own format without adding extra complexity to the network or CPE, and have a longer transmission distance. The GPON solution can cover a range of 20 miles from the central office, almost twice that of competing technologies, and can support redundant topology to provide protection in the event of a fiber break.

4. Shorter payback period. In areas where fiber has already been installed, the investment cost payback period of the GPON solution is 9 to 16 months, depending on the number of buildings and users covered by the service; when new feeder lines and feeder fiber stations are required, the payback period of the entire network is about 12 to 24 months.