Using the most suitable solution to achieve the last few miles of access is critical to the interests of network operators, and the high bandwidth, high efficiency, and scalability of passive optical network (PON) technology make it highly competitive force. This article analyzes the characteristics of PON technology in access network applications in detail.
Choosing the most suitable solution to achieve the last few miles of network access is a challenge for network service providers. Whether using wireless links or copper or fiber links, 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. This choice is usually very risky, because any decisions involving large-scale investments will affect the company ’s operations for many years to come.
This problem 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 advantages and disadvantages of different fiber access methods, service providers in Asia are actively deploying and experimenting with this technology as a solution to many of their problems.
Passive optical network standard
The target market for access networks is usually densely populated areas in urban areas, with a large number of residents and business users in some buildings. In addition, the expansion of traditional PSTN circuits is limited in many regions, giving service providers ample opportunity to introduce new advanced multimedia and broadband services in "new markets" without being hindered by the need to support outdated technologies.
PON is the ideal solution for such applications. Specifically, a PON is composed of an optical line terminal (OLT) located at the central office and a group of associated optical network terminals (ONT) located at the client, and between them is composed of optical fiber and passive optical splitter or connector Optical Distribution Network (ODN) (see Figure 1).
In the PON topology, an OLT can have multiple PON modules, and each PON module drives a separate PON network through an inexpensive passive splitter, and is connected to multiple ONTs by distributed optical fibers. Optical fiber and passive optical devices eliminate the need for active electronic devices and related maintenance to access network distribution equipment.
The transmission process of the downstream data stream and the upstream data stream in the PON is different. The downstream data is broadcast from the OLT to each ONT. Each ONT determines and processes the relevant data through the address matching 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. The upstream data is transmitted according to the control mechanism in the OLT, using Time Division Multiple Access (TDMA) protocol, which allocates a dedicated transmission time slot to each ONT. These time slots are synchronized, so data streams from different ONTs will not collide.
Like earlier DSLs 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, when an organization composed of major network operators set up a full-service access network (FSAN) alliance, the purpose of the organization was to develop a common standard for PON equipment. The standard gradually evolved into B-PON, and used ATM as its transmission protocol. In addition, IEEE also established the First Mile Ethernet Group (EFM) in 2001, focusing on the standardization of 1Gbps Ethernet dedicated symmetric systems. These two organizations have launched APON and EPON technical standards respectively.
During this period, the FSAN team began to standardize PONs operating at rates above 1Gbps. More importantly, while covering basic transmission issues, this standard also provides support for multiple services, as well as management, maintenance, and configuration functions in a highly scalable manner. This work eventually produced the Gigabit rate PON standard. While achieving high rates, it also supports the transmission of IP and TDM format data at extremely high efficiency, 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, or Gigabit Service Requirements (GSR) standard, and GSR has now become the main task Standards ensure network compatibility and interoperability.
Key characteristics of GPON technology
As defined by GSR, the key features of GPON include:
1. Full business support. Including voice (TDM, SONET / SDH), Ethernet (10 / 100BaseT), ATM, leased lines, etc .;
2. The physical coverage is at least 20 kilometers, the protocol supports a logical coverage distance of 60 kilometers;
3. Support various rate options using the same protocol, including symmetric 622Mbps, symmetric 1.25Gbps, 2.5Gbps downstream data flow and 1.25Gbps upstream data flow;
4. Powerful OAM & P function, providing end-to-end business management;
5. Realize the security protection of the downstream data stream at the protocol layer through the PON multicast feature.
Specifically, these features provide network operators with three key advantages of gigabit rate, maximum performance and network efficiency, excellent flexibility, and scalability:
1. Speed ​​and flexibility. GPON offers a wider range of data rates than its competing technologies, up to 2.488Gbps downstream data flow and 1.244Gbps upstream data flow. It also uses a brand new transmission convergence layer (TC), using a GEM and frame-based protocol for service mapping, which is an optimized version of ITU-T G.7041 GFP. Because GEM uses a simple and efficient general mechanism for transmitting different services, GEM is an important foundation for the GPON TC layer. In addition, the adoption of GEM clearly supports TDM services.
2. Efficiency. Broadband is a limited resource for operators, so it must achieve maximum network utilization efficiency and obtain maximum benefits under limited bandwidth conditions. However, because different PON technologies have different characteristics, the overall cost of the solution must be considered comprehensively. The cost of devices of different PON technologies is similar. Taking the EPON system as an example, the system requires the use of VoIP and other related equipment, so these equipment must be included in the cost. In addition, considering the cost per bit and the revenue that can be obtained from the infrastructure, this 100% efficiency at 1.25Gbps is obviously more attractive than a network with only 622Mbps and 50% efficiency.
The 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 shows us the available bandwidth defined as "revenue" in the network, as shown in Figure 3.
3. Scalability. The key issue that must be considered in the solution used by the access layer is how to carry an increasing number of protocols and technologies. We must currently support many TDM and data services, and we must also support emerging storage area networks (SAN) and data video. application.
GPON provides a clear conversion path through the adaptive method of GFP to add these services to the PON, and will not destroy existing equipment or change the transmission layer. This is in sharp contrast to APON and EPON technologies, both of which require specific adjustment methods for each service, especially when dealing with emerging services.
Benefits of GPON for operators
1. Economically provide Ethernet services and protect 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. The GPON solution can provide a single optical wavelength up to 2.5Gbps on the access network, a multi-wavelength rate up to 20Gbps in a single fiber optic cable, and provide economical T1 / E1 and Ethernet connections. Competing technologies usually have a rate of only 622 Mbps or lower, so it is difficult to sustain any new applications.
3. Voice and data are fully integrated in a single fiber. Transmitting voice and data in its own format will not increase the complexity of the network or CPE, and has a longer transmission distance. The GPON solution can cover a range of 20 miles from the central office, which is almost twice that of competing technologies. It can support redundant topologies to provide protection in the event of fiber disconnection.
4. Shorter payback period. In areas where optical fiber has been installed, the investment cost recovery period of the GPON solution is 9 to 16 months, depending on the number of buildings and users covered by the business; when new branch lines and branch fiber stations are needed, the recovery period of the entire network is about For 12 to 24 months.
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