Computer Networking Questions

1
Computer Networking Questions

• How is information transmitted from point A to
  point B?
• What, physically, are the three basic types of
  communications media?
• Fiber, copper wire, and free space (Chapters
  14-16)
• Whats a LAN? Whats a WAN? (Chapters 18 and 19)
• How does the Internet work? (Chapters 2 and 20)

?
NEW TOPIC
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Computer Networks

• A computer network is a system for communicating
  between two or more computers and associated
  devices
• A popular example of a computer network is the
  Internet, which allows millions of users to share
  information
• Computer networks can be classified according to
  their size
• Local area network (LAN)
• Metropolitan area network (MAN)
• Wide area network (WAN)

3
LAN Basics

• Types of LANs
• Types of LANs include
• Ethernet
• Token Ring
• FDDI
• Topology
• Nodes (defined to be any device connected to the
  network. This could be a computer, a printer
  etc.) in a LAN are linked together with a certain
  topology. These topologies include
• Bus
• Ring
• Star
• Branching tree
• Transmission media
• LANs can either be made wired or wireless
• Twisted pair, coax, or fiber optic cable can be
  used in wired LANs
• Air is used as the transmission medium in
  wireless LANs

4
Local Area Network (LAN)
A LAN is a computer network that provides access
to shared services in a limited area.

• Spans a limited area such as a building
• Typically spans no more than a mile
• Varies from serving a few users to thousands of
  users
• Usually serves a single organization
• Supports desktop computers, laptops, servers,
  personal devices
• Allows access to many resources
• Printers
• File Servers
• Internet Access
• Mainframe
• Storage

5
LAN Speeds

• LANs are very fast, moving data at speeds from 10
  Mbps to Gbps
• Why so much faster than WAN speeds?
• A sizable percentage of computer communications
  (in business) still occurs within a local
  environment
• Some estimate the percentage to be 80 versus 20
  outside LAN (?)
• Lower costs (ease of installation, shorter
  transmission distances, etc.)

WAN Connection Speeds 56 Kbps 1.544 Mbps
100 Mbps LAN
100 Mbps LAN
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LAN Characteristics

• Requires very little wiring, typically with a
  single cable connecting to each device
• Uses twisted pair, fiber, coax, or wireless
• A NIC inside a computer attaches to the network
  wiring
• Different types of LANs use different wiring
  configurations like ring, bus, and star
• Software facilitating operation of the LAN
• Windows NT Server, Novell NetWare
• Physical LAN equipment includes intelligent
  wiring hubs, bridges, LAN switches, routers
• Different types of LANs use different access
  methods

Physical Medium
Network Interface Card (NIC)
Topology
Network Operating System (NOS)
LAN Equipment
Access Control
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LAN Physical Building Blocks
Theres a lot behind the walls.
Wiring Closets Cable termination Equipment
racks

• Requires consistent and hierarchical wiring
  structures
• Compliance with specs.
• Flexibility/modularity/reconfigurability!
• Spare capacity for growth
• Secured access

Wallplates/jacks
Conduit/ inter-building pathways
Physical Medium Copper.. Fiber..
8
LAN Topologies

• LAN Topology describes how the network is
  constructed and gives insight into its strengths
  and limitations
• Bus
• Star
• Branching Tree
• Ring
• Note theres a difference between topology and
  physically how the network is wired. For example,
  a single wire could extend from a hub to each
  computer in a star configuration, but still
  logically act as a ring because of how its
  connected at the hub. (Logical versus Physical
  topology)

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Bus Topology
10
Star Topology
11
Branching Tree
12
Ring
13
Access Control Methods

• Like in a noisy room -- its difficult to
  communicate if every computer tries to
  simultaneously transmit
• Two primary access control methods
• Non-Contentious Access Token Access
• Contentious Access Carrier Sense Multiple Access
  with Collision Detection (CSMA/CD)

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Token-Based Access
Deterministic Approach

• Used in Bus and Ring topologies (e.g. Token Ring)
• A token is placed on the network and passed to
  each network node
• The token consists of a specific bit pattern
• The computer in possession of the token may
  transmit information
• The message is sent to all other members of the
  network
• The member the message is addressed to hears
  the message and all others ignore the message
• Once the information is delivered, the token is
  available for use

15

• Lets see an animation of the token ring
• Another example animation

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CSMA/CD Access
Non-Deterministic

• Carrier Sense Multiple Access with Collision
  Detection (CSMA/CD)
• Usually used in a bus topology Used in
  Ethernet LANs
• All stations can send whenever they have data to
  transmit
• First, a station listens to the network, if idle
  (that is, no one is talking), data is transmitted
• But, it is possible for two stations to transmit
  simultaneously, thinking that the channel is
  clear
• When this happens, a collision occurs
• The first station to detect a collision sends a
  special signal
• The stations in contention then wait a random
  time to again attempt transmission
• Performance degrades as network traffic increases

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CSMA/CD
A
B
C
D
A
B
C
D
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Examples of LANs
Popularity?
Medium?

• Ethernet
• Token Ring
• FDDI

Cost?
Access Method?
Distance?
Topology?
Speeds?
Devices Supported?
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Ethernet
MOST POPULAR LAN IMPLEMENTATION

• Developed by Xerox in 1976 (PARC - Palo Alto
  Research Center)
• Robert Metcalfe considered the inventor of
  Ethernet
• Eventually became an IEEE standard (IEEE 802.3)
• Known as 10Base-T
• Bus Topology
• CSMA/CD access method
• Medium - coaxial cable, twisted pair, fiber, or
  wireless
• Originally used coax, now primarily over UTP
• Speeds range from 10 Mbps to Gigabit speeds

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Types of Ethernet LANs

• 10Base-T (First digit is speed in Mbps T means
  UTP)
• Operates at 10 Mbps
• IEEE 802.3
• Fast Ethernet
• Operates at 100 Mbps
• Referred to as 100 Base-T
• Same IEEE 802.3 frame format, size, and
  error-detection mechanism
• Gigabit Ethernet
• 1 Gbps
• Uses multimode fiber
• Wireless (11-54 Mbps speed range)
• IEEE 802.11
• 2.4 GHz frequency range
• Also uses CSMA/CD access method

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Wireless LANs

• Whats a WLAN?
• WLAN Wireless Local Area Network
• Generic term for a LAN that uses radio frequency
  communication rather than copper cables or fiber
  optic cables.

2.4 GHz
What are the pros and cons of WLANs? Whats
Wi-FI??
22
Ethernet Frame Structure
Ethernet is based on the datagram or frame
electronic telegram
6 bytes Source Address
46 to 1500 bytes Data
8 bytes Preamble
4 bytes Frame Check Sequence
6 bytes Destination Address
2 bytes Type Field
Note the 26 bytes of overhead
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Ethernet Frame Structure

• Preamble Repeating Flag that IDs the sequence as
  an Ethernet frame (10101010 7 times followed by
  10101011)
• Destination Address 6-byte address uniquely
  identifies the recipient (NIC) of the datagram
• Source Address 6-byte address that uniquely
  identifies the sender (NIC)
• Type Field 2-byte identifier of what kind of
  datagram is being received (IP, UDP, etc) and
  length of data
• Data Actual transmitted information (46 to 1500
  bytes)
• Frame Check Sequence 4-byte field used for error
  detection

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Ethernet Concepts Padding Overhead

• If a message has less than 46 bytes of data,
  padding is added
• If only 42 bytes of data require transmission, 4
  bytes of padding are added.
• Bytes extraneous to the data we are interested in
  sending are called overhead
• Ethernet has 26 bytes of overhead in each
  datagram (frame)
• If you had 100 bytes of data to send, youd have
  to send 126 bytes of data
• How much overhead is transmitted within the 126
  bytes of data?
• 26/126 21

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Ethernet NIC

• 48 - bit unique address
• Permanently attached to NIC
• IEEE assigns addresses
• Organizationally Unique Identifiers (OUIs)
• Also known as MAC (Medium Access Control) Address
• Represented in Hexadecimal e.g. 02608CBBDCA7

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Token Ring IEEE 802.5

• Developed by IBM in 1970s
• Token passing network
• Logical topology - ring
• Medium - UTP, STP
• Deterministic possible to calculate the maximum
  time that will pass before any end station will
  be able to transmit
• Usually 16 Mbps

A
B
C
D
E
T
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Token Ring IEEE 802.5

• Data limited by ring token holding time
• Frame Check Sequence (FCS)
• End Delimiter End of Frame
• Frame Status 1 byte terminating a command/data
  frame

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Fiber Distributed Data Interface FDDI

• Consists of a dual ring
• Uses the token passing access method
• Operates on Multimode fiber optic cable
• Achieves speeds of 100 Mbps
• Supports up to 1000 nodes
• Up to 200 km

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FDDI Frame

• Preamble 8 byte frame synchronization pattern
• Start Delimiter
• Frame Control
• Source and Destination Address
• Data
• Frame Check Sequence
• End Delimiter
• Frame Status

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Campus Networks

• Sometimes a LAN that operates over a campus or
  multi-building environment is referred to as a
  campus network.
• Interconnects multiple LANs
• Is still privately operated and used by a single
  organization
• High speed networks like gigabit Ethernet are
  used in campus networks.

A business housed in a multi-building, proximate
setting would install a campus network.
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LAN Interconnection Devices

• Repeater
• Bridge
• Router
• Gateway

Its helpful to discuss these relative to the
OSI Model Whats the OSI Model?
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The OSI Model

• The Open Systems Interconnection (OSI) model is a
  theoretical framework for understanding and
  explaining networking protocols.
• Originally an effort by the ISO (International
  Standards Organization) to standardize network
  protocols.
• TCP/IP became the dominant set of standards but
  the OSI model is widely used to help understand
  protocols.
• The OSI model defines 7 layers of functional
  communications protocols.

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The OSI Model
7. Application Layer
Provides a network interface for applications
6. Presentation Layer
Translates data to standard format
5. Session Layer
Establishes sessions between computers
4. Transport Layer
Provides error control and flow control
3. Network Layer
Supports logical addressing and routing
2. Data Link Layer
Interfaces with network adapter
Converts information into transmitted pulses
1. Physical Layer
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Repeater

• Regenerates and propagates all electrical
  transmissions between 2 or more LAN segments
• Layer 1 of the OSI model

Workstation A
Workstation B
Repeater
Physical
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Bridge

• Connects 2 or more LAN segments and uses data
  link layer addresses to make data forwarding
  decisions
• Layer 2

Workstation A
Workstation B
Bridge
Data Link 23-01-88-A8-77-45
Data Link 53-F1-A4-AB-67-4F
Data Link
Data Link


Physical 1
Physical 2
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Router

• Connects 2 or more networks and uses network
  layer addresses (like IP address) to make data
  forwarding decisions

Workstation A
Workstation B
Higher Layers
Higher Layers
Router
Network 145.65.23.102
Network 137.22.144.6


Network
Network
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical 1
Physical 2
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Gateway

• Connects 2 or more networks and provides protocol
  conversion so that end devices with dissimilar
  protocol architectures can interoperate

137.22.144.6
Netware
Gateway
TCP/IP
145.65.23.102
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WANs

• A network spanning a large geographical area
• Connects internal company LANs may connect
  business to other businesses, suppliers, and
  customers
• Also called Enterprise Networks if they support
  communications for a large organization
• Provided by a common carrier (ATT, Sprint, MCI,
  etc) or several carriers.
• Even large companies cant afford to install high
  capacity circuits everywhere in the world
• Companies lease service
• Fiber Optic, Satellite, cable, microwave carries
  the service
• At capacities needed T1, T2 (6.176 Mbps), T3
  (44.736 Mbps), OC1 (51.84 Mbps), OC192 (9,953.28
  Mbps)
• From and to locations desired to implement the
  WAN
• WAN services include Internet, frame relay, ATM
  (Asynchronous Transfer Mode)

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Example
Sprint Network
LA Runs a 100 Mbps LAN
Sprint provisions a T-1 connection (1.544 Mbps)
into their network
DC Runs a 1Gbps LAN
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Whats a T1? The T-Carrier System

• Format is called DS1, carrier is called T1
• 24 channels (DS0 64kbps) multiplexed together
• Each channel 8 bits
• 7 bit for data
• 1 bit for control
• DS1 Frame is 193 bits every 125µsec 1.544Mbps
• 24 x 8 bits 192 bits
• 1 bit for frame synchronization pattern
  010101010101.
• DS1 used for data
• Only 23 channels - data
• 24th channel synchronization pattern

Channel 1
Channel 2
Channel 2
Channel 24
Channel 23
Channel 22
Bit 1 is a framing code
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DS3 - Whats a T3?

• 44.736 Mbps
• 7 x DS2
• 672 DS0
• T3

DS1
. .
DS2
. . .
DS3
DS1
DS2