Ubiquitous Computing Requirement

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OPERATIONS VIEW (OV)
Ubiquitous Computing Requirement
IPV 6 Provisions real number space for every
communicating entity or object
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Network Symbols
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Equipment
FIRE WALLs
Cluster
Networks

Blade Server
WAN LINKS
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BRIDGE i.e. WAP
HUB
ROUTER
SWITCH
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- Carrier Class Core Router - ATM Switch - Frame
Relay Switch
SWITCH Image
SWITCH
HUB
Switch
Switch ATM, ?
BRIDGE i.e. WAP
Unknown
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Some Networking Symbols

• Serial Link
• Ethernet
• Network Cloud (X.25, Frame Relay, ATM)
• Token Ring Network

• Host
• Internetworking Device
• Router
• Switch
• Bridge

Internetworking Device
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ROUTER
Router
Router Image
Communication Server
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WANs and WAN Devices
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End User Devices
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LANs technologies
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Network Devices
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Pipe Analogy for Bandwidth
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Network Definitions
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End user devices

• End-user devices that provide users with a
  connection to the network are also referred to as
  hosts
• These devices allow users to share, create, and
  obtain information.
• Host devices are physically connected to the
  network media using a network interface card
  (NIC)

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Network devices

• Network devices provide transport for the data
  that needs to be transferred between end-user
  devices. Network devices provide extension of
  cable connections, concentration of connections,
  conversion of data formats, and management of
  data transfers.

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Networking Devices

• Equipment that connects directly to a network
  segment is referred to as a device. These devices
  are broken up into two classifications.
• Network Devices
• end-user devices

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Local-area networks (LANs)

• LANs consist of the
• following components
• Computers
• Network interface cards
• Peripheral devices
• Networking media
• Network devices

• Some common LAN
• technologies are
• Ethernet
• Token Ring
• FDDI

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Local Area Networks (LAN)

• Businesses needed a
• solution that would
• successfully address the
• following three problems
• How to avoid duplication of equipment and
  resources
• How to communicate efficiently
• How to set up and manage a network

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Wide-area networks (WANs)

• WANs interconnect LANs, which then provide access
  to computers or file servers in other locations.

• Some common WAN
• technologies are
• Modems
• Integrated Services Digital Network (ISDN)
• Digital Subscriber Line (DSL)
• Frame Relay
• US (T) and Europe (E) Carrier Series T1, E1,
  T3, E3
• Synchronous Optical Network (SONET)

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Wide-area networks (WANs)

• A way for information to move efficiently and
  quickly
• WANs could connect user networks over large
  geographic areas

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Metropolitan-area networks (MANs)

• A MAN is a network that spans a metropolitan area
  such as a city or suburban area. A MAN usually
  consists of two or more LANs in a common
  geographic area .

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Storage-area networks (SANs)

• A SAN is a dedicated, high-performance network
  used to
• move data between servers and storage resources
• SANs offer the following features
• Performance SANs enable concurrent access of
  disk or tape arrays by two or more servers at
  high speeds.
• Availability SANs have disaster tolerance built
  in, because data can be mirrored using a SAN up
  to 10 kilometers (km) or 6.2 miles away.
• Scalability Like a LAN/WAN, it can use a
  variety of technologies. This allows easy
  relocation of backup data, operations, file
  migration, and data replication between systems.

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Storage-area networks (SANs)
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An understanding of the following key points
should have been achieved

• A SAN provides enhanced system performance, is
  scalable, and has disaster tolerance built in
• A VPN is a private network that is constructed
  within a public network infrastructure
• Three main types of VPNs are access, Intranet,
  and Extranet VPNs
• Intranets are designed to be available to users
  who have access privileges to the internal
  network of an organization
• Extranets are designed to deliver applications
  and services that are Intranet based, using
  extended, secured access to external users or
  enterprises

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Repeater

• A repeater is a network device used to regenerate
  a signal. Repeaters regenerate analog or digital
  signals distorted by transmission loss due to
  attenuation.

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Repeaters - Hubs
Repeaters

• A repeater regenerates and retimes network
  signals at the bit level to allow them to travel
  a longer distance
• 5-4-3 Rule for 10-Mbps Ethernet should be used to
  limit latency
• Too much latency on the LAN increases the number
  of latecollisions and makes the LAN less efficient

Hubs

• Hubs are actually multiport repeaters
• Change the network topology from a linear bus to
  a star
• Three basic types
• Passive no boost, no clean and no power
• Active - needs power to amplify the incoming
  signal
• Intelligent - microprocessor chip and diagnostic
  capabilities

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Routers

• Routers have all the capabilities listed above.
• regenerate signals
• concentrate multiple connections
• convert data transmission formats, and manage
  data transfers
• They can also connect to a WAN, which allows them
  to connect LANs that are separated by great
  distances

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Switches

• Workgroup switches add more intelligence to data
  transfer management.
• They can determine whether data should remain on
  a LAN or not
• They can transfer the data only to the connection
  that needs that data.

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Switches
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Bridges Switches
Bridges

• Bridges and switches operate at the Data Link
  layer.
• Destination MAC address is looked up in the
  bridge table to determine whether to filter,
  flood, or copy the frame onto another segment.

• Switches

• A switch has many ports with many network
  segments connected to them.
• A switch chooses the port to which the
  destination device is connected.
• Alleviates congestion in LANs by reducing the
  size of collision domains, reducing traffic and
  increasing bandwidth.
• Two basic operations
• Switching data frames.
• Build and maintain switching tables and search
  for loops.
• Switches operate at much higher speeds than
  bridges and can support new functionality, such
  as virtual LANs.

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Bridges

• convert network transmission data formats as well
  as perform basic data transmission management.
• provide connections between LANs.
• perform a check on the data to determine whether
  it should cross the bridge or not. This makes
  each part of the network more efficient

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Bridges
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Network topology

• Network topology defines the structure of the
  network. One part of the topology definition is
  the physical topology, which is the actual layout
  of the wire or media. The other part is the
  logical topology, which defines how the media is
  accessed by the hosts for sending data

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Different topologies
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Logical topology

• The logical topology of a network is how the
  hosts communicate across the medium
• - broadcast
• Ethernet
• - token passing
• Token Ring
• Fiber Distributed Data
  Interface (FDDI)

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A protocol

• A protocol is a formal description of a set of
  rules and conventions that govern a particular
  aspect of how devices on a network communicate.
• Protocols determine the format, timing,
  sequencing, and error control in data
  communication

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Protocols

• Protocols control all aspects of data
• communication, which include the following
• (IEEE, ANSI , TIA , EIA , ITU )
• How the physical network is built
• How computers connect to the network
• How the data is formatted for transmission
• How that data is sent
• How to deal with errors

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Virtual private network (VPN)

• A VPN is a private network that is constructed
  within a public network infrastructure such as
  the global Internet. Using VPN, a telecommuter
  can access the network of the company
  headquarters through the Internet by building a
  secure tunnel between the telecommuters PC and a
  VPN router in the headquarters

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Types of VPNs

• Access VPNs Access VPNs provide remote access
  to a mobile worker and small office/home office
  (SOHO) to the headquarters of the Intranet or
  Extranet over a shared infrastructure.
• Intranet VPNs Intranet VPNs link regional and
  remote offices to the headquarters of the
  internal network over a shared infrastructure
  using dedicated connections
• Extranet VPNs Extranet VPNs link business
  partners to the headquarters of the network over
  a shared infrastructure using dedicated
  connections

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Benefits of VPNs

• A VPN is a service that offers secure, reliable
  connectivity over a shared public network
  infrastructure such as the Internet.
• They are the most cost-effective method of
  establishing a point-to-point connection between
  remote users and an enterprise customer's network

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Intranets and extranets

• Intranets are designed to permit access by users
  who have access privileges to the internal LAN of
  the organization.
• Extranets refer to applications and services that
  are Intranet based, and use extended, secure
  access to external users or enterprises.

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Network Comparisons
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Throughput

• Throughput refers to actual measured bandwidth,
  at a specific time of day, using specific
  Internet routes, and while a specific set of data
  is transmitted on the network

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Limitations

• Bandwidth varies depending upon the type of media
  as well as the LAN and WAN technologies used. The
  physics of the media account for some of the
  difference.
• The actual bandwidth of a network is determined
  by a combination of the physical media and the
  technologies chosen for signaling and detecting
  network signals.

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Measurement

• In digital systems, the basic unit of bandwidth
  is bits per second (bps). Bandwidth is the
  measure of how much information, or bits, can
  flow from one place to another in a given amount
  of time, or seconds.

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WAVEFORM ANALYSIS
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Digital versus analog

• Electromagnetic waves are called analog because
  they have the same shapes as the light and sound
  waves produced by the transmitters
• Analog bandwidth is measured by how much of the
  electromagnetic spectrum is occupied by each
  signal. The basic unit of analog bandwidth is
  hertz (Hz), or cycles per second.
• In digital signaling all information is sent as
  bits, regardless of the kind of information it
  is. Voice, video, and data all become streams of
  bits when they are prepared for transmission over
  digital media.
• Unlimited amounts of information can be sent over
  the smallest or lowest bandwidth digital channel.

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Bandwidth

• Analog bandwidth
• Refers to the frequency range of an analog
  electronic system.
• The units of measurement is Hertz
• Digital bandwidth
• Digital bandwidth measures how much information
  can flow from one place to another in a given
  amount of time.
• The unit of measurement is bits per second
  (bps).

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Distance and bandwidth
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Importance of bandwidth

• Bandwidth is defined as the amount of information
  that can flow through a network connection in a
  given period of time.

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Highway Analogy for Bandwidth
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File Transfer Time Calculations
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An understanding of the following key points
should have been achieved

• Understanding bandwidth is essential when
  studying networking
• Bandwidth is finite, costs money, and the demand
  for it increases daily
• Bandwidth is measured in bits per second, bps,
  kpbs, Mbps, or Gbps
• Limitations on bandwidth include type of media
  used, LAN and WAN technologies, and network
  equipment
• Throughput refers to actual measured bandwidth,
  which is affected by factors that include number
  of users on network, networking devices, type of
  data, users computer and the server
• The formula TS/BW (transfer time size of file
  / bandwidth) can be used to calculate data
  transfer time
• Comparison of analog and digital bandwidth

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Networking Models

• The concept of layers is used to describe
  communication from one computer to another
• As the data passes between layers, each layer
  adds additional information that enables
  effective communication with the corresponding
  layer on the other computer
• The OSI and TCP/IP models have layers that
  explain how data is communicated from one
  computer to another.

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OSI layers

• The OSI reference model explains how packets
  travel through the
• various layers to another device on a network
• Dividing the network into seven layers provides
  the following
• advantages
• It breaks network communication into smaller,
  more manageable parts.
• It standardizes network components to allow
  multiple vendor development and support.
• It allows different types of network hardware and
  software to communicate with each other.
• It prevents changes in one layer from affecting
  other layers.
• It divides network communication into smaller
  parts to make learning it easier to understand.

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OSI model
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The OSI Model - Layer 1
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The OSI Model - Layer 2
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The OSI Model - Layer 3
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The OSI Model - Layer 4
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Layer Communication

• Layer 4 on the source computer communicates with
  Layer 4 on the destination computer. The rules
  and conventions used for this layer are known as
  Layer 4 protocols.

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The OSI Model - Layer 5
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The OSI Model - Layer 6
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The OSI Model - Layer 7
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TCP/IP model

• Application layer handles issues of
  representation, encoding, and dialog control.
• The transport layer deals with the quality of
  service issues of reliability, flow control, and
  error correction
• Internet layer divides TCP segments into packets
  and send them from any network
• Network layer is concerned with all of the
  components, both physical and logical, that are
  required to make a physical link

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Common TCP/IP Protocols
The relationship between IP and TCP is an
important one. IP can be thought to point the way
for the packets, while TCP provides a reliable
transport
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Comparing TCP/IP with OSI
TCP
IP
Ethernet
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Peer-to-peer communications
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An understanding of the following key points
should have been achieved

• Network communication is described by layered
  models
• The OSI and TCP/IP are the two most important
  models of network communication
• The International Organization for
  Standardization developed the OSI model to
  address the problems of network incompatibility
• The seven layers of the OSI are application,
  presentation, session, transport, network, data
  link, and physical
• The four layers of the TCP/IP are application,
  transport, internet, and network access
• The TCP/IP application layer is equivalent to the
  OSI application, presentation, and session layers
  
• Fundamental networking devices are hubs, bridges,
  switches, and routers
• The physical topology layouts include the bus,
  ring, star, extended star, hierarchical, and mesh
  
• A WAN consists of two or more LANs spanning a
  common geographic area

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Detailed encapsulation process

• If one computer (host A) wants to send data to
  another computer (host B), the data must first be
  packaged through a process called encapsulation.
• Encapsulation wraps data with the necessary
  protocol information before network transit.

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Data Encapsulation
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Data Encapsulation example
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Physical Network
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6.2.1 Media Access Control (MAC)

• The specific technologies for each are as
  follows
• Ethernet logical bus topology (information flow
  is on a linear bus) and physical star or extended
  star (wired as a star)
• Token Ring logical ring topology (in other
  words, information flow is controlled in a ring)
  and a physical star topology (in other words, it
  is wired as a star)
• FDDI logical ring topology (information flow is
  controlled in a ring) and physical dual-ring
  topology (wired as a dual-ring)

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6.2.1 Media Access Control (MAC)

• There are two broad categories of Media Access
  Control, deterministic (taking turns) and
  non-deterministic (first come, first served).
• deterministic protocols include Token Ring and
  FDDI.
• Non-deterministic MAC protocols use a first-come,
  first-served approach. CSMA/CD is a simple
  system. The NIC listens for an absence of a
  signal on the media and starts transmitting.

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6.2.2 MAC rules and collision detection/backoff

• The access method CSMA/CD used in Ethernet
  performs three functions
• Transmitting and receiving data packets
• Decoding data packets and checking them for valid
  addresses before passing them to the upper layers
  of the OSI model
• Detecting errors within data packets or on the
  network

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IP defines private intranet address
ranges 10.0.0.0 - 10.255.255.255 (Class
A) 172.16.0.0 - 172.31.255.255 (Class
B) 192.168.0.0 - 192.168.255.255 (Class
C) Addresses reused by many organizations Addresse
s cannot be used for communication on Internet
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6.1.4 Naming

• Ethernet uses MAC addresses that are 48 bits in
  length and expressed as twelve hexadecimal
  digits. The first six hexadecimal digits, which
  are administered by the IEEE, identify the
  manufacturer or vendor. This portion of the MAC
  address is known as the Organizational Unique
  Identifier (OUI). The remaining six hexadecimal
  digits represent the interface serial number, or
  another value administered by the specific
  equipment manufacturer. MAC addresses are
  sometimes referred to as burned-in addresses
  (BIA) because they are burned into read-only
  memory (ROM) and are copied into random-access
  memory (RAM) when the NIC initializes.

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Various types of cabling

• Shielded twisted-pair (STP)
• STP cable is more expensive, more difficult to
  install, and less frequently used than UTP.
• Unshielded twisted pair (UTP)
• UTP contains no shielding and is more
  susceptible to external noise but is the most
  frequently used because it is inexpensive and
  easier to install.

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Ethernet Media

• Ethernet technologies can be used in a campus
  network in several different ways
• 10 Mbps at the user level to provide good
  performance.
• 100 Mbps for clients or servers that require
  more bandwidth.
• Fast or Gigabit Ethernet between backbone
  devices.

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Network Connections
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DTE/DCE

• When the connection is made directly to a service
  provider, or a device that provides signal
  clocking such as a channel/data service unit
  (CSU/DSU), the router will be a data terminal
  equipment (DTE) and use a DTE serial cable.
• When the local router is required to provide the
  clocking rate it will use a data communications
  equipment (DCE) cable

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Peer-to-peer communications

• Each layer of the OSI model at the source
  communicate with its peer layer at the
  destination
• The protocols of each layer exchange information,
  called protocol data units (PDUs).
• Each layer depends on the service function of the
  OSI layer below it.
• The lower layer uses encapsulation to put the PDU
  from the upper layer into its data field then it
  adds whatever headers and trailers the layer
  needs to perform its function.

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RJ-45 PINOUTS
The Ethernet standard specifies that each of the
pins on an RJ-45 connector have a particular
purpose. A NIC transmits signals on pins 1 and
2, and it receives signals on pins 3 and 6. The
wires in UTP cable must be connected to the
proper pins at each end of a cable.
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Interconnecting Devices

• Use straight-through cables for
• Switch to router
• Switch to PC or server
• Hub to PC or server

• Use roll-over cables to
• Connect a terminal to a console port

• Use crossover cables for
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• PC to PC
• Router to PC

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Router Connections

• Cisco router physical connectivity is provided by
  serial connections
• The first type of serial connections is a 60-pin
  connector
• The second is a more compact smart serial
  connector
• The provider connector will vary depending on the
  type of service equipment

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Router Connection Points
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Ethernet
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6.1.1 Introduction to Ethernet

• From its beginning in the 1970s, Ethernet has
  evolved to meet the increasing demand for high
  speed LANs. the same protocol that transported
  data at 3 Mbps in 1973 is carrying data at 10
  Gbps.
• The success of Ethernet is due to the following
  factors
• Simplicity and ease of maintenance
• Ability to incorporate new technologies
• Reliability
• Low cost of installation and upgrade

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6.1.1 Introduction to Ethernet

• This work later formed the basis for the Ethernet
  access method known as CSMA/CD.

• The original idea for Ethernet grew out of the
  problem of allowing two or more hosts to use the
  same medium and prevent the signals from
  interfering with each other. This problem of
  multiple user access to a shared medium was
  studied in the early 1970s at the University of
  Hawaii. A system called Alohanet was developed to
  allow various stations on the Hawaiian Islands
  structured access to the shared radio frequency
  band in the atmosphere.

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6.1.1 Introduction to Ethernet

• The differences between the two standards were so
  minor that any Ethernet network interface card
  (NIC) can transmit and receive both Ethernet and
  802.3 frames. Essentially, Ethernet and IEEE
  802.3 are the same standards.

• In 1985, the Institute of Electrical and
  Electronics Engineers (IEEE) standards committee
  for LANs published standards. They started with
  the number 802. Called Ethernet 802.3. This had
  to be compatible with the ISO/OSI model. To do
  this, the IEEE 802.3 standard had to address the
  needs of Layer 1 and the lower portion of Layer 2
  of the OSI model. As a result, some small
  modifications to the original Ethernet standard
  were made in 802.3.

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6.1.3 Ethernet and the OSI model

• maps a variety of Ethernet technologies to the
  lower half of OSI Layer 2 and all of Layer 1.
  Ethernet at Layer 1 involves interfacing with
  media, signals, bit streams that travel on the
  media, components that put signals on media, and
  various topologies. Ethernet Layer 1 performs a
  key role in the communication that takes place
  between devices, but each of its functions has
  limitations. Layer 2 addresses these limitations.

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6.1.3 Ethernet and the OSI model

• Data link sublayers contribute significantly to
  technology compatibility and computer
  communication. The MAC sublayer is concerned with
  the physical components that will be used to
  communicate the information. The Logical Link
  Control (LLC) sublayer remains relatively
  independent of the physical equipment that will
  be used for the communication process.

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6.1.3 Ethernet and the OSI model

• Ethernet operates in two areas of the OSI model,
  the lower half of the data link layer, known as
  the MAC sublayer and the physical layer.

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6.1.3 Ethernet and the OSI model

• A collision domain is then a shared resource.
  Problems originating in one part of the collision
  domain will usually impact the entire collision
  domain.

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6.1.3 Ethernet and the OSI model

• Layer 1 involves media, signals, bit streams that
  travel on media, components that put signals on
  media, and various topologies. Each of its
  functions has its limitations. Layer 2 addresses
  these limitations.
• For each limitation in Layer 1, Layer 2 has a
  solution.
• Layer 1 cannot communicate with the upper-level
  layers Layer 2 does that with logical link
  control (LLC).
• Layer 1 cannot name or identify computers Layer
  2 uses an addressing (or naming) process.
• Layer 1 can only describe streams of bits Layer
  2 uses framing to organize or group the bits.
• Layer 1 cannot choose which computer will
  transmit binary data, from a group in which all
  computers are trying to transmit at the same
  time Layer 2 accomplishes this by using a system
  called Media Access Control (MAC).

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6.1.2 IEEE Ethernet naming rules

• Ethernet is not one networking technology, but a
  family of networking technologies that includes
  Legacy, Fast Ethernet, and Gigabit Ethernet.
  Ethernet speeds can be 10, 100, 1000, or 10,000
  Mbps. The basic frame format and the IEEE
  sublayers of OSI Layers 1 and 2 remain consistent
  across all forms of Ethernet.

• The abbreviated description consists of
• A number indicating the number of Mbps
  transmitted.
• The word base, indicating that baseband signaling
  is used.
• One or more letters of the alphabet indicating
  the type of medium used (F fiber optical cable,
  T copper unshielded twisted pair).

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6.1.6 Ethernet frame structure
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6.1.7 Ethernet frame fields

• fields permitted or required in an 802.3 Ethernet
  Frame are
• Preamble - is an alternating pattern of ones and
  zeroes used for timing synchronization in the
  asynchronous 10 Mbps and slower implementations
  of Ethernet.
• Start Frame Delimiter - one-octet field that
  marks the end of the timing information, and
  contains the bit sequence 10101011.
• Destination Address
• Source Address
• Length/Type
• Data and Pad
• FCS - contains a four byte CRC value that is
  created by the sending device and is recalculated
  by the receiving device to check for damaged
  frames.
• Extension

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6.2.3 Ethernet timing

• The electrical signal takes time to travel down
  the cable (delay), and each subsequent repeater
  introduces a small amount of latency in
  forwarding the frame from one port to the next.
  Because of the delay and latency, it is possible
  for more than one station to begin transmitting
  at or near the same time. This results in a
  collision.
• Full-duplex operation also changes the timing
  considerations and eliminates the concept of slot
  time. Full-duplex operation allows for larger
  network architecture designs since the timing
  restriction for collision detection is removed.
• In half duplex, assuming that a collision does
  not occur, the sending station will transmit 64
  bits of timing synchronization information that
  is known as the preamble. The sending station
  will then transmit the following information
• Destination and source MAC addressing information
  
• Certain other header information
• The actual data payload
• Checksum (FCS) used to ensure that the message
  was not corrupted along the way
• Stations receiving the frame recalculate the FCS
  to determine if the incoming message is valid and
  then pass valid messages to the next higher layer
  in the protocol stack.

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6.2.7 Ethernet errors

• The following are the sources of Ethernet error
• Collision or runt Simultaneous transmission
  occurring before slot time has elapsed
• Late collision Simultaneous transmission
  occurring after slot time has elapsed
• Jabber, long frame and range errors Excessively
  or illegally long transmission 
• Short frame, collision fragment or runt
  Illegally short transmission
• FCS error Corrupted transmission
• Alignment error Insufficient or excessive
  number of bits transmitted
• Range error Actual and reported number of
  octets in frame do not match
• Ghost or jabber Unusually long Preamble or Jam
  event

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6.2.9 Ethernet auto-negotiation

• 10BASE-T required each station to transmit a link
  pulse about every 16 milliseconds, whenever the
  station was not engaged in transmitting a
  message. Auto-Negotiation adopted this signal and
  renamed it a Normal Link Pulse (NLP). When a
  series of NLPs are sent in a group for the
  purpose of Auto-Negotiation, the group is called
  a Fast Link Pulse (FLP) burst. Each FLP burst is
  sent at the same timing interval as an NLP, and
  is intended to allow older 10BASE-T devices to
  operate normally in the event they should receive
  an FLP burst.
• Auto-Negotiation is accomplished by transmitting
  a burst of 10BASE-T Link Pulses from each of the
  two link partners. The burst communicates the
  capabilities of the transmitting station to its
  link partner.

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Other
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6.1.5 Layer 2 framing

• Framing helps obtain essential information that
  could not, otherwise, be obtained with coded bit
  streams alone.
• Which computers are communicating with one
  another
• When communication between individual computers
  begins and when it terminates
• Provides a method for detection of errors that
  occurred during the communication
• Whose turn it is to "talk" in a computer
  "conversation"

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6.1.5 Layer 2 framing

• The frame format diagram shows different
  groupings of bits (fields) that perform other
  functions.
• The names of the fields are as follows
• Start frame field
• Address field
• Length / type field
• Data field
• Frame check sequence field 

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6.1.5 Layer 2 framing

• All frames contain naming information, such as
  the name of the source node (MAC address) and the
  name of the destination node (MAC address).
• In some technologies, a length field specifies
  the exact length of a frame in bytes. Some frames
  have a type field, which specifies the Layer 3
  protocol making the sending request.
• Data
• The Frame Check Sequence (FCS) field contains a
  number that is calculated by the source node
  based on the data in the frame. This FCS is then
  added to the end of the frame that is being sent.
  
• There are three primary ways to calculate the
  Frame Check Sequence number
• Cyclic Redundancy Check (CRC) performs
  calculations on the data.
• Two-dimensional parity adds an 8th bit that
  makes an 8 bit sequence have an odd or even
  number of binary 1s.
• Internet checksum adds the values of all of the
  data bits to arrive at a sum.

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6.2.4 Interframe spacing and backoff

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.

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6.2.5 Error handling
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6.2.6 Types of collisions

• Collisions typically take place when two or more
  Ethernet stations transmit simultaneously within
  a collision domain. A single collision is a
  collision that was detected while trying to
  transmit a frame, but on the next attempt the
  frame was transmitted successfully.
• Three types of collisions are
• Local - collision on coax cable (10BASE2 and
  10BASE5), the signal travels down the cable until
  it encounters a signal from the other station.
• Remote - a frame that is less than the minimum
  length, has an invalid FCS checksum, but does not
  exhibit the local collision symptom of
  over-voltage or simultaneous RX/TX activity. This
  sort of collision usually results from collisions
  occurring on the far side of a repeated
  connection.
• Late - Collisions occurring after the first 64
  octets. The most significant difference between
  late collisions and collisions occurring before
  the first 64 octets is that the Ethernet NIC will
  retransmit a normally collided frame
  automatically, but will not automatically
  retransmit a frame that was collided late.

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6.2.8 FCS and beyond

• A received frame that has a bad Frame Check
  Sequence, also referred to as a checksum or CRC
  error, differs from the original transmission by
  at least one bit. In an FCS error frame the
  header information is probably correct, but the
  checksum calculated by the receiving station does
  not match the checksum appended to the end of the
  frame by the sending station. The frame is then
  discarded.

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6.2.10 Link establishment and full and half duplex

• Link partners are allowed to skip offering
  configurations of which they are capable. This
  allows the network administrator to force ports
  to a selected speed and duplex setting, without
  disabling Auto-Negotiation.