LIS508 lecture 7:introduction to networks

Thomas Krichel

2003-11-19

Structure

• General things about networks

• History of Internet

• LANs and Ethernet

• The TCP/IP model

• discussion mostly based on Tanenbaum’s “computer networks” classic book….

Computer network

• Definition:– A computer network is a collection of autonomous

computers– The distributed nature of the system is apparent to the

user

• Motivation– Resource sharing– Increase reliability– Improve scalability– Money savings through use of PCs

Progress in computers

• Moore’s law: the power of computers doubles roughly every 18 months.– 1985 - 386 Processor - 275,000 transistors – 1989 - 486 DX Processor - 1,180,000 transistors – 1993 - Pentium Processor - 3,100,000 transistors – 1997 - Pentium II Processor - 7,500,000 transistors – 1999 - Pentium III Processor - 24,000,000 transistors – 2000 - Pentium 4 Processor - 42,000,000 transistors

Power input in networks

• The speed of network connections has improved much faster than that.

• But since computer network links computers, progress in network speed has been held up by computers being slow!

• The conclusion should be that progress in network technology will go on and will affect more of our lives.

• We are only at the beginning of the wired age!

Computer networks for communication

• Access to remote information– Data sources– Software– E-commerce– Video on demand

• Person to person communication– Email– Video conferencing– Interactive television

Types of networks by technology

• Broadcasting network– any message issued by one computer is

received by all computers on the network

• Point-to-point network– Circuit switched: there is one physical link that

is being made between two machines that need to communicate. ex.: telephone

– Packet switched: the communication is broken into pieces. Each piece is sent individually. ex: Internet

Types • LAN

– Bounded size,• bus topology: one cable from first to last machine • ring topology: one cable in circular for• star topology: all cables go to a central machine.

– High speed and reliability

• WAN– Interconnection of a large number of hosts– Has transmission lines and routers– Has an irregular topology, often with subnets

WAN example

• The university campus it Westchester has one LAN linking all the computers together. Student and office facilities are separate.

• At the center, there is a gateway machine that send all the traffic to the Brooklyn campus.

• There, there is another set of machines that send out-of-campus traffic out of campus

an internet

• An internet is a collection of networks that can be used together. – It does not have a known topology.– Parts of it are controlled by different

organizations.– It uses interconnects many types of networks.

• The most famous internet is the Internet.

Network modeling

• Usually networks are modeled as layers

• Purpose of layer is to carry out services for the higher layer in a way that is transparent to the higher layer.– Layers communicate with their peers

according to known protocols– Between layers in the same machine there is

an interface.

Service types

• Connection oriented / connectionless– phone conversation– datagram

• Reliable / non-reliable– Online video– File transfer

example

• http is an application protocol on the Internet. – It is the protocol that web servers and web

clients use– It is also used in some other instances.

• http relies on a transport service to transport data from one machine to another. This transport protocol is called TCP. This happens on a lower layer.

The Internet

• The Internet is kept together by two key protocols– The Internet protocol IP.– And the transmission control protocol TCP.

• TCP relies on IP to provide services, thus IP is on a lower layer than TCP.

• Both are commonly referred to as TCP/IP.

Origins of TCP/IP

• 1957: USSR launches the Sputnik

• US worried about command and control structure after a nuclear attack

• Early 60s Paul Baran promotes packet switching rather than circuit switching.

• Mid 60s: Pentagon says that it wants this, gives grants to ARPA.

ARPAnet original design

• Network nodes have host computers and router computer

• Hosts sends messages smaller than 8061 bits• routers breaks it up into packets smaller than

1008 bits• all router are connected by transmission lines• each router connected to 2 others• store and forward principle

implementation• 12k times 16 bit words memory minicomputer

without hard disk as routers connected by 56kbps leased phone lines

• life with four hosts in 1969–12• over 30 hosts in 1972–09• protocol research leads to TCP/IP in 1974• integrated into Berkeley UNIX, freely available• Internet research group developing protocols• 1979 Internet research group reorganized to

Internet Control and Configuration Board

More history

• 1980: MILNET split off the ARPAnet• CSNET set up

– links researchers at non-ARPA contract institutions to the ARPAnet

– ran on a single box with dial-up lines since late 70s

• 1984 NSF links adds a router to it supercomputers, sets up NSFnet,– the first purely TCP/IP network, on 56kbps.– additional funding for (eventually 20) regional

networks connect to the backbone

• 1990 ANS (MERIT,MCI,IBM) take over NSFnet

The Internet now

• Connects millions of computers world-wide.

• Each computer is part of a local network. This network is sometimes very small.

• If you have problems connecting to the Internet, it is most likely a local problem, rather than the Internet being “down”.

LAN

• A LAN generally operates a broadcasting network.

• Something that is transmitted by one station is heard by all other stations

• An individual message is called a frame. • This creates a media access control

(MAC) problem. When one station issues a frame, it destroys a frame from another station, through frame collision.

Ethernet

• comes from the term "luminiferous ether".• Today the name for a family of LAN

protocols implementing CSMA/CD protocol for the MAC problem.

• CSMA/CD stands for carrier sensitive media access with collision detection.

• Initially only referred to IEEE 802.3 standard

• Can run over a variety of physical devices.

IEEE 802.3 history

• 1980 first version following work by DEC, Intel and XEROX

• 1982 version two released

• 1983 Novell creates its own version…

• 1990 support for twisted pair cable added

• Over time, a whole set of standards have emerged to deal with other applications such as wireless devices.

10baseT

• This is most widely used physical infrastructure today.

• It runs over twisted-pair cable and RJ-45 jack.

• Cables are cheap, transmission is fast, .59c

• Maximum length of cable about 150m, thus not good for bus architecture.

Fiber optic cable

• This type of cable is a lot faster.

• It uses light to transmit signal, so it is almost as fast as the speed of light.

• It has no problems of loss of signal over long distances.

• But it its more expensive.

• Only used for transmissions of lots of data, between hubs.

other devices

• “repeater” is a device that amplifies an electric signal so that it can travel over some more wire.

• “hub” is a repeater with a lot of outgoing wires. Hubs are dump, or smart. A smart hub can be inspected by a monitoring device to see what the traffic is doing there.

• A “bridge” or “switch” is a device that links different LANs together. In normal “promiscuous” mode, it receives signals from all LANs. It will know which MAC address is on which LAN and transmit echo signal to the right LAN.

MAC address

• A physical address burned onto each network interface card used on a LAN.

• Forms an address for your computer on a LAN

• It is 6 bytes long, written in hex, usually written with each byte separated by “:”.

• 2 power 48 addresses, that is about 11000 addresses for every inhabitant of the planet.