Internet  Service Provider (ISP)

This  supper fast  network  spanning  the world  from  one major  metropolitan area to another  is provided  by a handful of national internet  service  providers (ISPs). These  organizations  connections  running at approximately 45Mbps linked up at  specified interconnection points called national access points  ( which  are located in major metropolitan areas). Local ISPs  connect  to this  backbone  through  routers so that  data can  be carried  through the  backbone  to its destination.

Each ISP is equipped with own  contingency backbone  network  or is  at least equipped  with an  outsourced backup. These  smaller networks are interlinked and  intertwined to provide the multi  faceted backup  redundancy needed to keep the  web  intact in case  of partial  failure through  peering and transit agreements.

                                              Overview  of the internet

 Let us assume that a client  calls his  or herI SP ( Internet  Service Provider ) over a dialup telephone line as shown in  figure 2.18. the  modem  within  the PC that converts the  digital signals. The computer  processes to  analog signals that  can pass unhindered over the telephone systems. These  signals  are transferred to the ISP POP (Point of Presences) where  they are removed from the  telephone systems and get entered into the  ISP regional  networks. From  this point  the systems  is fully  digital  and packet switched.

The ISP regional  networks  consists of interconnected  routers  in the various  cities  the  ISP serves. If  the packet is  destined for a host  served directly by the ISP the packet will delivered to the host. Otherwise  it is handed over the ISP backbone  operator.

The major  backbone  operator  companies  like AT & T and sprint operate will large  international  backbone  networks with thousands of routers connected by high and  bandwidth fiber optics. Large corporations and hosting services that run server farms( Machines that can serve thousands of web pages per second) often  connect  directly to the backbone. Backbone  operators encourage this direct connection by  renting space which  are called as carrier hotels, basically  equipment racks in the same room  as the router to allow short fast  connections between  server forms and the backbone.

If  a packet  given to the backbone is destined for ands ISP company  served by the  backbone, the  packet  is sent to  the closest router and handed off there. However many  backbone of varying sizes, exist in the world  so a packet may have  to go to  a competing  connects at the NAPs. Basically a NAP is a room full of routers at least one per  backbone. A LAN in the  room connect  all the routers so packets can  be forwarded from  any backbone to nay other backbone. In  addition  to being  interconnected at NAPs the  large  backbones  have numerous direct connections between  their routers. This  techniques  is known as private peering.