1 it-212, how computers work lan, connections, email electrical and computer engineering spring 2002

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1 IT-212, How Computers Work LAN, Connections, email Electrical and Computer Engineering Spring 2002

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1

IT-212, How Computers Work

LAN, Connections, email

Electrical and Computer EngineeringSpring 2002

2

Internet

Constantly Evolving, Reconfiguring Access

Actual path may be very different from one day to the next, but knowable using “route” command

Content Asynchronous changes Difficult to reference in documents

Network of Networks

3

Local Area Networks

Cyber Interconnection of Physically Local Computers or Devices (nodes) Not just computer-to-computer Printers/plotters with network cards Connectivity

Electrical: twisted-wire cables, phone lines Optical: fiber optics, IR RF: Local Area Wireless Networks (LAWNs)

4

LAN Configurations

Peer-to-Peer No central server All computers act as clients to others No unique operating system required

Client/Server Coordination of multiple requests from

nodes Resource Contention Directed Communication

5

Clients

Fat Client Run programs from own drives Minimal network services

Thin Client Server to supply programs and data

Diskless Workstation Server supplies O/S, programs, and data

Terminal Only terminal capabilities, remote display X-terminal

6

Servers (1)

File Server High performance Computer

Large Hard Disk and fast I/O Often use Network Operating System

(NOS) Windows NT, Unix, Linux, Novell NetWare

Maintains list of file access permissions, logons, etc.

a.k.a., host computer

7

Servers (2)

Mail Server SMTP, POP, IMAP

Printer Computer connected to net, printer

connected to computer Printer itself may have internet

connection built-in as well as server capability

8

Wide Area Networks

Physically Separated e.g., separate buildings

High Speed Phone Line Connections T1, T2, T3, ATM

Virtual Private Network (VPN) Encryption to maintain privacy among

nodes on WAN

9

Network Connections to LAN

Network Interface Card (NIC) Dominant standard: Ethernet, 10BASExxx

Wiring Coaxial connector (BNC)

Daisy Chain via T-connectors Sensitive to cable breaks 50 Ohm Terminator required at both ends 10BASE-2

Twisted pair wiring (RJ-45 connector) Connect to hub 10BASE-T or 100BASE-T

10

Ethernet 10base2 Networks

All Nodes Connected on Bus NetworkEach Node Interfaces Via TransceiverEach Node Has Unique AddressEach Message Contains Address of intended destination Address of source Data to be transmitted Error checking data (optional)

11

Ethernet Networks

On Receipt, Recipient Sends AcknowledgementIf Simultaneous Transmission, “Network Collision” First sender detecting error sends

network block signal Each sending node to wait random

amount of time before trying again

12

Hub Networks

XC

VR X

CV

RX

CV

R

A

B

C

Hub

13

Hub Networks

Hub Provides “Cross-point” Switch Which Allows Nodes to Communicate With Each Other Without InterferenceNode Sends MessageHub Consecutively Polls Each Connection Searching for Data Transmissions Since only one connection is polled at a time,

simultaneously initiated messages do not collide

14

Phone and Power Line LANs

Use Existing Wires Within Building to Serve As LAN Connectors High frequency (2MHz +) signals

carried on on 60 Hz power lines Phone conversations

More bandwidth than 300-3kHz used by phone

15

P & PL LAN

Phone Jack May Not Be Available in All RoomsCannot Span Phone Lines i.e., Cannot Go Through PBXNoise on Power LinesFeatures Power line networks can control

power to other appliances

16

Network 7-Layer Model

Application LayerPresentation LayerSession LayerTransport LayerNetwork LayerData-link LayerPhysical Layer

17

Application & Presentation Layer

Application Layer Data conversion Header attachment

Presentation Layer Conversion to ASCII Compression Encryption

18

Session & Transport Layer

Session Layer Boundaries for message start/stop Communication protocol

Half-duplex: transmit or receive Full-duplex: simultaneous transmit/receive

Transport Layer Segments data Creates checksums Backup copy of data Header enumerating segments, checksums,

and position within message

19

Network & Data-Link Layer

Network Layer Selects route for data Turns segments into packets Header created enumerating sequence of

packets and receiving computer address

Data-link Layer Supervising actual transmission Confirms checksum prior to transmission Addresses and duplicates packets Duplicate is kept until receipt confirmation

20

Physical Layer

Physical layer Encodes binary data into medium for

transmission Sends packets along medium

21

Network Communications

Intermediate Nodes Calculate and verify checksum Allowed to reroute

Receiving Node Processes Received Packets in Reverse Order

22

Internet Data Travel

Connection to Network NIC to ethernet, or Modem to ISP

LAN to HUB to Connect Multiple NodesHUB to Router or Firewall Router: no protection, only isolates local traffic Firewall: traffic only permitted by port # or IP

adx

Firewall to Gateway Forwards non-local to traffic to WAN

23

Internet Data Travel

Gateway to Network Routers Evaluates current traffic and available

paths/connections to destination May use Internet backbone

Dissimilar Networks May Require Bridge Protocol conversion to connect

24

Digital Subscriber Line: DSL

Splitter

Phone Company Switching Office

25

Digital Subscriber Line

xDSL: “x” for various forms of DSLADSL (asynchronous) Upstream (away from PC): 640 Kb/s Downstream (toward PC): 8 Mb/s 18,000 feet max from phone office

VDSL (Very high speed) Downstream: 10-26 Mb/s 4,500 feet max

26

Digital Subscriber Line

Universal DSL (G-Lite) Upstream: 512 Kb/s Downstream: 1.5 Mb/s 18,000 feet max No splitter required

RADSL (rate adaptive) Upstream: 128 Kb/s to 1 Mb/s Downstream: 600 Kb/s to 7 Mb/s 21,000 feet max

27

DSL Properties

Point-to-Point Connection Messages not broadcast to all nodes

Connection Made Via NIC in PC and xDSL Modem Always on (no dial up)

Shares Existing Line Voice call and xDSL transmit on

separate frequencies

28

Cable Modems

Splitter

29

Cable Modems

Download Data transmitted via cable is segmented

into 110 bands, each 6 MHz wide Connection medium is hybrid coaxial cable Data uses IP packets with error correction and

header identifying target PC Download speed: 3-10 Mb/s

Upload Only 7 upload bands Typical speed 2 Mb/s

30

Cable Modems

Data Is Broadcast to All PC on Same Hub Reduced Security

Cable “modem” ignores messages not for this PC (checks header), reassembles packets for this PC, passes to Network Interface card in PC

Reduced Performance

Splitter in Home Splits Data and TV From Single Coax

31

Cable Modems

Fiber Optic Connects Distribution Hub to Cable NetworkCable Network Is Connected to Internet Via 45 Mb/s T3 ConnectionCable Supplier May Maintain Local Copy of High Use Web Pages To Improve Performance

32

Email

Initially LAN Based Local communications Meetings, interoffice memos

Now Internet Based Internet client/server software AOL, Netscape, Outlook, Pine, etc.

33

Email Mechanics

Typical Send Using Simple Mail Transfer Protocol

(SMTP) Receiving

Post Office Protocol (POP) IMAP

America on Line (AOL) Uses proprietary protocols Interfaces to Internet standard protocols via

gateway software

34

Email

Message contents Text within body Attachments

“Native format” (e.g., spreadsheet) translated into ASCII often using Multipurpose Internet Mail Extensions (MIME)

ASCII transmitted over net Translated from ASCII to Native format at

receiving PC

35

Email vs. USPS Mail

Email Timely delivery Confirmation of delivery No per-message cost Mailing lists

USPS Temporal delay Daily delivery (no Sunday) Cost for “return receipt requested”

36

Email Etiquette

Message tone More formal than spoken word Less formal than written memo Experience suggests one topic per email

Reflectors Send message to single destination List Server, e.g.,MajorDomo, retransmits to

all subscribed members

Spam Internet junk mail

37

Sending Email

Sender uses email client software to send message Attachments encoded in ASCII

MIME, uuencode, BINHEX common methods Long attachments may also be compressed

Client connects to internet via SMTP server Server either OKs transmission of message

or asks that transmission be deferred If OKed, message sent to SMTP server software,

confirmation sent back to sender

38

email

SMTP Uses Domain Name Server (DNS) to Determine How to Route MessageTransmission Via Internet RoutersRecipient’s SMTP Server Software Transfers Message to POP or IMAP ServerMail Is Held at Recipient’s Server Until Recipient Requests ItClient Software Reconstitutes Attachments Uncompress, if necessary Decode from ASCII back to native format