What is Access Network?
ITU-T definition:
An access network (AN) is the so-called last mile, is a series of transmission entities (such as line and transmission facilities) between service node interface (SNI) and user network interface (UNI), which is an implementation system providing bearer capabilities for telecom service.
Based on transmission media, AN is divided into wired access network and wireless access network. The wired access network includes the commonly used coaxial cable access (for CATV) or copper twisted pair access (for broadband services accessed to home using ADSL modems), and the optical access that will be presented in this post.
The wireless access technologies include the fixed wireless access and mobile wireless access. The fixed wireless access refers to the WLAN technologies. Such as WiFi and the mobile wireless access refers to the 2G/3G technologies used in mobile phone services. Currently, fixed network access services use the copper twisted pair access, that is, DSL access mode. After fiber technologies are fully recognized, large carriers begin to deploy optical networks in a large scale. The current AN development trend can be summarized as optical-in copper-out. This figure displays the optical-in copper-out process.
You may ask: “Why not replace copper lines with fibers”? The implementation of technologies is encumbered by live networks where enormous copper lines exist. To utilize existing resources to the maximum, coexistences of copper lines and fiber is inevitable. FTTH will be the main access mode in the future.
To following illustrates the reason why PON technologies are used for the optical access by comparing the copper line access an optical access. For the early copper line access, the point to point (P2P) mode is used. Specifically, a user is connected to a port of the device. When the optical access is used to provide services to users. The effective distance of a fiber to reaches tens of kilometers, and each optical port provides a 2.5 Gbit/s bandwidth. If the P2P mode is still used, fibers and bandwidths will be severely wasted. Then, let´s talk about the point-to-multipoint (P2MP) roadside switch solution. Compared with the P2P mode, this solution greatly improves the resource utilization. But, the operation and maintenances of roadside switches turn out to be a problem because they are active devices that require stable power supplies. Therefore, the PON access turns out to be the optimal solution for the optical access. On the entire line, passive devices are connected in the P2MP mode, resolving the resource utilization problem of the first mode, and the operation and maintenance problem of the second solution.
