Hello Jianjingzhang,
So - What is the "big deal" about packet switching and routers?
The short answer is that these are the methods and equipment used to
run the Intenet.
However, this is a pretty open ended question, so let me start by
comparing the basic characteristics between circuit switching and
packet switching, then describe how routers work, and finish with a
series of references for more information.
Packet Switching (compared to Circuit Switching)
Circuit switching dominates the "Public Switched Telephone Network"
(or PSTN) - the system used to connect analog phones together for a
telephone call. The network sets up a "circuit" - a specific set of
connections between two locations, and maintains that connection for
the duration of the call. This method was set up initially by
telephone operators and then later large switching units in the
telephone company.
Packet switching dominates data networks like the Internet. With
packet switching, the network determines the "best route" for each
message transmitted between two systems. The Internet Protocol (or
IP), defines the method used for splitting a message into pieces,
sending it across the network, and reassembling it at the destination.
The primary advantage is that each line is used by many different
messages and the utilization is far higher than with a circuit
switched network. The primary disadvantage is the added complexity to
send and receive messages, as well as the unpredictable delays (or
latency) that occur from a variety of causes.
In most cases, a router is simply a dedicated computer that receives
IP messages and determines based on a "routing table" where it should
send it next. As mentioned above, the decision it makes for one
message may be different than the next, based on criteria such as
- errors seen on an interface
- speed of an interface
- utilization of an interface
- cost to use an interface
- "quality of service" levels
and so on. To use an example, I ran the traceroute program on my
system to www.google.com over a few minute period and had the
following results:
#1
1 dal-tgn-tkw-vty254.as.wcom.net (216.192.232.254) 151.028 ms
105.991 ms 103.206 ms
2 dal-big2-eth01.wan.wcom.net (206.175.224.65) 109.245 ms 107.108
ms 106.754 ms
3 dal-ppp2-fas2-0-0.wan.wcom.net (206.175.225.5) 107.861 ms
106.306 ms 107.218 ms
4 dal-peer1-fas1-0-0.wan.wcom.net (205.156.223.84) 107.858 ms
105.841 ms 106.319 ms
5 0.so-4-0-0.hr3.dfw9.alter.net (157.130.151.117) 106.713 ms
106.994 ms 105.762 ms
6 0.so-1-1-0.xl1.dfw9.alter.net (152.63.103.194) 108.587 ms
108.669 ms 104.97 ms
7 0.so-7-0-0.xr1.dfw9.alter.net (152.63.101.250) 106.718 ms
106.572 ms 103.444 ms
8 185.atm6-0.br4.dfw9.alter.net (152.63.100.217) 109.147 ms
108.101 ms 107.513 ms
9 204.255.168.82 (204.255.168.82) 108.459 ms 109.063 ms 108.944
ms
10 * agr3-loopback.dallas.cw.net (208.172.130.103) 113.196 ms
agr4-loopback.dallas.cw.net (208.172.130.104) 113.066 ms
11 dcr1-so-0-2-0.dallas.cw.net (208.172.139.9) 106.904 ms 109.401
ms dcr2-so-1-3-0.dallas.cw.net (208.172.139.157) 108.149 ms
12 dcr2-loopback.santaclara.cw.net (208.172.146.100) 157.585 ms
152.597 ms 151.608 ms
13 bhr2-pos-0-0.santaclarasc3.cw.net (208.172.156.214) 152.697 ms
155.026 ms 151.309 ms
14 csr01-ve241.sntc03.exodus.net (216.33.153.188) 151.581 ms
152.116 ms 153.482 ms
15 google-exodus.exodus.net (64.68.64.210) 152.994 ms 159.235 ms
154.973 ms
[to read this, starting at my machine connected to Worldcom's network,
it went through a few machines, passed on to alter.net machines, to
cw.net machines, and finally to exodus.net machines used by google]
#2
1 dal-tgn-tkw-vty254.as.wcom.net (216.192.232.254) 148.275 ms
105.717 ms 101.014 ms
2 dal-big2-eth01.wan.wcom.net (206.175.224.65) 103.17 ms 99.545 ms
103.534 ms
3 dal-ppp2-fas2-0-0.wan.wcom.net (206.175.225.5) 103.401 ms 98.576
ms 106.555 ms
4 dal-peer1-fas1-0-0.wan.wcom.net (205.156.223.84) 107.706 ms
106.48 ms 102.306 ms
5 0.so-4-0-0.hr3.dfw9.alter.net (157.130.151.117) 107.927 ms
105.967 ms 108.297 ms
6 0.so-1-1-0.xl1.dfw9.alter.net (152.63.103.194) 108.672 ms 105.77
ms 100.097 ms
7 0.so-7-0-0.xr1.dfw9.alter.net (152.63.101.250) 109.591 ms
106.155 ms 103.155 ms
8 185.atm6-0.br4.dfw9.alter.net (152.63.100.217) 106.497 ms
107.506 ms 111.946 ms
9 204.255.168.82 (204.255.168.82) 110.567 ms 105.415 ms 107.52 ms
10 agr4-loopback.dallas.cw.net (208.172.130.104) 111.652 ms
agr3-loopback.dallas.cw.net (208.172.130.103) 110.3 ms *
11 dcr1-so-0-2-0.dallas.cw.net (208.172.139.9) 108.064 ms 105.715
ms dcr2-so-1-3-0.dallas.cw.net (208.172.139.157) 107.857 ms
12 dcr2-loopback.washington.cw.net (206.24.226.100) 145.174 ms
dcr1-loopback.washington.cw.net (206.24.226.99) 142.583 ms 155.736
ms
13 bhr1-pos-10-0.sterling1dc2.cw.net (206.24.238.166) 141.597 ms
bhr1-pos-0-0.sterling1dc2.cw.net (206.24.238.34) 143.117 ms
bhr1-pos-10-0.sterling1dc2.cw.net (206.24.238.166) 144.536 ms
14 csr11-ve241.sterling2dc3.cw.net (216.109.66.90) 146.46 ms
144.092 ms 145.348 ms
15 218-google-exodusdc.exodus.net (216.109.88.218) 151.022 ms
149.591 ms 147.674 ms
[a similar sequence, note at step 10, the paths diverged. Apparently a
link through Washington DC was better than a more direct path from
Texas to California.]
A good search phrase
packet switching routers
A few good references
http://www.rad.com/networks/1998/packet/ps.htm
has a pretty nice explanation at the beginning of packet switching.
http://www.privateline.com/Switching/packet.html
a look at changes in wireless telephone systems & how radio telephones
(circuit switched) and cellular phones (packet switched) differ.
http://www.nwfusion.com/links/Encyclopedia/P/647.html
a more encyclopedia definition and links from Network World Fusion
magazine.
and a few thousand more hits from that phrase.
--Maniac |
