Some time ago we discussed Layer 1, or physical layer in PON communication systems. Now, in terms of OSI model, access network, and PON networks are access networks at it’s best, exists at Layer 2. Because when we talk about Layer 2 – we usually mean Virtual Local Area Networks, or VLAN’s, which is a core terms for Ethernet based last mile access networks. VLAN is specified in IEEE 802.1Q. So, when you install & configure or yet design a GPON or EPON based network – you need to consider all the hardware, software and management network requirements, that will later allow you split traffic in VLAN’s, so that each traffic comes to the right place and possibly with right QoS – Quality of Service applied to it.
So, even there is a customers who likes to filter traffic down to a MAC adress level, the more common will be traffic filtering based on VLAN tags assigned at each of ONU’s. Having that said, it does not means that access network based on PON, can not have also some layer 3 functions.
It must have, and have, because now, when CATV services using a 1550 nm separate wavelength are less and less popular, almost all new installations are using IPTV and for this you need to deal with multicast management. IPTV is a service that nowadays means much to operator, and triple play services are not imaged without a large portion of multicast packets delivered over newly built EPON network.
And here’s a good news – EPON/GPON is ideally suited for multicast applications, because the single copy of a multicast packet can be “read” by as many CPE’s – in this case ONU’s as needed! And each ONU will “read” (extract) only those packets, that carries a video channel that enduser wants to watch, and ignore others.
To find out, which multicast groups are requested at any given time, ONU must support IGMP snooping function, and in case of Fiberbit ONU’s – it supports. What is IGMP snooping?
It’s a constant monitoring (from CPE’s end) transmissions, which then ONU compares to requested channels with local access control list (ACL). If the requested stream is authorized, and is already present in PON, then ONU passes it over to set top box.
Here we also need think in terms of hierarchy. OLT in this case acts more like a proxy, and determines, whether it a given multicast group is already available, or, if it should be requested from a higher authority. So, that dramatically reduces unnecessary requests directly to the source.
