1 it-212, how computers work lan, connections, email electrical and computer engineering spring 2002
TRANSCRIPT
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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
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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)
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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Network 7-Layer Model
Application LayerPresentation LayerSession LayerTransport LayerNetwork LayerData-link LayerPhysical Layer
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Application & Presentation Layer
Application Layer Data conversion Header attachment
Presentation Layer Conversion to ASCII Compression Encryption
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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
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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
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Physical Layer
Physical layer Encodes binary data into medium for
transmission Sends packets along medium
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Network Communications
Intermediate Nodes Calculate and verify checksum Allowed to reroute
Receiving Node Processes Received Packets in Reverse Order
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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
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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
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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
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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
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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
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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
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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
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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
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Initially LAN Based Local communications Meetings, interoffice memos
Now Internet Based Internet client/server software AOL, Netscape, Outlook, Pine, etc.
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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
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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
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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”
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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
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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
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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