PON (Passive Optical Network) is the main technology to realize fiber to the home(FTTH), providing point-to-multipoint fiber access. It consists of OLT (Optical Line Terminal) on the bureau side, ONU (Optical Network Unit) and ODN (Optical Distribution Network) on the user side. Generally, in PON system, TDM(time-division multiplexing) broadcast mode is adopted for downlink transmission and TDMA (time division multiple access) mode is adopted for uplink transmission, forming a point to multipoint tree topology. The biggest highlight of PON technology is “passive”, which means the ODN does not contain any active electronic components and electronic power supply; it is all composed of passive components such as optical splitter, with low management, maintenance and operation cost.
Introduction to the development of PON
In 1987, British Telecom researchers first put forward the concept of PON. In 1995, the FSA (Full Service Access Networks) alliance was established to jointly define a common PON standard. In 1998, ITU-T issued G.983 series APON (ATM PON) standard based on 155Mbit/s ATM technology. At the same time, various telecom equipment manufacturers also developed APON products, and now there are practical applications of APON products in North America, Japan and Europe. At the end of 2000, some equipment manufacturers established EFMA (Ethernet in the First Mile Alliance), which proposed the concept of Ethernet-based PON—EPON( Ethernet PON), and led the IEEE to establish EFM (Ethernet in the First Mile) in 2001 to start the formal study of EFM-related standards, including 1.25Gbit/s EPON.
GPON (Gigabit Capable PON) was proposed by FSAN in September 2002, and then ITU passed G.984.1 and G.984.2 protocols in March 2003. The G.984.1 protocol specifies the overall characteristics of GPON access system; The G.984.2 protocol specifies the physical medium related sub-layer of ODN (Optical Distribution Network) of GPON. In June 2004, ITU passed the G.984.3 protocol, which stipulates the relevant requirements of GPON transmission convergence layer (TC).
For many years, the industry has always believed that PON is the future development direction of access network. On the one hand, the bandwidth it provides can meet the needs of various broadband services now and in the future, so PON is ideal in solving the problem of broadband access; On the other hand, the overall cost is relatively low in terms of equipment cost and operation&maintenance cost.
GPON and EPON have their own advantages as two ways of optical network access. Next, this paper will comprehensively analyze and compare the two from four aspects: technical specification, cost, level of development, application and security.
- Technical specifications
Compared with GPON, EPON is simpler and more direct. In pure Ethernet transmission, the two packaging methods of GPON and ATM support will not play much role. However, in access network services, EPON is only suitable for services with only data transmission, while GPON can provide three in one services. Therefore, EPON is an all Ethernet solution, which fully conforms to the characteristics, performance and specifications of Ethernet protocol, while GPON uses synchronous optical fiber network and synchronous digital architecture technology and general framing protocol for Ethernet transmission.
GPON vs. EPON: Technical Specification
Overall, the technical differences between GPON and EPON are very small. The difference between the two is mainly on the interface, while the switching, network element management, user management are similar, or even the same. In comparison, GPON has more advantages in multi-service bearing and full-service operation, which is mainly due to the GPON standard is developed by FSAN, which is an operator-led organization.
Various factors influence the cost, such as: technical complexity, scale production and market application scale.
From the technical point of view, the EPON “entry threshold” is very low, which can easily attract a large number of vendors to join the EPON industry alliance. At present, there are 5-6 professional vendors in the market who can provide EPON core chips (excluding system vendors who design their own chips). During the discussion of the standard, these vendors have started the design and verification of the chips, so when the 802.3ah standard is officially promulgated, most of them have already launched the second generation of chips that are fully compatible with the standard and can quickly support the large-scale deployment of EPON systems. The core chips for EPON are now available. Nowadays, the core chip of EPON has been developed to the third generation SoC (system on chip) stage, which integrates MAC and PHY of multi-port standard Ethernet, SRAM, CPU and PON-specific SerDes, such as the EPON OLT SFP PX20++ module that is popular among the current market. With this chip as the core, only a few peripheral devices (such as modules and interface circuits) need to be added to form a complete system. In addition, the industry chain of EPON optical transceiver modules has been complete and started to promote each other with the development of EPON system, and the two have started to step into a virtuous cycle: the price of ONU module has dropped from more than 200 USD at the beginning to less than 50 USD at present, and the price level has been comparable with the common Gigabit Ethernet optical module.
However, on the contrary, GPON’s technical standards are quite complex, and it is more difficult to achieve fully standard-compliant devices at an acceptable cost on the basis of existing technologies. In addition to GPON chips that designed independently by GPON equipment vendors, there are no professional chip vendors to launch commercial GPON core chips. Besides, the optical modules used in GPON equipment are dedicated independent or collaboratively developed modules, and there are no professional module vendors to provide samples, let alone mass production. For example, there are very few manufactures can provide the GPON OLT SFP module that compatible to Calix switch. Compared with EPON, the industry chain of GPON is not yet complete. Moreover, as the optical module of GPON has to meet the high burst synchronization index (at 1.244Gbit/s rate, the GPON standard stipulates that the burst synchronization physical layer overhead is only 12byte), and the requirements for the driver and amplifier chips in the module are very high; it also has to meet the power budget of three types of ODN, and has high requirements for ONU transmitter power and OLT receiver sensitivity. Only DFB transmitters and APD receivers can be used, and their cost is almost six times that of the traditional FP transmitters and PIN receivers used in EPON modules. Therefore, the cost of GPON optical modules is obviously higher than that of EPON optical modules.
However, there are opposing voices that argue that GPON cannot simply be said to be more costly than EPON. GPON is more expensive than EPON when measured in terms of individual optical ports. However, operators mainly focus on the network construction cost, not the equipment cost. This is because in terms of bandwidth and service support capability, each GPON port can access 64 or even 128 users, while each EPON port can only access 32 users. In addition, in the actual network construction, the cost of office-end equipment accounts for less than 10%, while OLT accounts for 50%. Therefore, GPON is cheaper in terms of overall network construction cost.
- The level of development
EPON system manufacturers have developed early systems and started small-scale field tests in the process of standard formulation. On the one hand, they follow up in time with the evolution of the standard, on the other hand, they also accumulate relevant engineering and service experience for large-scale deployment in the future. In this way, all aspects have been prepared when the EPON standard is promulgated. Once the demand breaks out, large-scale deployment can be started immediately. The reason why Yahoo BB in Japan was able to deploy FTTH with EPON on a large scale shortly after the EPON standard was officially promulgated is because the engineering technology of EPON is mature. This in turn has greatly promoted the development of EPON industry.
At present, the price of EPON optical module is close to that of traditional module, while the current deployment scale of GPON is still very small, so it is difficult for the price of GPON optical module to decline rapidly. Moreover, due to the leading commercial process, EPON has a comparative advantage in interconnection. EPON Technology is simple, effective, mature and stable, especially for medium and low-end customers and residential users. It has great advantages over ADSL (asymmetric digital subscriber line).
- Application and security
The GPON standard was developed by the FSAN organization and promulgated by the ITU-T. The FSAN (Full Service Access Networks) consortium is an operator-led forum for optical access standards, whose members are mainly network operators, equipment manufacturers and senior industry experts. GPON technology provides triple play service capabilities for voice, data and video. Because GPON is a standard promoted and established by operators, more consideration is given to bandwidth, service carrying, management and maintenance. GPON has higher bandwidth and wider coverage than EPON. It can carry more business types and improve operation and maintenance functions. Due to the consideration of more high-level service support, the initial cost of GPON is high. Moreover, GPON can provide high bandwidth efficiency, has the ability to transmit clock, and can meet operators’ higher requirements in network operation and maintenance as well as comprehensive network management.
Although GPON and EPON have different scope of application, they are more complementary to each other. In FTTH application scenarios, especially in full service operation scenarios GPON has more advantages (higher bandwidth capability: 2.55 times the EPON downlink bandwidth, 1.37 times the EPON uplink bandwidth; higher splitting ratio of 1:128), so GPON networking costs are lower than EPON. In fact, in the process of building and organizing the network, the construction mode of GPON and EPON is not very different, but is only the technical choice faced by operators in broadband access that matter. Because of the different technical characteristics, EPON and GPON technology is actually two different market applications, EPON technology is more suitable for the type of application of Internet access, GPON technology is more suitable for full-service operations and triple-play application types.
When we are talking about GPON vs EPON, the first thing is that GPON has larger bandwidth than EPON. Its service carrying capacity is more efficient and its optical splitting ability is stronger. It can transmit services with larger bandwidth, realize more user access, and pay more attention to multi-service and QoS guarantee. However, the implementation of GPON is more complex, which leads to its higher cost compared with EPON. Luckily, with the large-scale deployment of GPON technology, the cost difference between GPON and EPON is gradually narrowing.
All in all, EPON and GPON have their own advantages. In terms of performance indicators, GPON is better than EPON, but EPON has the advantages of time and cost. The good thing is that GPON is catching up. Looking forward to the future broadband access market, it may not be who replaces who, but should coexist and complement each other.