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The first step is to create an I/O blade with the PON OLT MAC. AT&T is working with the Open Compute Project to develop an open specification for a GPON MAC 1RU “pizza box”, as shown in Figure 1. This blade includes the essential GPON Media Access Control (MAC) chip under control of a remote control program, which is, in turn, controlled from high level applications via OpenFlow. This replaces an existing closed and proprietary OLT chassis (not shown) that integrates this GPON MAC chip with GPON protocol management, 802.1ad-compliant VLAN bridging, and Ethernet MAC functions. These other functions from the legacy device are being unbundled and implemented in software.   The box shown in Fig.1 is frequently referred to as a white-box OLT, but its is important to note that each port on the box is a separate PON OLT with 32 or 64 subscribers.

     Figure 1. GPON OLT IO Blade.


Subscriber traffic is identified by two VLAN tags within the central office. The ONT and OLT hardware is responsible for tagging/untagging this traffic as it flows from/to the subscriber. ONOS instructs the OLT which VLANs to use through OpenFlow messages.

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Figure 4. VLAN architecture of PON accessPacket walkthrough RG -> vSG

Figure 4 shows where VLAN tagging happens as traffic flows from a subscriber’s home to the Internet. Within the home there are no VLAN tags. A default priority VLAN tag of (VLAN ID 0) is added at the physical home CPE/RG in order to carry Ethernet precedence bits. The OLT instructs the ONT (via OMCI messaging sent from the vOLT agent) to add/remove the appropriate C-Tag to the traffic inside the PON, and  the OLT adds an S-Tag to the packet in order to uniquely identify the subscriber’s traffic in the fabric (This is 1:1 VLAN mapping in GPON systems. Other mappings (eg. N:1) are also possible). The inner tag (C-tag) identifies the specific subscriber within the OLT device they are attached toPON. The outer tag (S-tag) identifies the OLT device itselfPON. Taken together, these two tags can identify a subscriber uniquely across all OLT devices in the system. The fabric maps the two outer VLAN tags to a pseudowire which forwards the traffic between the OLT and the vSGserver which hosts the vSGs for all the subscribers in the PON (i.e outer Tag). In a future release, we will remove this limitation by adding the capability to place the vSG for a subscriber anywhere in the cloud. The vSG strips the VLAN tags, performs its functions, and then forwards the packet along the service graph. VLAN tags are not needed after the vSG because the vSG also NATs the traffic to the vSG’s WAN address, which is unique in the system.