Throughout my career I have implemented a pretty large number of standard Cisco campus LAN designs. You know the model; a hierarchical one with access switches aggregating to a distribution layer, which then aggregates to a fast core switching layer. Pieces of the hierarchy can be collapsed into one another in situations where, for example, due to the size of the network it makes little sense to have separate access and distribution layers.
As a design framework this has a lot of inherent advantages, but sometimes it feels a little restrictive – like you’re attempting to shoehorn this in to somewhere it doesn’t really fit. This couldbe for a number of reasons; corporate standards, business agreements or even just the time pressures being too great to really try and work with something different.
Recently I came across an alternative campus LAN design approach, something that was completely new to me. A high end hotel undertaking a project to refresh its network needed new endpoint connectivity to all the rooms and management areas, and in doing this they preferred to ditch the traditional network design instead opting to implement a GPON solution.
A typical requirement in this space is for between 6-10 individual UTP connections per room. These are used for phones, multiple TVs, wireless, internet access, room management systems etc. Personally, I’ve never stayed in a hotel room with four TVs and automated curtains, but hey, I don’t move in those circles. Sometimes it’s nice to see how the other half live.
As an example, let’s say there are 30 rooms per floor and 10 connections per room, giving us 300 cables per floor. A hotel has 5 floors, and at 300 cables per floor, that’s a requirement for 1,500 data ports. Bear in mind that this is also just for the hotel guest rooms, there’s nothing included here for staff connections, conference rooms, building management and security etc. Basically, we’re talking a lot of copper.
Physically, a traditional network design would likely see these UTP cables backhauled from the individual rooms to a comms. room on each floor acting as a distribution point. Each of these distribution points then connect back to a central comms. room that contains the core switching environment.
An additional consideration is that a comms. room requires power and cooling and is therefore an ongoing cost. It also requires space, and is a comms. room per floor the best use of space in a hotel?
AN ALTERNATIVE APPROACH
For those unfamiliar (I certainly was), PON (Passive Optical Networking) is a technology that involves using passive optical splitters to divide a single optical connection to serve multiple endpoints. GPON is a particular flavour of PON that is Gigabit capable. The key here is that it’s passive, you use splitters to increase the available connections instead of switches. Splitters essentially being a bunch of mirrors that, unlike switches, don’t require power.
Taking the design requirements above, you can pass a single fibre from an OLT (Optical Line Terminal) port in the main comms. room to each floor and split this off using a 1:32 splitter into 32 different connections. Each of these 32 connections has an endpoint device on it, an ONU (Optical Network Unit), from which we can offer Ethernet/PoE/POTS connectivity to the endpoint devices in the room.
Think of GPON as a wrapper, IP endpoints sit either side of the GPON network and have their traffic encapsulated in a GPON wrapper for transit. Layer 3 services are provided by non GPON elements of the network.
I can certainly see the appeal of this to the hotel. From an environmental and cabling standpoint the benefits are clear. Also, consider expasion. Need more ports? Swap out the ONT for one with more ports. No need for running extra cables, adding extra switches to the distribution point, etc.
As always however, nothing is perfect. There’s some points about a GPON network that require clarification.
GPON technology involves a shared bandwidth of 2.5Gb downstream and 1.25Gb upstream per OLT port. Remember, each OLT port is divided by a splitter, so that’s a shared bandwidth between multiple ONUs on the OLT port.
This bandwidth can increase with 10G-GPON or other framing types, the theoretical limit being that of the Single Mode Fibre, but it remains a shared medium.
Traffic is broadcast downstream, and uses multiplexing upstream. ONUs have a specific FSAN number, a unique ID analogous to a MAC address. Although traffic is broadcast to every ONU on an OLT port, it is tagged with the destination FSAN, meaning only the target ONU can decrypt it.
I’m by no means advocating GPON as something suitable for every scenario. There are a number of design considerations with a GPON solution, and a shared bandwidth combined with downstream broadcast traffic raises some scalability concerns. Nevertheless, to me it’s an interesting alternative to the norm, and to my mind an option worth considering as a play in niche situations where the customer requirements just don’t fit the standard campus LAN design.