In reply to a post by jimbof:

In reply to a post by candlerb:

Downstream and upstream are simultaneous, as they're on different wavelengths, but individual ONTs on the same splitter have to take it in turns to transmit upstream. This is controlled by the OLT, which allocates timeslots.

The service data rate you get is controlled by the OLT shaping traffic. Roughly speaking, delays are inserted between packets, to ensure that overalll you don't receive or send data faster than the service you paid for.

What's to stop you giving everyone else on your splitter a "really bad day" by abusing the fibre and preventing them from being able to establish connections? How are Openreach able to deal with such situations? Thinking it could be a faulty ONT, or could be a malicious attack to take someone offline.

1. VLAN tagging
2. Prioritisation and queuing
3. Traffic shaping

In reply to a post by Pheasant:

1. VLAN tagging
2. Prioritisation and queuing
3. Traffic shaping

Those are all IP layer measures. I mean, at a more fundamental level, given this is - if I understand correctly - effectively a shared light pipe from 32 houses to one port at the exchange, what's to stop that being ruined by a badly behaved device at one end flooding the fibre with light all the time?

ONTs only transmit when they're given permission by the OLT. In theory, it's possible for an ONT to go off the rails and blat over everyone as you say. But in practice I've never heard of ONTs failing in that way.

As I understand it, the transmit control is quite sophisticated, and even takes into account the different distances (and hence round-trip times) between each ONT and the OLT.

I did see similar problems occasionally in the old days of 10base2 networks (we're talking 1990's here), when a NIC went bad and started transmitting continuously. But shared ethernet is different: devices just wait until they see a quiet line before they transmit, and are not being given explicit permission to transmit.

In reply to a post by witchunt:

Except if there is a subtended headend in the DSLAM.

Being pedantic, I'd say "cabinet" there. A "DSLAM" provides a DSL (copper) service.

Edited by candlerb (Sat 27-Feb-21 19:56:20)

In reply to a post by jimbof:

In reply to a post by Pheasant:

1. VLAN tagging
2. Prioritisation and queuing
3. Traffic shaping

Those are all IP layer measures. I mean, at a more fundamental level, given this is - if I understand correctly - effectively a shared light pipe from 32 houses to one port at the exchange, what's to stop that being ruined by a badly behaved device at one end flooding the fibre with light all the time?

I can imagine a couple of brute force scenarios, but the reality is the absolute risk is very low and could be quite easily traced. Even if a single PON was “attacked” - the total affected customers is still relatively low.

In reply to a post by candlerb:

....in the old days of 10base2 networks (we're talking 1990's here), when a NIC went bad and started transmitting continuously. But shared ethernet is different: devices just wait until they see a quiet line before they transmit, and are not being given explicit permission to transmit.

Thank goodness both 10Base2 and 10Base5 were dying out as I got into work. One poor terminator or dicky BNC connector could bring down an entire segment.

The other day someone mentioned “broken ring” as a moniker for TR. I had a good chuckle.

UTP and switched Ethernet transformed local area networking.

In reply to a post by Pheasant:

Thank goodness both 10Base2 and 10Base5 were dying out as I got into work. One poor terminator or dicky BNC connector could bring down an entire segment.

I used to admin a school network, the kids would steal the BNC's, it was a nightmare

The other day someone mentioned “broken ring” as a moniker for TR. I had a good chuckle.

UTP and switched Ethernet transformed local area networking.

My company was 16Mbps TR for ages, mostly retrofit over structured cabling for the newer buildings, the older ones had the IBM Type1 ports, and we all had adaptors. Even with laptops and a PCMCIA TR card. Thankfully this was all ripped out for switched Ethernet.

16 Mbps TR was slightly faster than 100 Mbps hub Ethernet, it was switching and volume that killed TR. I read IBM sold the patents to Cisco, whom promptly buried them

Oh yeah I remember good old IBM Type 1 thick as your middle finger and about as flexible and the “self healing” connectors to physically close the ring on the TR hubs.

We were ripping it out by the truckload in CBD high rise offices in Sydney in the early nineties and replacing it with new fangled Category 3 and 5! We still kept the TR going and just used Type 1 plug adapters into the structured cabling Those were the days. I guess that was all old world big blue before they fancied themselves and refashioned themselves from a hardware company to a “services” organisation.....

I remember as a summer job after leaving school I setup my uncles office, must’ve been around 1990/91 and cabling it with thinnet. A few years later we ripped out the coax and star wired with Krone Cat 5 as I recall and used HP 100VG AnyLAN gear. Must’ve been around 93/94 or something like that.

In reply to a post by jimbof:

In reply to a post by Pheasant:

1. VLAN tagging
2. Prioritisation and queuing
3. Traffic shaping

Those are all IP layer measures. I mean, at a more fundamental level, given this is - if I understand correctly - effectively a shared light pipe from 32 houses to one port at the exchange, what's to stop that being ruined by a badly behaved device at one end flooding the fibre with light all the time?

Nothing.

Which is probably why Openreach don’t allow third party ONTs for residential customers; they always provide and manage the ONT.

It’s worth noting that GPON is a huge improvement relative to ADSL/VDSL2 services, where a malicious actor or completely unrelated hardware can disrupt service for hundreds of subscribers with simple RF interference. Just look at the recent news story about a village that kept losing broadband services that was finally traced to a single malfunctioning television!

GPON has two key advantages versus DSL; physical access to the fibre is required to interfere with the signal, and all data is encrypted. It’s not perfect, but it’s a massive leap forwards for residential services.