Level of ISP Internet Access Redundancy

Murphy is out there, ready to make your life miserable. Whether because of bad weather, carrier problems, or bad luck, an ISP's connection to a NAP or another POP might go down from time to time, with the potential for leaving customers without service. A redundant network will enable traffic to take another route until the problem is fixed. A well-designed ISP network is a network that has its POPs connected to multiple NAPs or to multiple other POPs, as illustrated in figure 2-3.

Figure 2-3  A redundant network provides more reliable connectivity.

Distance to Destinations

Customers should be concerned with the number of hops—that is, the number of interim networks—needed to reach a destination network through their ISP. In general, the more hops, the more potential for traffic to get delayed, misdirected, or garbled.

The distance to destinations depends on how the provider is connected to the NAPs. As you know by now, the Internet is a set of overlapping backbones from different providers, with the NAPs being the crossover between one provider and another. Small providers might connect to only one NAP or might not connect to any NAPs at all. In the latter case, a provider could offer service by being the customer of another provider who is connected to the NAP.

In general, providers that claim less than five hops to a destination are those who are connected to most of the NAPs and have a widespread backbone. Smaller resellers tap into major providers, and the traffic might end up hopping a couple of different backbones before reaching the final destination.

Traffic Exchange Agreements

It is important that an ISP be part of all traffic exchange agreements, such as the CIX. Every ISP has to pay its due; there is no free lunch out there. If an ISP is not part of an interexchange agreement with other ISPs, its traffic might be denied when it reaches other providers' networks.

Demarcation Point

Finally, in addition to pricing and backbone issues, customers will want to consider demarcation point (DP) issues in selecting an ISP and forming an agreement. A demarcation point is the point that differentiates the provider's network from the customer's network, as illustrated in figure 2-4. It is important to differentiate between the areas of responsibility of both parties, provider and customer. Demarcation points are defined down to the cables and connectors to make sure that no arguments occur in case of equipment or network problems.

Figure 2-4  Demarcation point.

Different providers define the DP differently depending on who is paying for the equipment and the access line.

Customer Premises Equipment (CPE)

Customer Premises Equipment (CPE) usually includes the router, the CSU/DSU, the cabling, and probably an analog modem for monitoring. ISPs typically offer customers the choice of buying the CPE and the access line, buying just the access line, or just paying a monthly fee with all equipment and access needs taken care of by the ISP. Any arrangement is available at a price. ISPs usually are responsible for maintaining equipment or packages that they provide. An ISP might have a predefined package that includes CPE and/or access. If the customer does not want to take the package, then the customer would be required to choose equipment that is pre-approved by the ISP. The customer would then be responsible for troubleshooting and maintaining its own equipment. The provider is always available to solve problems at an extra charge. Figures 2-5 through 2-7 illustrate some examples of ISP packages.

Figure 2-5  Example: ISP provides access and CSU/DSU; customer provides router.

Figure 2-6  Example: ISP provides access, router, and CSU/DSU.

Figure 2-7  Example: customer provides everything.

In the scenario illustrated in figure 2-5, the ISP is responsible for the access line and the CSU/DSU all the way up to the CSU's serial connector at the customer site. Restrictions might be imposed on the customer premises routers to meet some memory or software revision guidelines.

In the scenario illustrated in figure 2-6, the ISP has provided everything and its responsibility ends at the LAN port of the router at the customer's premises.

In the scenario illustrated in figure 2-7, the customer provides the CPE and the access line; the provider's responsibility will end at the POP's wiring closet where the ISP interconnects with the carrier's central office (CO).