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A Basic ISP POP Diagram

Members of the ISP-Tech list describe in the simplest language what it takes to build a point of presence (POP) to provide dialup Internet access.

[February 14, 2003]

On the ISP-Tech list in February, MA asked a fundamental question:

"I was wondering what you need to make your own point of presence for dialup users. I really cannot find a basic, well thought out diagram of a point of presence (POP) and how the POP relates to the data center. "

Respondents started with the basics.

[TH replied] "Our POP design is very simple. It is a Cisco 2500 with a P2P T-1 providing backhaul to our data center. It uses a number of Channelized T-1s in a number of Cisco Access Servers to provide lines for individual customers. All of this plugs into a hub serving Ethernet to each device. We also throw a Bay Tech RPC with Ethernet, modem, and console card to give us remote management via Ethernet, or if we lose the T-1 to it via modem to verify power. Then all we do is either rent or trade for Internet access an empty office or closet and throw everything out there."

[PS agreed] "That's about it. Usually there isn't a lot to a POP unless it's serving a really large number of customers. The only item I can think of not mentioned already is UPS and surge protection and the presumption that you'll need an actual rack to drop this stuff into.

Most of the 'work' is done back at the data center. Billing system, RADIUS, Web servers, e-mail servers, and more are all at the data center, not the POP.

Are you referring to starting to offer dialup service for the first time in your business? This would be a different story. A POP is usually a remote location that backhauls to a central data center. Could you clarify what it is that you wish your business to offer?"

[TH added] "We do have a rack and a APC 1400RM in each location. The CSU/DSUs we use are ADTRANs for remote customers who want T-1 access. In locations with high traffic or customer base increase your T-1s accordinly. Some locations will have multiple Cisco 2500s if we have a lot of dedicated or a ton of dialup. Adding duplicate items for each location is simple this way. We use Bay Tech for cabling. A quick and dirty text diagram looks like this:"

MA asked for a definition of the phrase "channelized T-1" and received two responses:

[PS explained] "We have customers that get quite confused by the buzzword of "T-1". A T-1 connecting in a channelized fashion to a 5300 access server as an example refers to 23 or 24 channels per T-1 so four T-1 lines would probably be 96 lines (depending on line setup and customer needs).

However, a T-1 connection can also be used as a 1.544 Mbps circuit to backhaul data.

You may have already understood this but it's the most common confusion I run across when talking T-1 lines."

[GC added] "Channelized means that the 24 channels of the T-1 are aggregated either as a whole or a fraction of the 24 to deliver the package. Each channel is a DS-0 comprised of 64 Kbps. Normal voice channels use 56 Kbps with 8 Kbps as overhead. Channelizing the data allows full utilization of the 24 64 Kbps DS-0s or their aggregation for voice or data. A fractional T-1 would be a number of DS-0s channelized together and carried over a full T-1 circuit.

In the context of the example above, the channelized T-1s were all labeled T-1/Ethernet, so the assumption is that they were some form of bridged Ethernet at a T-1 channelized data rate or 24 x 64 Kbps each. Don't do the math as it'll come up to 1.536 Mbps, not 1.544 Mbps. It is still correct. The remainder is required overhead. "

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