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
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
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
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
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
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
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.
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
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.