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GPON: VLANs and GEM Ports

GPON: VLANs and GEM Ports

This article discusses traffic flows in GPON. As discussed in GPON Fundamentals article, the OLT is the first aggregation point in GPON access network. The OLT terminates the GPON Transmission Convergence (GTC) layer on the user side and forwards Ethernet frames to Ethernet layer on the network side. Figure 1 shows the termination points for ONU/ONT scenario.

The U reference point represents the customer-facing interface of the ONU/ONT. It is possible that the U reference point can be within the ONU/ONT device when ONT and RG devices are combined into a single device.

The R/S reference point represents the OLT facing ONT interface. The S/R reference point represents the Optical Distribution Network (ODN) connecting GPON interface on the OLT. The S/R and R/S interfaces contain all the protocol elements necessary to allow communication between OLT and one or more ONTs over ODN.

The V reference point represents the network-facing interface of the OLT.

Relationship between T-CONT and GEM Ports


T-CONT: A traffic bearing object within an ONU/ONT that represents a group of logical connections, and is treated as a single entity for the purpose of upstream bandwidth assignment on the PON. In the upstream direction, it is used to bear the service traffic. Each T-CONT corresponds to a service traffic of one bandwidth type. Each bandwidth type has its own QoS feature.

ALLOC_ID: Each T-CONT is identified by the ALLOC_ID uniquely. The ALLOC_ID ranges from 0 to 4095. It is allocated by OLT i.e. a T-CONT can only be used by one ONU/ONT per PON interface on the OLT.

GEM Port: A GPON Encapsulation Method (GEM) port is a virtual port for performing GEM encapsulation for transmitting frames between the OLT and the ONU/ONT. Each different traffic-class (TC) per UNI is assigned a different GEM Port. Each T-CONT consists of one or more GEM Ports. Each GEM port bears one kind of service traffic i.e. a T-CONT type.

GEM Port ID: Each GEM Port is identified by a port ID uniquely. The Port ID ranges from 0 to 4095. It is allocated by the OLT i.e a GEM port can only be used by a single ONU/ONT per PON interface on the OLT.

Figure 2 shows the relationship between T-CONT and GEM Ports.

Between the ONT and OLT is the ODN, and Ethernet frames are carried over it through the use of GEM Channels. GPON has GEM channels as part of its GTC layer. The GEM channels carry variable-length Ethernet frames. GEM channels are identified by GEM Port IDs. This identifier is assigned by OLT upon creation of a new channel and is only valid during the entire life-cycle of the channel. Each GPON interface for a given ONT can have several GEM Ports. A GEM Port ID is unique per GPON interface and represents a specific traffic or group of flows between the OLT and the ONT.

There are 2 types of GEM Channels:

  • Downstream-only GEM Channels – These channels are used to transmit downstream broadcast/multicast traffic from OLT to all ONTs. The ONTs identify traffic meant for them based on GEM Port ID.
  • Bi-directional GEM Channels – These channels are used for upstream and downstream traffic between the OLT and the ONT. The frames are transmitted from the OLT into the GPON interface and are forwarded only on the U interface of the ONT on which that GEM Port has been assigned.

GEM Ports are used to differentiate among traffic classes (TCs). A U interface may have several GEM Ports associated with it that support different TCs. Thus, within a GPON interface, each GEM Port carries one or more traffic flows associated with a specific TC.

On U interface, traffic is classified into VLANs with various Ethernet priorities based on: Physical Port, VLAN ID, 802.1p bits, &/or DSCP. Once the traffic has been assigned a VLAN and COS (802.1p) values, these two values are used to select an upstream GEM Port so that QoS can be applied to the flows carried by the GEM Port. A GEM Port always belongs to a single T-CONT. In downstream direction, the ONT forwards the traffic received by GEM Ports to appropriate U interface.

1:1 VLAN

In a 1:1 VLAN architecture, the ONT maps each 1:1 VLAN into a unique U interface. There are 2 variations on tag assignment at V interface in upstream direction – the traffic at V interface could be double-tagged or single-tagged.

  • For double-tagged VLANs at V, the ONT can either assign a C-VLAN ID or translate a C-VLAN ID. The OLT adds the S-VLAN ID. (Subscriber 1 in Figure 3)
  • For double-tagged VLANs at V, the ONT can assign S-C VLAN IDs to incoming traffic, and the OLT passes through the traffic. (Subscriber 2 in Figure 3)
  • For single-tagged VLAN at V, the ONT adds the S-VLAN ID or translates an incoming tag to S-VLAN ID, and the OLT passes through the traffic. (Subscriber 3 in Figure 3)

In the downstream direction, the OLT removes the outer tag or passes through the traffic to proper GEM port based on the tag value and priority bits. The ONT removes the tags and forwards frames from the GEM port to its associated U interface.


For N:1 VLAN model, the ONT always adds the S-VLAN ID or translate an incoming tag to S-VLAN ID for upstream traffic. The OLT will pass-through any upstream traffic with S-VLAN ID on them. In the downstream direction, the OLT will pass-through traffic with S-VLAN ID to ONT by determining GEM Port based on MAC address and priority bits. If the GEM Port cannot be determined, then the frame is flooded using the unidirectional GEM Port associated with the S-VLAN ID. The ONT will remove the tag and forward frames from the GEM Port to appropriate U interface. For N:1 model, traffic is always single-tagged at V interface.

QoS and Traffic Management

As seen from figure 1, the GPON link connects the OLT and ONTs to transport Ethernet services. Please note that GPON can also encapsulate ATM and TDM (E1, E3) services. The GTC Adaptation sublayer maps Ethernet frames into GPON GEM frames. A QoS mechanism is required in GEM to support Ethernet QoS (i.e. 802.1p bits). In order to provide QoS, two mechanisms are employed-

  • Classification of traffic into traffic classes
  • Forwarding the traffic classes into GEM Ports and T-CONTs configured to emulate Ethernet QoS service

Upstream Traffic Management

Figure 5 shows a sample model of upstream traffic management. It shows 4 T-CONTs per PON interface where each T-CONT represents a specific traffic class (TC). The Classifier receives traffic from U interface and maps to queues as per configuration using associated GEM Ports. If a second UNI interface is present on the same ONT, it would also perform classification and map the traffic to TC(s). As mentioned above, a GEM Port must bear one or more T-CONTs. Other upstream traffic from other ONTs is mapped to other 4 T-CONTs according to the TC.

At the OLT, each TC is mapped into a separate queue. T-CONTs from various ONTs that share the same TC are mapped to the same queue, and a scheduler is used among the queues towards the network-facing port i.e. V interface.

Downstream Traffic Management

Figure 6 shows a sample model of downstream traffic management. In downstream direction, T-CONT is not used. Traffic received from the V interface at the OLT is assigned to queues according to the TCs. It is then transmitted in the downstream direction to the PON interface by using a scheduler. At the ONT, the traffic is classified again and placed into appropriate queues for each U interface. A scheduler is used to transmit frames to the U interface.


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