With the development of broadband services, people are increasingly aware of the serious bandwidth "bottleneck" in the network access part (the last mile). The two ends of the access part have reached the rate above the gigabit level, such as the internal transmission rate of the widely used PC at the client end has reached the gigabit rate; as the other end of the access part, the metropolitan area network or the backbone network The rate has also reached 2.5-10Gbit/s, which is at least 3 orders of magnitude higher than the access part. With the implementation of triple play, it is becoming more and more urgent to break through the bottleneck of the access network. Only by breaking through the bandwidth "bottleneck" of the access part can the entire network effectively play the role of broadband and truly promote the development of various services. Bring economic and social benefits to operators.
Technically speaking, there are three ways to break through the bottleneck of the access network, one is very high-speed digital subscriber line (VDSL); the other is fiber-to-the-home (FTTH) based on passive optical network (PON); the third is high-speed wireless access. EPON is a passive optical network technology based on Gigabit Ethernet. It inherits the low cost and ease of use of Ethernet and the high bandwidth of optical network. It is the most cost-effective among many technologies for FTTH. With the official release of the EPON international standard - IEEE802.3ah in 2004, the EPON industry alliance has attracted the active participation of many manufacturers. From EPON core chips, optical modules to systems, the EPON industry chain has become increasingly mature.
1. EPON technology and features
(1) Development of EPON
Optical fiber access can be technically divided into two categories: active optical network (AON, ActiveOpticalNetwork) and passive optical network (PON, PassiveOptical Network). PON is a pure media network. Since the active equipment between the central office and the user end is eliminated, it can avoid the electromagnetic interference and lightning effects of external equipment, reduce the failure rate of lines and external equipment, and improve system reliability. It can save maintenance costs and is a long-awaited technology for the telecommunications maintenance department. The service transparency of PON is good, and it can be applied to signals of any standard and rate in principle. At present, the practical technologies based on PON mainly include APON/BPON, GPON, and EPON/GEPON. The main difference lies in the adoption of different two-layer technologies. Specifically shown in Figure 1.
(2) Composition of EPON system
In an EPON, without any complex protocol, the optical signal can be accurately transmitted to the end user, and the data from the end user can also be centrally transmitted to the central network. At the physical layer, EPON uses 1000BASE Ethernet PHY, and at the same time, in the transmission mechanism of PON, through the newly added MAC control command to control and optimize the burst data between each optical network unit (ONU) and optical line terminal (OLT) communication and real-time TDM communication. Since the ONU sends datagrams in its own time slot, there is no collision and no need for CDMA/CD, thus making full use of the bandwidth. In addition, EPON provides QoS similar to APON/GPON by implementing 802.1p in the MAC layer.
(3) EPON transmission principle
EPON transmits data downstream from OLT to multiple ONUs and transmits data upstream from multiple ONUs to OLT is very different. The different upstream/downlink technologies adopted are shown in Figure 3 respectively.
When the OLT is started, it will periodically broadcast information such as time slots allowed to be accessed on this port. After the ONU is powered on, according to the access information broadcast by the OLT, it actively initiates a registration request, and the OLT passes the authentication of the ONU (this process is optional), allows the ONU to access, and assigns an OLT port to the ONU that requests registration. Logical Link Identifier (LLID).
2. EPON radio and television network solutions and last 100-meter coaxial broadband access technology
(1) EPON Broadcasting Network Solution
The EPON system uses a single-mode optical fiber to transmit bidirectional data using two different wavelengths (uplink wavelength 1310nm and downlink wavelength 1490nm) on one core fiber.
There are mainly the following three ways to enter the home after FTTB is realized by using EPON.
·FTTH (fiber-to-the-home), the client configures ONU to receive data information.
After LAN and ONU arrive at the building, use twisted-pair wires to enter the home, and the user bandwidth can be adjusted by controlling the OLT output port and the second-floor switch of the building.
·EOC, ONU to the building, the last 100m of the user end still uses the coaxial cable to enter the home, reducing the scope of transformation as much as possible, and the user end configures the EOC module to exchange data with the ONU.
EOC (Ethernet Over Cable) can be mainly divided into three categories: baseband transmission, modulation transmission, and 2.4GHz extended application, among which many specific standard/non-standard technologies can be subdivided, such as baseband, MoCA, coaxial Wi-Fi, CableRan, UcLink wait.
The EOC solution uses the original coaxial resources to solve the access problem of the last 100 meters, avoids the huge two-line home renovation project, and provides data uplink and downlink transmission functions without affecting the original downlink broadcast and TV signals.
EPON + passive coaxial broadband access for the last 100 meters is the most suitable two-way transformation mode for radio and television networks. Telecom currently mainly promotes PON + twisted pair access for the last mile.
(2) Last 100-meter coaxial broadband access technology
·Baseband transmission
The in-band frequency of the coaxial cable is 0-1000MHz, and the cable TV system works at 5-860MHz, among which 5-65MHz is used for the uplink channel. However, in practical applications, the 5-20 MHz frequency band cannot be used by the CMTS/CM communication system that adopts the frequency band transmission mode due to serious spurious signal interference. And Ethernet is a baseband transmission system. When transmitting at a rate of 10Mbit/s (10BASE-T), the power spectrum of the Ethernet signal is mainly concentrated in the range of 0.5-15 MHz. This provides the possibility of frequency resources for establishing Ethernet in a coaxial cable network. In fact, today's data exchange chip and electronic technology can realize 10BASE-T communication without relay at a distance of 100 m through coaxial cable in the HFC network of cable TV at low cost.
3. Conclusion
UCLink and CableRan technologies have been used by a small number of radio and television users in the first two years, but the effect is not very satisfactory, and they have not been adopted on a large scale. Baseband transmission and coaxial Wi-Fi technology has entered the commercial stage, and many places are conducting pilot work. It is believed that more and more EPON+EOC networks will appear in the near future.
Passive Ethernet EPON uses the topology of PON to realize Ethernet access. It has the advantages of high bandwidth, easy maintenance, and low cost. It can comprehensively access multiple services such as voice, data, and video through a single platform. The combination of EPON technology and HFC network provides a new solution for the rapid development of broadband access services for radio and television networks.
