Networking
Analogy
Networking
A network is two or more computers connected together so that they can exchange information
Networks extend the bus, they convey 0’s and 1’s among computers
Networking
Networks began with the military
Army, Navy, and Air force all had computers, and wanted to communicate with each other
Networks and Protocols were created
The URI Network
Envision Lab
Gateway Gateway
INTERNET
URI Network
What do you need to communicate?
Physical way to send message Smoke signals drum
What do you need to communicate?
What does the message mean?
When will the message start?
When will it end?
Who is it meant for?
What kind of message is it?
Does it require a reply?
What code do you use?
Basic Components of a Data Communication System
Sending Device
Communications Link
Receiving Device
Sending Device - Modem
Modem = modulate/demodulateConverts digital signal to analog and vice versa User who connects to the computer via telephone servicesspeeds 56k (56,000 bps)directly connected to the internet (unlike LAN)
Sending Device - Modem
US Robotics Internal PCI Faxmodem
$34.99
Sending Device – Cable Modem
Sold by the cable company
Requires Ethernet
Puts home computer on LAN
Speeds > 1 Mbps (1 million bytes per second)
Sending Device – Cable Modem
Linksys Cable Modem
$99.99
Up to 42 Mbps download speeds, 10 Mbps upload
“Always on” connection
Sending Device – Cable Modem
D-Link Fast Ethernet PCI Network Adapter
$19.99
10/100 Mbps Ethernet or Fast Ethernet compatible
Sending Device - DSL
Alternative to cable for faster internet access then dial-up
Offered by phone company (via phone line)
Data transfer of 128,000 – 256,000 bps
(128k – 256k)
Sending Device - DSL
Zoom ADSL Bridge Modem 5515 - DSL modem
~ $60.00
Protocols
To send a message, you need an agreed upon way to pulse the bits (0s and 1s) of the message
Protocol - A set of rules for the exchange of data
Protocols make sure that everything is in order before it is sent
Protocols
How fast to pulse the bits?
How to mark the start and end of a message?
What computer the message is for?
What kind of message is it?
Does the message require a reply?
Is the whole message coming or just part of it?
What part of the message is this?
Terminology
LAN – Local Area NetworkA network of computers directly connected
together by the same network management
URI Dorms, CS Dept, network within a business
Terminology
WAN – Wide Area NetworkA network of computers, usually physically
separated from each other by great distances
Sometimes under the same management, sometimes under different management
ISO’s 7 Layer Model
Physical Layer
The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.
How media gets from one place to another
Speed
10baseT LAN = 10Mbits/sec
100baseT LAN = 100Mbits/sec
ISDN = 64Kbit/sec
Modem = 56Kbits/sec
Cable Modem/DSL = 1.2 Mbits/sec and up
T1 = direct connection at 1.5 Mbits/sec
T3 = 24 parallel T1 lines
Wireless – 11 Mbs
Connection Medium – Wire Pairs
Twisted pair copper (phone lines)
Inexpensive, already installed
Susceptible to noise
Connection Medium – coaxial copper
Faster, less prone to noise
Conductive wires in an enclosure
Laid underground
Carry cable TV
Connection Medium - Fiber
Glass fibers over which light is pulsed
No metal
Faster then coaxial
Materials less expensive and lighter
Connection Medium - Microwave
Fast, cost effective
Line of sight transmission
Connection Medium - Radiowave
Used for long range transmissionsa station on earth sends signal to a satellite orbiting the earth, which has a transponder that receives the signal, amplifies the signal, changes the frequency and sends it back down to another earth station
Hardware – NIC or Modem
NIC – network interface card
Collection of hardware and software resources that allow a computer to communicate to networks
Hardware - NIC
Microsoft Networking 10/100 Ethernet PCI Adapter
$6.99
Hardware - Gateways
Gateways connect two dissimilar networks or two computers with different interface cards
Transfers data from one network to another
Hardware - Bridge
Connects similar networks
Hardware - Router
Hardware that looks at IP addresses and sends data to another router
Finds a path from one computer to another
Hardware - Router
Much of the work to get a message from one computer to another is done by a routerEnsures info doesn’t go where it’s not
neededMakes sure info reaches its destination
Hardware - Router
Linksys Wireless Router
$ 99.99
Hardware - Router
Netgear 4-Port Cable/DSL Router
$59.99
Data Link Layer
The Data Link Layer describes the logical organization of data bits transmitted on a particular medium.
Determines speed at which data is pulsed over the medium and adds special bits to indicate the start and stop of transmission
PPP
Point to Point protocol
Used by modems
No collisions
Windows has PPP built into dial-up adapter software
Ethernet
Protocol used on LANs and by cable modems
Collisions all computers use the same cableThe node “listens” to see if the cable is in
useCSMVCD – carrier sense multiple access
w/ collision detection
Internet or Network Layer
The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Allows computers on different networks to talk to each otherAdds bits of data to determine where data is headed
IP Address
Each computer is assigned a unique IP address
ISP gives you a dynamic IP address
LAN gives you a fixed IP
We are currently using IPv4 which was developed 20 years ago
IPv4
IIPv4, IP addresses have 4 bits fields
Each bit field is represented using a binary number and is referred to as an octet
They each have eight positions when viewed in binary form
If you add all the positions together, you get 32, which is why IP addresses are considered 32-bit numbers
IPv4
Since each of the eight positions can have two different states (1 or 0) the total number of possible combinations per octet is 28 or 256
So each octet can contain any value between 0 and 255, Eg. homepage.cs.uri.edu is 131.128.81.37
Classes
The four octets are used to create classes of IP addresses
Classes can be assigned to a particular business, government or other entity based on size and need
Classes
There are 5 classes in IPv4 plus some special addresses
The five IP classes are: A,B,C,D, and E
Net and Host
The five octets are split into two sections: Net and Host
Net always contains the first octetUsed to identify the network that a
computer belongs to
Host always contains the last octetUsed to identify the actual computer on the
network
Default Network
0.0.0.0
Class A
Used for very large networks
IP addresses with a first octet from 1 to 126
The other three octets are used to identify each host
There are 126 class A networks with 16,777,214 possible hosts
2,147,483,648 unique IP addresses
115.24.53.107
Loopback
127.0.0.1
Used as a loopback address
Used by the host computer to send messages back to itself
Commonly used for troubleshooting and network testing
Class B
Used for medium sized networksIP addresses with an octet from 128 to 191 The second octet is also part of the Net IdentifierThe other two octets are used to identify each host16,384 class B networks with 65,534 possible hosts1,073,741,824 unique IP addresses145.24.53.107
Class C
Used for small to mid-sized businessesIP addresses with a first octet from 192 to 223Second and Third octets are also used as part of the Net identifierLast octet is used to identify the host2,097,152 class C networks each with 254 possible hosts536,870,912 unique IP addresses195.24.53.107
Class D
Used for multicasts
The first four bits are 1110
The other 28 bits are used to identify a group of computers the multicast message is intended for
268,435,456 addresses
224.24.53.107
Class E
Used for experimental purposes
They have never been documented or utilized in a standard way
First four bits are 1111
240.24.53.107
Broadcasts
Messages that are intended for all computers on a network are sent as broadcasts
These messages always use the IP address 255.255.255.255
IPv4
Combine the four octets and you get 232 or a possible 4,294,967,296 unique valuesWe are running out of IP addresses!In the next 5 – 15 years we will be switching to IPv6, which uses 128 bit IP addressesUntil then we use NAT
NAT
Network Address Translation
Allows your home network to share a single internet connection over a single IP address
Also, NAT keeps your home network fairly secure
DNS
Domain Name Server
Translates names like homepage.cs.uri.edu to IP addresses
You can use a name or an IP address in a web browser http://homepage.cs.uri.edu/courses/fall2004/csc101s1 http://131.128.81.37/courses/fall2004/csc101s1
Transport Layer
The Transport Layer describes the quality and nature of the data delivery.
Ensures that messages are received error free and in the right order
Packets
Data is broken up into fixed sized packets
Each packet is marked with a sequence number
TCP
Transmission Control Protocol TCP enables two hosts to establish a connection and exchange streams of dataTCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent
UDP
Uniform Data Protocol
UDP provides very few error recovery services, offering instead a direct way to send and receive data over an IP network
It's used primarily for broadcasting messages over a network
Session Layer
The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call.
Presentation Layer
The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating point numbers can be exchanged between hosts with different math formats.
Application Layer
The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations.
HTTP
HyperText Transfer Protocol
HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands
For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page
SMTP
Simple Mail Transfer Protocol
a protocol for sending e-mail messages between servers
Adds the date, and header (to from) to message
POP and IMAP
POPPost office protocolDownload messages to local serverStores e-mail messages in a text file
IMAP Internet Message Access Protocol Your e-mail stays on the server
FTP
File Transfer Protocol
The protocol for exchanging files over the Internet
FTP is most commonly used to download a file from a server using the Internet or to upload a file to a server
Telnet
Terminal Emulation program
For logging into remote servers
To start a Telnet session, you must log in to a server by entering a valid username and password
Your computer acts as a terminal by remotely logging into another computer
Telnet
Telnet is not very secure, it sends your password unencrypted
Many system administrators block the use of Telnet
SSH
Developed by SSH Communications Security Ltd., Secure Shell is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another
It provides strong authentication and secure communications over insecure channels.
SSL
Secure Sockets Layer
Encrypts the data before it goes to TCP/IP and decrypts upon receiving
Allows secure transmission of data, used when you buy stuff online, and when you transmit passwords
OSCAR
Used for AOL instant messanger
• A business manager writes a long report and wants to send it by US Mail.
• Data being e-mailed from one computer to another.
• The manager hands the report to a secretary who generates mailing labels using standard addressing conventions of to address and from address.
• The computer adds bits to the e-mail message which represent the “from”, “for”, and “subject” fields. The structure and placement of this data is determined by SMTP.
• The secretary hands the labels and the reports to a mail clerk.
• The e-mail application sends the data to the TCP/IP stack in the computer’s operating system.
• The clerk adds the zip code to the mailing labels.
• The operating system adds does a DNS lookup of the e-mail’s host name address to get the IP address. It adds bits representing the IP address to the data.
• The clerk knows the weight limit of the US postal service, so he breaks the report into several envelopes and puts an address label on each one. He also puts a sequence number (1,2,3…) on each letter.
• The operating system breaks the e-mail message into TCP packets.
• The clerk puts the envelopes in the outbox of the company.
• The computer sends the data to the network card or modem.
• The US postal service comes by and picks up the envelopes at certain times.
• The network card broadcasts the data over the data link layer. The data link layer dictates when the packets can be broadcast.
• The US postal service uses hand carriers, trucks, and airplanes to get the envelopes to their destination.
• The data travels over many types of network media, such as wire pairs, microwave, and satelite.
• The envelopes go to the local post office, then the regional post office, then the central post office, then the local post office of the receiver. Each post office has a set of rules as to where to send certain zip codes.
• The data goes through various gateways and routers. Each router has rules as to where to send certain IP addresses.
• A clerk on the receiving end watched the mail truck go by and continually asks, “Anything for out company?”
• The network card in the receiving computer constantly watched traffic on the network looking for packets with their IP address.
• The receiving clerk collects envelopes off the local mail trucks until he has all the envelopes that make up the report. If any envelopes are missing, he calls the sending clerk and asks him to resend that envelope.
• The receiving computer’s TCP stack collects all packets and puts them into order. If any are missing, it signals the sending computer to re-send the missing packet.
• When all the envelopes have arrived, the clerk gives them to the secretary. The secretary takes out the pieces, puts a nice “to” “from” cover letter on the report, and hands it to the manager.
• The receiving computer uses an e-mail program that understands SMTP to display the e-mail header and the text on the e-mail.