EPON and GPON: Only 1 article is needed to ample fully understand them

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The difference between GPON and EPON

As the two main members of optical network access, EPON and GPON have their own merits, compete with each other, complement each other, and learn from each other. With the latest EPON equipment, we ensure seamless internet access for our customers. Let’s compare them in various aspects:

rate

EPON provides fixed uplink and downlink 1.25 Gbps, uses 8b/10b line encoding, and the actual rate is 1Gbps. GPON supports multiple rate levels. It can support uplink and downlink asymmetric rates, downlink 2.5Gbps or 1.25Gbps, and uplink 1.25Gbps or 622 Mbps. Determine the uplink and downlink rates according to actual needs, select the corresponding optical module, and improve the rate-price ratio of optical components. The EPON power board efficiently supplies energy to the network infrastructure.

Split ratio

The so-called splitting ratio refers to how many ONUs (users) an OLT port (central office) carries. The EPON standard defines the splitting ratio as 1:32. The GPON standard defines the following splitting ratios: 1:32; 1:64; 1:128. In fact, technically the EPON technology system can also achieve higher split ratios, such as 1:64, 1:128, and the EPON control protocol can support more ONUs.

The splitting ratio is mainly limited by the performance indicators of the optical module. A large splitting ratio will cause the cost of the optical module to increase significantly. In addition, PON insertion loss is 15-18dB, and a large splitting ratio will reduce the transmission distance. Sharing the bandwidth with too many users is also the price of a large split ratio.

EPON offers high-speed connectivity for residential and business users.
EPON offers high-speed connectivity for residential and business users.

QOS

QOS (Quality of Service) is the level of service. EPON adds a 64-byte MPCP multipoint control protocol (multipoint control protocol) to the MAC layer Ethernet header. MPCP controls access to the P2MP point-to-multipoint topology through messages, state machines and timers, and implements DBA dynamic bandwidth allocation.

The contents involved in MPCP include the allocation of ONU transmission time slots, automatic discovery and joining of ONUs, and reporting congestion conditions to higher layers for dynamic bandwidth allocation. MPCP provides basic support for the P2MP topology, but the protocol does not classify the priorities of services, and all services compete randomly for bandwidth.

GPON has a more complete DBA and excellent QoS service capabilities. GPON divides service bandwidth allocation methods into 4 types. The priorities from high to low are fixed bandwidth (Fixed), guaranteed bandwidth (Assured), non-guaranteed bandwidth (Non-Assured) and best-effort bandwidth (BestEffort). The DBA also defines a traffic container (T-CONT) as the upstream traffic scheduling unit, and each T-CONT is identified by Alloc-ID.

Each T-CONT can contain one or more GEM Port-IDs. T-CONT is divided into 5 service types. Different types of T-CONT have different bandwidth allocation methods, which can meet different QoS requirements of different business flows on delay, jitter, packet loss rate, etc. T-CONT type 1 is characterized by fixed bandwidth and fixed time slots, corresponding to fixed bandwidth (Fixed) allocation, and is suitable for delay-sensitive services, such as voice services.

Type 2 is characterized by fixed bandwidth but uncertain time slots, corresponding to guaranteed bandwidth (Assured) allocation, and is suitable for fixed-bandwidth services that do not have high jitter requirements, such as video on demand services. Type 3 is characterized by a minimum bandwidth guarantee, the ability to dynamically share excess bandwidth, and a maximum bandwidth constraint.

Corresponding to Non-Assured bandwidth allocation, it is suitable for services with service guarantee requirements and large burst traffic, such as download services. Type 4 is characterized by best effort (BestEffort), no bandwidth guarantee, and is suitable for services with low latency and jitter requirements. Such as WEB browsing business; type 5 is a combination type. After the guaranteed and non-guaranteed bandwidth is allocated, the additional bandwidth requirements are allocated on a best-effort basis.

OAM

OAM (Operation Administration and Maintenance) refers to operation, management and maintenance. EPON does not consider OAM too much and simply defines ONT remote fault indication, loopback and link monitoring.

And it is optional support. GPON defines PLOAM (Physical Layer OAM) at the physical layer, and OMC (I ONT Management and Control Interface) at the higher level, and performs OAM management at multiple levels. PLOAM is used to implement functions such as data encryption, status detection, and error monitoring.

The OMCI channel protocol is used to manage services defined by the upper layer, including ONU functional parameter sets, T-CONT service types and quantities, and QoS parameters. Request configuration information and performance statistics, automatically notify the system of operating events, and realize the OLT’s management of ONT configuration, fault diagnosis, performance and security.

Business carrying

EPON follows the simple Ethernet data format, but adds a 64-byte MPCP point-to-multipoint control protocol to the Ethernet header to implement bandwidth allocation, bandwidth polling, automatic discovery, ranging and other tasks in the EPON system. Not much research has been done on the support of services other than data services (such as TDM synchronization services). Many EPON manufacturers have developed some non-standard products to solve this problem, but they are not ideal and difficult to meet carrier-grade QoS requirements.

GPON is based on a completely new Transmission Convergence (TC) layer. This sub-layer can complete the adaptation of high-level diversity services. It defines ATM encapsulation and GFP encapsulation (general framing protocol), and you can choose one of the two for service encapsulation.

In view of the fact that ATM applications are not popular at present, a GPON.lite device that only supports GFP encapsulation emerged at the historic moment, which removes ATM from the protocol stack to reduce costs. GFP is a universal link layer protocol applicable to a variety of services, and is defined by ITU as G.7041.

A small amount of modifications have been made to GFP in GPON. Port ID is introduced in the header of the GFP frame to support multi-port multiplexing; Frag (Fragment) segmentation indication is also introduced to improve the effective bandwidth of the system. And it only supports the data processing mode for variable-length data and does not support the data transparent processing mode for data blocks. GPON has strong multi-service carrying capabilities.

The TC layer of GPON is essentially synchronous and uses standard 8kHz (125µm) fixed-length frames, which allows GPON to support end-to-end timing and other quasi-synchronous services. In particular, it can directly support TDM services, which is the so-called NativeTDM. GPON has “natural” support for TDM services.

The EPON technology revolutionized broadband access networks
The EPON technology revolutionized broadband access networks