GEM ports description and usage in the GPON technology

GEM ports description and usage in the GPON technology: First I would like to discuss about the GEM,What is the meaning of GEM? One by one I will focus on the important things related to the GEM topic but for that you have to read the entire article carefully. GEM-GPON Encapsulation Method- It is connection-oriented data frame transport mechanism that also supports fragmentation of the frames into variable sized transmission data fragments. GEM Port-GPON Encapsulation Method Port- It represents a logical connection associated with a specific traffic flow. GEM Port ID- GPON Encapsulation Method Port ID- It is a 12-bit value assigned by the OLT to the individual logical connections on the GPON interface. It is carried in the header of all the GEM frames of the logical connection.

In this article I have tried to describes generic details about GEM Port usage in the GPON technology. The GPON technology has introduced the GEM channel as part of its GPON Transmission Convergence (GTC) layer. In fact the GEM channels carry variable-length frames and also including Ethernet frames. GEM channels are identified by a uniquely assigned identifier, the GEM Port ID. This identifier is assigned automatically by the OLT upon creation of a new channel and remains constant during the entire life-cycle of the channel. GEM port IDs are virtual port identifiers that have the same importance within the ODN or have significance within a single ODN. Each GPON interface for a given ONU can have several GEM Ports. A GEM Port ID is unique per GPON interface and represents a specific traffic flow or group of flows between the OLT and one or more ONUs.

GPON provide the permission to the OLT (through OMCI) to determine the allowed transmission directions (i.e. upstream and / or downstream) for each and every GEM channel during the configuration process. I would like to write here about upstream and downstream.

Upstream- In the upstrea the traffic is received at the PE-Agg node from the OLT on a VLAN. The VLAN is terminated at the PE-Agg and the traffic over this VLAN is mapped into an IP-VPN in the IP/MPLS network. The p-bit in the VLAN header is translated in to EXP bits of the MPLS frame.

Downstream- In the downstream direction the traffic flows from the internet towards the end user crossing the IP/MPLS network, the GPON network and the Base Station. The traffic is received from the internet at the PDN-GW which checks the destination IP address of the IP packet and determines the eNodeB. PDN-GW setup an IP Tunnel between it and the eNodeB across the IP/MPLS network and GPON network. It then sends the IP packet over this tunnel after encapsulating it into outer IP packet. The PDN GW sends this IP packet to the PE-Agg facing to it. The PE-Agg receives this IP packet over a sub-interface which is configured into an IP-VPN. It consults with the relevant VRF of the IP-VPN and determines the remote-end PE-Agg. It then encapsulates the IP packet into the MPLS frames with EXP bits marked and send it to the relevant PE-Agg-node.

Once the traffic is received at the remote end PE-Agg node facing to the OLT, it uses the MPLS tag to determine the VPN association. It then removes the MPLS tag and takes out the IP packet. It then checks the destination IP address, consults with the relevant VRF and determines outgoing interface. It then encapsulates the IP packet with the VLAN header, translates the DSCP-bits into the corresponding p-bits in the VLAN header and sends it to the OLT.

The OLT receives the VLAN tagged with p-bit marked traffic from the PE-Agg node. With the help of the VLAN ID and p-bit, the OLT determines the outgoing Shelf/Rack/slot/port / GEM port. The OLT sends the traffic to the relevant PON port and GEM port.

The OLT determines the Traffic Class based on p-bit. The TC assignment for the traffic flows is applied by a classifier in the OLT.


In fact GPON allows or permit the OLT (through OMCI) to determine the allowed transmission directions (i.e. upstream and / or downstream) for each and every GEM channel during the configuration process of such type of network. There are two types of GEM channels:

Uni-directional GEM channel for broadcast traffic

Downstream (only) GEM channels – Such type of channels can be used for the to achieve the transmitting downstream flooded, broadcast, or multicast traffic. The frames are transmitted from the OLT into the GPON interface to all the ONUs, and are then selectively forwarded to UNI interfaces by those ONUs which are configured with that GEM port. The application of this type of GEM Channel is IPTV.

GEM channel for uni-directional broadcast/multicast traffic

GEM channel for uni-directional

Bi-directional GEM channel for unicast traffic

PON systems are a broadcast medium in the downstream direction, so all ONUs receive all the downstream traffic for every GEM port, however ONUs silently discard traffic that is not addressed for them. Additionally, AES encryption can be (and typically is) applied over the bidirectional GEM ports. A different key per ONU is used by the OLT for encryption and by the ONU for decryption.

There are 3 types of Bi-directional GEM channels.
➤One GEM port per UNI interface 
➤Multiple GEM Ports per UNI interface 
➤One GEM port for multiple UNI interfaces

One GEM port per UNI interface

These channels are used for both upstream and downstream traffic between that ONU and the OLT. They are uniquely assigned per UNI interface on an ONU. The frames are transmitted from the OLT into the GPON interface and are forwarded only on the UNI interface of the ONU on which that GEM port has been assigned.

One GEM Port

One GEM Port per UNI port

Multiple GEM ports per UNI interface

In such network GEM ports can also be used to differentiate among traffic classes. A given UNI interface may have several GEM ports associated with it that support different traffic classes. This types of arrangement can be explained as follows: within the GPON interface, each and every GEM Port carries one or more traffic flows associated with a specific traffic class going to a specific UNI interface on a specific ONU.

On UNI interface ingress, traffic is classified into VLANs with various Ethernet priorities based on a number of criteria: physical port, VID, VLAN P-bits, EtherType and/or DSCP. In fact any combination of such types of criteria can be used to determine the Ethernet priority.

The VID and EtherType can be used to determine the new VID. Once the traffic has been assigned a VLAN and Ethernet precedence, these two Ethernet header components are used to select an upstream GEM Port so that proper QoS can be applied to the flows. A GEM Port is mapped into one and only one T-CONT. Similarly for egress, the ONU is responsible for forwarding traffic received from GEM ports on the PON to the appropriate UNI interface.

➥Within the GPON, a bidirectional GEM Port MUST be able to carry one or more traffic flows associated with the same traffic class going to a specific UNI interface on a specific ONU.

➥ OLT and the ONU MUST support one bi-directional GEM Port for each traffic class configured for a specific UNI interface on a specific ONU.

Multi GEM port

Multiple GEM ports per U interface

One GEM port for multiple UNI interfaces

In this case, if multiple UNI interfaces belong to the same service, then these UNI interfaces are mapped into a single GEM port. This scenario is normally used for the EBU services.

One GEM port for multiple

One GEM port for multiple UNI interfaces