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I would like to clarify my understanding of the difference between an ISP Point of Presence and an ISP data center. Perhaps more importantly, I'm trying to understand at what kind of granularity each is generally deployed, and what effect that has on Content Delivery Networks such as Akamai's ability to plonk down one of their caching servers in one or the other.

My understanding of the difference between POPs and data centers is this:

  • An ISP has more Points of Presence than they do data centers.
  • A Point of Presence is often somewhere where the ISP has leased space from a telco provider to put their hardware.
  • An ISP data center is a larger facility that connects many ISP points of presence.

Regarding POPs, my questions are:

  • What do these things actually look like? Are they a box owned by the ISP that sits in a telephone exchange somewhere? I'm trying to get a visceral understanding of what these things manifest as in the real world.
  • Typically, how granular is the deployment of these things? Does an ISP typically have a POP every 10 miles, 100 miles, 1000 miles?
  • What level of the network stack do POPs operate at? Do they have IP addresses, or is it something lower than that?

The reason I'm interested in this, and relating it back to content delivery networks, is that I've recently been watching presentations given by Van Jacobson on PARCs Content Centric Networking, which is proposed as an IP alternative. There's a part in one of his presentations where he states that one of the problems with existing CDNs is that they can't get into ISPs POPs to deploy their caching servers, because most of the POPs are down manholes, and that it is easier for ISPs to offer a caching service at their POPs because of this. So a statement like this would seem to indicate that

  1. CDNs must be deploying their caching servers somewhere other than down POP manholes (at an ISP data center perhaps?) and that
  2. the routers/machines that an ISP has at their POP operate at least at the Network Layer

How accurate is my understanding of how this stuff all works?

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As with many IT terms, there isn't typically a precise definition, in this case it'll vary depending on the ISP. – Ward Jun 10 '11 at 5:45

First off, there is no exact formal definition of what a "Point of Presence" (POP) or a "Datacenter" (DC) is.

Next, I would say you're somewhat looking at this from the wrong perspective. What's called POP and DC is often not defined by the tech group of the ISP. They're often chosen by, or chosen with significant input from, the sales/marketing group. So don't rely too much on these words.

My personal rough take on these words is that:

  • An ISP POP is a place where the ISP has router(s) or edge switches, and would accept a cross-connect link from a customer.
  • An ISP datacenter is a place where the ISP has the above, plus could optionally co-locate several servers (say, half a rack or more) on behalf of their customer.

problems with existing CDNs is that they can't get into ISPs POPs to deploy their caching servers

I don't quite agree. A CDN also has a commercial need pushing their design in the opposite direction -- the more POPs the CDN has, the more duplication they have of content (i.e. more storage is required, and objects are served fewer times from cache). CDN's mainly want to add POPs when:

  1. The network latency becomes 'too high' to reach a given customer goal, or
  2. When adding POPs enables attractive settlement-free peering deals (free of charge peering with other networks).
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