Introduction to the optical fiber access network system

With the rapid development of the optical fiber technology, the optical fiber access network system is also improved. Compared with other technologies, the optical fiber access network has many advantages, so I studied the characteristics and performance of the optical fiber access network system. I would like to share with you here, hoping to help you. The optical access network (OAN) technology can be divided into two categories: Active Optical Network (AON) and Passive Optical Network (PON ). AON can be divided into SDH-based AON and PDH-based AON. PON can also be divided into ATM-based PON (APON) and Ethernet-based PON (EPON ).

1. Broadband OAN System

1.1 SDH-based AON
With the increasing transmission capacity of backbone networks, the access methods of transmission networks are becoming more and more diversified. technically speaking, the access layer requires less bandwidth and needs to provide integrated service transmission such as IP, TDM, and ATM. the SDH-based optical fiber access network system that provides IP and ATM transmission and processing will be the main method to solve the access layer transfer problem. This method can provide point POP for a business at a low cost) provides access, transmission, and protection for high-quality leased lines, ATMs, and IP addresses.

The main advantage of applying SDH in access networks is that SDH can provide ideal network performance and business reliability; the inherent flexibility of SDH makes it especially suitable for SDH systems in the rapidly developing fiber-optic access network system of cellular communication. of course, considering the high cost sensitivity of the access network and the bad performance of the operating environment, SDH devices suitable for access networks must be a new system with high compact power consumption and low cost.
The Latest Development Trend of SDH for access networks is to support IP access. At present, at least Ethernet interface ing is required. In addition to carrying voice traffic, some SDH net loads can be used to transmit IP services, so that SDH can also support IP Access. there are multiple supported methods. In addition to the existing point-to-point communication protocol (PPP) methods, there are also link access procedures (LAPS) and general frame formation procedures (VPNs) on SDH.

The Ethernet over SDH system implements an IP packet multi-switched wide area network. In essence, it uses a connectionless network mechanism. The internal full-mesh Connection provides a connectionless network mechanism suitable for Distributed communication, saves a lot of light bandwidth for the service provider. ethernet over SDH combines the L2 switching flexibility and resource optimization capabilities of Ethernet with the large capacity, high bandwidth efficiency, and low protocol overhead of the existing SDH optical network, this provides a high-speed and Economical Data Access solution. the frame ing process of the Ethernet over SDH system uses the LAPS protocol frame instead of the PPP frame. it first encapsulates the MAC frame of the Ethernet into the LAPS protocol frame, and then maps the LAPS protocol frame to the SDH frame. ethernet over SDH technology overcomes the limitations of traditional SDH in data transmission and opens up a new world for SDH access.

1.2PON
PON is a pure media network that can avoid electromagnetic interference and lightning impact on external devices, reduce the failure rate of lines and external devices, improve the reliability of the optical fiber access network system, and save maintenance costs, it is a technology that has long been expected by the Telecommunications maintenance department. PON has good business transparency and can be applied to signals of any standard and speed in principle. in particular, APON can share optical fiber and optical line terminals by using the centralized and Statistical Multiplexing of ATM and the passive splitter, the cost is expected to be 20% ~ lower than the traditional PDH/SDH Access System Based on circuit switching ~ 40%.

APON's business development is implemented in stages. In the initial stage, it is mainly the VP leased line service. compared with ordinary leased line services, APON provides low cost, small size, power saving, reliable and stable optical fiber access network systems, and has some advantages in cost-effectiveness. step 2: implement the simulation of the circuit of one group and the secondary group, and provide the intranet connection and telephone and data services for the enterprise. step 3 implements Ethernet interfaces and provides Internet and VLAN services. in the future, it will gradually expand to other services and become a fully-qualified access network system. an important factor in whether APON can be widely used is price. currently, the service supply capability of APON products is limited and the cost is too high. The market prospect of APON products is uncertain due to the global failure of ATM, but its technological advantages are obvious. especially when considering the overall operation and maintenance costs, the PON system is laid in newly-created areas, highly competitive areas, or areas that need to replace the old copper system ), fiber-to-Building (FTTB) is a far-sighted choice.

1.3 EPON
EPON is a combination of several optimal technologies and network structures. EPON uses a point-to-multiple-point structure and Passive Optical Fiber Transmission Mode to provide multiple services over Ethernet. at present, IP/Ethernet applications account for more than 95% of the total LAN communication, and EPON has become the most effective communication method for end users to connect to the access network because of the above cost-effective and efficient structure. the emergence of 10 Gbit/s Ethernet trunk and Metro rings will also make EPON the best solution for all-optical networks in the future.

In an EPON, optical signals can be accurately transmitted to end users without any complicated protocols, and data from end users can also be centrally transmitted to the central network. in the physical layer, EPON uses a-base Ethernet PHY. In the PON transmission mechanism, it uses new MAC control commands to control and optimize each optical network unit (ONU) in the second layer of the protocol, EPON adopts mature full-duplex Ethernet technology and uses TDM, because ONU sends data packets within its own time slot, there is no collision and no CDMA/CD is required to make full use of the bandwidth. in addition, EPON implements 802.1p In the MAC layer to provide QoS similar to APON.

2. Several optical access methods

2.1 Optical Fiber Distributed Network (FDN)
FDN is divided into active and passive optical fiber networks.
The difference between a passive optical fiber network and an active optical fiber network is that it replaces a remote Optical Fiber Device (ROLT) in an active optical fiber network with a pair of optical passive splitter ). the transmission protocols used by the two are also different. The active optical fiber network adopts PDH or SDH transmission protocols, while the passive optical fiber network must adopt TDMA or TCM. therefore, the transmission distance and capacity of the passive optical fiber network must be limited. in addition, the active optical fiber network features simple technology, easy implementation, and strong networking capabilities. the advantage of the passive optical fiber network is that it does not require data centers and reduces the maintenance workload and costs.

2.2 fiber-optic coaxial hybrid network (HFCs)
It has evolved from a cable TV (CATV. it can provide CATV services as well as voice, data, and other types of services. A highly distributed smart Broadband User Access Network Based on analog frequency division multiplexing technology and integrated application of analog and digital transmission technology, optical fiber and coaxial cable technology, and RF technology, it is a product of the combination of CATV and telephone network. the coverage of the hfc-based network can reach 100 km, and the transmission signal attenuation is small and the noise is low. It is an ideal CATV network transmission technology.

The typical structure of HFCs uses optical fiber cables as the trunk transmission network of CATV. The cables are distributed in star or ring shapes at the front ends of cable TV stations and extend to residential areas, office areas, and suburban counties and villages in the city, many optical nodes are formed. from the Optical Node, the cable TV signal is sent to the end user through the traditional coaxial cable. in this way, the original CATV network can be fully utilized, and the network construction is fast and the cost is low. At the same time, due to the large bandwidth of the coaxial cable, it can be used as the access platform for broadband integrated services. it is a technology favored by cable TV operators.

2.3 Optical Network Unit (ONU)
According to the location of ONU in OAN, OAN can be divided into FTTC, FTTB, FTTO and FTTH types.
In the FTTC structure, The ONU is set on the side of the road or the wire rod side of the box. A twisted pair copper cable is used between ONU and various users. For broadband image service, coaxial cable is used. one of the main features of FTTC is that existing copper cable facilities can still be used in the introduced line, which can delay the introduction of some optical fiber investment. at present, FTTC is the most realistic and economical solution for OAN to provide narrowband services below 2 Mbit/s. however, this structure is not ideal if comprehensive narrow band and broadband services are to be provided.

In the FTTB structure, ONU is directly placed in the building and then distributed to users through multiple pairs of twisted pair wires. FTTB is more fiber-oriented than FTTC, so FTTB is more suitable for users with high density and need to provide comprehensive narrowband and broadband services. both the FTTO and FTTH structures are equipped with a non-source optical splitter on the side of the road, and The ONU is moved to the user's office or home, which is a truly transparent optical fiber network. they are not subject to any transmission standard, bandwidth, wavelength and transmission technology, and are the ideal mode and long-term goal of OAN's development.

3 access technology application and Comparison

The starting point of broadband access network construction is one of the main problems that currently plague telecom operators. Broadband is the development direction. This is a consensus in the industry. But where should we start? How can we ensure an effective ROI? There are many points worth exploring.

Although the growth rate of data services has reached 40% in recent years, which is much higher than the 10% annual growth rate of voice services, the actual business volume proportion of the entire traffic is still relatively low, about 3%. from the national perspective, the regional differences are still very large, and even a large imbalance exists in the area of a city, and the development of each point is not balanced. therefore, in the planning and construction of broadband data access networks, a fixed mode cannot be used. Instead, a corresponding solution should be adopted based on the actual situation, and the scalability of the optical fiber access network system should be fully considered. At present, there is a tendency in applications: IP acts as the backbone layer for transmission, such as IP over SDH, IP over DWDM, and IP over Optical, the next generation of the IP network will be able to support all services.

With the rapid development of OAN in recent years, FTTC and FTTB have been gradually implemented. All-optical networks are of course the final trend. However, at this stage, FTTH is not realistic, and the last few hundred meters connecting users, that is, the technology of the Home segment, is also brilliant. The main transmission media are: xDSL technology based on twisted pair wires, HFCs Technology Based on copper axis, wireless technology, APON/EPON technology based on optical fiber, and Ethernet technology. These five main technologies have their own technical characteristics, and also determine their respective application scenarios.

4 xDSL

Digital user line DSL is a broadband access technology based on common telephone lines. It transmits data and voice signals separately on the same copper line. data signals are not transmitted through telephone exchange equipment, this reduces the burden on telephone switches and keeps online without dialing. Therefore, DSL does not need to be used to access the Internet. "x" in xDSL represents various digital user line technologies, such as asymmetric digital user line ADSL), high-speed digital user line HDSL) and high-speed Asymmetric Digital user line VDSL. the main difference between them is that the symmetry of the uplink and downlink, as well as the transmission rate and effective distance are different. ADSL is a mature one among many DSL technologies. It has the advantages of large bandwidth, simple connection, and low investment, so it has developed rapidly. HDSL uses two or three pairs of Copper Twisted Pair wires to provide users with one-time PDH group rate Duplex Digital connections.

VDSL is a high-speed ADSL, and some people call it a broadband digital user line (BDSL ). the VDSL optical fiber access network system is generally used for the last household connection in the optical fiber access network. The transmission distance is only 100 Mbit/s to 1 km13 Mbit/s ), the downlink transmission rate can reach 13, 26, 52 Mbit/s, or even 155 Mbit/s. the combination of VDSL technology and Ethernet technology can solve the problem of IP access solutions. users do not have to re-deploy 5 categories of high-cost twisted pair wires, only through the VDSL terminal adapter, you can connect to the Internet and transmit high-speed Ethernet signals over a common telephone line. You don't have to worry about IP-based access.

1. Detailed comparison of two common access network technologies
2. Introduction to common ATM access methods
3. Comprehensive Explanation of ATM access knowledge
4. Dual-line access technology opens bridges between north and south
5. Considerations for dual-line access