Show 248 – GPON For Campus Use Cases

Every once in a while we like to talk about something completely different on Packet Pushers. Today is one of those days. A few weeks ago, Andy Burridge posted an article on our community blog feed, introducing the idea of Gigabit Passive Optical Networks (GPON) as an alternate campus networking strategy to traditional three-tiered Ethernet.

GPON isn’t exactly a technology that can be dropped in as a replacement for traditional Ethernet campus switching anywhere you like. For enterprise network architects, GPON is more like an interesting alternative in specific situations and use cases where traditional Ethernet might be difficult or expensive to deploy.

We’re joined by guests Andy Burridge, a network engineer for a solutions provider; and Wallace Chase, a network architect at Washington State University. We’ll talk about exactly what GPON is, potential use cases for the technology, and network design challenges.

Show Notes:

What Is GPON?

• Gigabit-capable Passive Optical Networks
• It is a (complex) access layer technology
• Carries multiple transport types, including Ethernet, ATM, TDM
• GigE -> Optical Line Termination (OLT) -> Splitter -> Optical Network Termination (ONT) -> Endpoints
  • OLT does not provide routing services. Core switching (hairpinning) may occur
  • Splitter is passive (no power required). Takes light power and splits into channels per user Attenuation of signal occurs
  • Upstream (to users from OLT) via broadcast but encrypted using AES via ONT identifier, multiplexed downstream via TDMA based on 125us time slices
  • ONTs are similar to cable modems in a sense. Ethernet on the one side, GPON on the other Possible coax termination for triple play. Tellabs, Motorola make ONTs
• Verizon FIOS (US) is based on GPON

GPON Use Cases

• Hotel
• Apartment & dormitories
• Triple play capability – Ethernet, phone, cable TV

Design Challenges Of GPON

• Locked into ONT – OLT data flow. Can’t mesh from the ONT to anywhere else like in a multi-tiered Ethernet design
• Some significant SPOFs
  • Splitter. Should be very low risk, not active item with moving parts that are likely to fail
  • Fiber cut. Nice to mux lots of ONT signals across a single piece of fiber, right until it gets cut
    • However, there are network protection modes that provide failover between the ONT and the core
    • Also optional to have a network protection mode for failover to the ONT. I.e., dual-homed OLT -> 2 ONTs. Expensive. Secondary fiber never gets used unless there’s a failure
• Bandwidth limitations between tiers. 2.5Gbps down / 1.25Gbps up. Not infinite just because it’s SMF Framing to 10G-PON could improve this, but the point is that this is shared bandwidth across all the ONTs using this OLT port
• Power redundancy for endpoints. How to supply power to phones when power goes out and the source was the ONT? Same question for the ONT itself. UPS complexity
• How is traffic classification & preferential treatment provided at the ONT?
  • Uses Dynamic Bandwidth Assignment (DBA). This is a packet scheduler
  • Transmission containers (T-CONT) consist of 5 types – fixed (voice, fixed bandwidth, delay sensitive), assured & non-assured (video, guarantee bandwidth), best effort (not sensitive, lower priority), and mixed (all sorts of traffic)
  • One scenario appears to be…VLAN tag -> GEM port -> T-CONT-> Scheduler. Each T-CONT gets a specific bandwidth allocation
• Can traffic classes be identified and given preferential treatment?
  • ONT -> OLT (Does the guaranteed 125us timeslice help with this?)
  • OLT -> ONT (This is broadcast, so…)

What Networking Gear Supports GPON?

• Big 3 globally – Huawei, Alcatel-Lucent, ZTE
• Zhone, Tellabs in the US
• Others include Ericsson, Fujitsu, NEC, Nokia Siemens, UT Starcom