Hardware Reference
In-Depth Information
handle the traffic, and many high-end computers now have one of these processors.
In this section we will first give a brief introduction to networking and then discuss
how network processors work.
Introduction to Networking
Computer networks come in two general types:
local-area networks
,or
LAN
s, which connect multiple computers within a building or campus, and
wide-
area networks
or
WAN
s, which connect computers spread over a large geographic
area. The most popular LAN is called
Ethernet
. The original Ethernet consisted
of a fat cable into which a wire coming from each computer was forcibly inserted
using what was euphemistically referred to a
vampire tap
. Modern Ethernets have
the computers attached to a central switch, as illustrated in the right-hand portion
of Fig. 8-14. The original Ethernet crawled along at 3 Mbps, but the first commer-
cial version was 10 Mbps. It was eventually replaced by fast Ethernet at 100 Mbps
and then by gigabit Ethernet at 1 Gbps. A 10-gigabit Ethernet is already on the
market and a 40-gigabit Ethernet is in the pipeline.
Application provider premises
Server
Internet
Router
Firewall
Telephone line
Packet
ISP
Switch
User's computer
Fiber-optic link
Figure 8-14.
How users are connected to servers on the Internet.
WANs are organized differently. They consist of specialized computers called
routers
connected by wires or optical fibers, as shown in the middle of Fig. 8-14.
Chunks of data called
packets
, typically 64 to about 1500 bytes, are moved from
the source machine through one or more routers until they reach their destination.
At each hop, a packet is stored in the router's memory and then forwarded to the
next router along the path as soon as the needed transmission line is available.
This technique is called
store-and-forward packet switching
.
Although many people think of the Internet as a single WAN, technically it is a
collection of many WANs connected together. However, for our purposes, that dis-
tinction is not important. Figure 8-14 gives a bird's-eye view of the Internet from
the perspective of a home user. The user's computer is typically connected to a
