Hello everyone!
In the previous section, we talked about the xDSL technology, which is an important technology in the access network. However, xDSL has a contradiction between bandwidth and effective transmission distance. To solve this problem, we adopt the "fiber-in copper-out" solution and introduce the application of optical fibers in the access network. The optical access technology solves the problem between distance and bandwidth.
Optical Fiber Access Solution
The point-to-point optical network has advantages of satisfying the bandwidth requirement of the customer, service isolation, and high security. Disadvantages: low fiber utilization and linear cost increase with the number of users. In addition, when users are densely distributed, factors such as the capacity of CO devices, cabling density, and occupation of the distribution equipment room must be considered.
Based on the above disadvantages, a tree-structured network topology, that is, a point-to-multipoint (P2MPM) optical network, is developed to solve this problem. A backbone optical fiber transmits signals and then connects to the user terminal. Compared with the point-to-point optical network, 50% of optical fibers and optical modules can be saved. The point-to-multipoint optical network currently includes an active optical network (AON) and a passive optical network (PON).
An active optical network is a network that uses active optical fiber are transmission devices, such as optical switches and active optoelectronic devices. between the CO device and the user distribution unit. The active devices need to be configured and maintained during network deployment. The maintenance cost is high. In addition, the roadside power supply is difficult.
A passive point-to-multipoint optical network. Optical fibers from the equipment room to users are passive. Passive optical splitters are used to physically split optical signals on the backbone line and then connect optical signals to users. Compared with the active point-to-point mode, this mode saves active optical modules and does not require optical switches. module means that the power consumption of the entire network is low. Outdoor passive devices improve anti-interference capability, provide high reliability, and are easy to maintain, which significantly reduces maintenance costs. Passive point-to-multipoint (PON) technology is the mainstream technology for optical access.
Progress of PON Technology Standardization
There are many types of PON technologies, such as TPON, APON/BPON, GPON, EPON, WDM PON, LR PON, 10GEPON, XG-PON. According to standards, PON technologies can be classified into the following three types:
ITU-T G.983
ATM Passive Optical Network (APON), which was renamed BPON in 1995 and 2001. Broadband Passive Optical Network (BPON) 2001: APON-based standard. This standard supports wavelength division multiplex module (WDM), dynamic and high-speed uplink bandwidth allocation, and durability. BPON also created a management interface standard OMCI, a hybrid vendor network between OLT and ONU/ONT. BPON: The BPON uses the ATM encapsulation mode. It is mainly used to carry ATM services and can receive voice, data IP, and video services at the same time. However, the efficiency of transmitting IP data is low and the ATM cost is high.
ITU-T G.984
GPON (Gigabit Passive Optical Network, 2001): development of BPON standards. GPON supports higher rates, enhanced security, and optional Layer 2 protocols (ATM, GEM, and Ethernet). GPON uses a single optical fiber to connect the OLT, optical splitter, and ONU. Different wavelengths are used to carry data in the upstream and downstream directions. Wavelengths ranging from 1260 nm to 1360 nm are used in the upstream direction, and wavelengths ranging from 1480 nm to 1500 nm are used in the downstream direction. Indicates the transmission line rate. For each user, the downstream rate is 2.488 Gbit/s and the upstream rate is 1.244 Gbit/s (physical layer). The GPON system uses the wavelength division multiplexing (WDM) principle to transmit data on the same ODN with different wavelengths in the upstream and downstream directions. In the downstream direction, data is transmitted in broadcast mode, and in the upstream direction, data is transmitted in TDMA mode by timeslot.
IEEE 802.3ah
EPON or GEPON (Ethernet Passive Optical Network, 2000) is an IEEE/EFM standard that encapsulates Ethernet data and encapsulates information into Ethernet frames for transmission. Signals are transmitted between the OLT and the ONU based on IEEE 802.3 Ethernet frames. The transmission line rate is 1.25 Gbit/s in the upstream direction and 1.25 Gbit/s in the downstream direction. Logical split ratio: 1:32(1:64)
We will explain the PON technologies in detail in the subsequent content. Now, you only need to know the mainstream technologies of the access network optical access.
Typical Application Scenarios of PON Access
Typical Application of PON Access in Telecommunication Networks
Broadband remote access server(BRAS)
Service router(SR)
Optical line terminal(OLT)
Optical network unit(ONU)
Optical network unit(ONU)
High-speed Internet(HSI)
Internet Protocol television(IPTV)
Video on demand(VOD)
Typical Application of PON Access on Campus Networks
Passive optical LAN(POL)
Data Encapsulation of PON Networks
When user data enters the PON system, PON encapsulation and decapsulation need to be performed. Data encapsulation processes of different PON technologies are similar. The following uses GPON as an example.
Data Encapsulation of PON System (Upstream)
When user data enters the PON system, PON encapsulation and decapsulation need to be performed. Data encapsulation processes of different PON technologies are similar.
Step1: Ethernet frames generated by the PC enter the ONU.
Step2: The Ethernet frame (destination MAC address to frame check sequence) is extracted and added with the GEM frame header. The dynamic bandwidth report and physical layer overhead are added to the GEM frame header to form a complete upstream frame. Performs electrical-optical conversion and transmits optical signals to the OLT through optical fibers at a specified time.
Step3: After processing the PON frame header information, the OLT extracts and restores the carried Ethernet frames. The Ethernet frame is added with fields such as the preamble and then transmitted to the upper-layer switch.
The PON encapsulation objects include Ethernet data frames, voice data, and T1/E1 data.
GEM: GPON Encapsula Mode/Method, GPON Encapsulation Mode.
PLI: Indicates the payload size. The value is 12 bits. That is, the payload of each GEM frame can contain a maximum of 4095 bytes.
Port ID: 12 bits, identifying 4096 GEM port IDs.
PTI: Indicates the 3-bit payload type. The last bit indicates whether the packet is the last segment.
HEC: 13 bits, header error control.
Data Encapsulation of PON System (Downstream)
Step1: PC3 communicates with PC1 and PC2 respectively. The OLT receives two Ethernet frames sent to PC1 and PC2 respectively. The source MAC address is MAC3, and the destination MAC addresses are MAC1 and MAC2.
Step2: When an Ethernet frame is combined and encapsulated into a downstream frame, the downstream physical control frame header is added.
Step3: After receiving the PON frames, ONU1 filters and extracts the GEM frames related to itself.
Step4: ONU1 extracts and restores Ethernet frames. Add a preamble and send it to PC1.
In the next article, I will introduce you to the knowledge of data communication.
Thanks for reading!
