IP Access
IP access into the Internet cloud can be accomplished in several ways. The most common is local access via dial-up (Local and Toll-Free), DSL, or cable. However, there are also gateways that allow access via Frame Relay, ATM, and X.25.
Local Access
Point-to-Point Protocol (PPP) with Dynamic IP Address Assignment
The large providers have rotaries that are located at sites in several hundred cities across the United States. At these sites, they have several hundred modems attached to terminal servers which are hubbed typically via an ethernet LAN to a hub router. This router has multiple T1 lines connected to an Access Concentration Router (ACR) at one of the nine backbone sites. The ACR is FDDI connected to the backbone routers. Analog lines come into the provider site via T1 facilities.
Toll-Free Access
Point-to-Point Protocol (PPP) with Dynamic IP Address Assignment
Point-to-Point
Protocol (PPP) with Fixed IP Address Assignment
Serial
Line Interface Protocol (SLIP) with Fixed IP Address Assignment
Compressed
Serial Line Interface Protocol (CSLIP) with Fixed IP Address Assignment
All toll-free rotaries have similar topologies. The toll-free numbers are located in one or two cities in the United States.
Toll-free sites are co-located with switch sites. Analog lines are provided using T1s from the DMS 250 switch to the terminal server which is co-located at the site.
Modems, terminal servers and hub routers are all usually monitored 24 hours-a-day, seven days-a-week.
The T1 connections from the hub routers are all serial connections. These serial connections are provisioned on a fiber-optic network which is designed with a loop topology so that the provider can reroute services around any breaks or failures within the network.
Frame Relay and ATM Gateways (data is contained in PVC's - Permanent Virtual Circuits)
The Frame Relay/Intranet gateway - if offered - is co-located with the Intranet backbone routers and backbone Frame Relay switches are located. Backbone routers are connected to a Customer Concentration Router (CCR) which is cross-connected to a Frame Relay switch. This Frame Relay switch is connected to the Frame Relay network. A Class C IP address on the CCR is mapped to a DLCI on the Frame Relay switch. A PVC is built from the Frame Relay gateway switch to a router at the customer’s site.ATM gateways work in a similar fashion. The gateway is simply a means of taking fragment IP packets in the form os frames or cells and assembling them back into packets, and plaing them onto the Internet.