Routed optical networks

Wed May 3 06:20:48 UTC 2023

> At that scale, DWDM in the metro will make sense
I would risk to say a little more on this.
Indeed, maybe the situation (in many countries) when the Carrier sells a lot of TDM services.
But in general, packet services are enough these days for many carriers/regions.

Additionally, I am sure that in many countries/Metro it is cheaper to lay down a new fiber than to provision DWDM, even if it is a pizza box. The colored interface is still very expensive.
Of course, there are some Cities (not “towns”) where it is very expensive or maybe even impossible to lay down a new fiber.
Yes, in the majority of cases, it is cheaper to lay down fiber.

Hence, the importance of DWDM for the Metro is overestimated.

Use only routers. Provision enough fiber. Have always 1 router hop to the aggregation (hub-spoke topology), no routers chaining in the ring.
If fiber is not enough – then use normal DWDM with an external transponder. Routers would be still in hub-spoke topology.

Ed/
From: NANOG [mailto:nanog-bounces+vasilenko.eduard=huawei.com at nanog.org] On Behalf Of Mark Tinka
Sent: Wednesday, May 3, 2023 7:09 AM
To: nanog at nanog.org
Subject: Re: Routed optical networks

On 5/2/23 07:28, Vasilenko Eduard via NANOG wrote:
The incumbent carrier typically has enough fiber strands to avoid any colored interfaces (that are 3x expensive compare to gray) in the Metro.
Metro ring typically has 8-10 nodes (or similar). 16-20 strands of fiber were not possible to construct anyway – any cable is bigger.
It is the same cost to lay down fiber on 16 strands or 32.
Hence, PTT just does not need DWDM in Metro, not at all. Hence, the DWDM optimization that you are talking about below is not needed too.

This may or may not always be the case. Especially for large carriers, where there could be a requirement to sell some of those dark fibre pairs to large customers (think the content folk coming into town, e.t.c.), they may no longer have the priviledge of having plenty of free fibre in the metro. Or if they did, the rate of traffic expansion means they burn through those fibre pairs pretty quick.

10Gbps isn't a lot nowadays, and 100Gbps may start to push the limits depending on the size of the operator, the scope of the Metro-E ring and the level of service that needs to be maintained during a re-route (two available paths in the ring could balance 100Gbps of traffic, but if one half of that ring breaks, the remaining path may need to carry a lot more than 100Gbps, and then packets start to fall flat on the floor).

At that scale, DWDM in the metro will make sense, at least more sense than 400G-ZR, at the moment.

If you rent a single pair of fiber then you need colored interfaces to multiplex 8-10 nodes into 1 pair on the ring.
Then the movement of transponders from DWDM into the router would eliminate 2 gray interfaces on every node (4 per link): one on the router side, and another on the DWDM side.
Overall, it is about a 25% cost cut of the whole “router+DWDM”.

Some operators would also be selling Transport services in or along the metro, and customers paying for that may require that they do not cross a router device.

It is still 2x more expensive compare to using additional fiber strands on YOUR fiber.

There are plenty of DWDM pizza boxes that cost next to nothing. At scale, the price of these is not a stumbling block. And certainly, the price of these would be far lower than a router line card.

By the way, about “well-defined stack of technologies”:
NMS (polished by SDN our days) should be cross-layer: it should manage at the same time: ROADM/OADM in DWDM and colored laser in Router.
It is a vendor lock up to now (no multi-vendor). Hence, 25% cost savings would go to the vendor that has such NMS, not to the carrier.
Technology still does not make sense because no multivendor support between the NMS of one vendor and the router or DWDM of another.
Looking at the NMS history, it would probably never be multi-vendor. For that reason, I am pessimistic about the future of the colored interfaces in routers (and alien lambdas in DWDM). Despite a potential 25% cost advantage in eliminating gray interfaces.

OpenROADM is a good initiative. But it seems it's to be to Transport equipment vendors what IPv6 and DNSSEC is to the IP world :-).

Mark.
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