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Chapter 4 - Network Access

4.0 Network Access

4.0.1 Introduction >4.0.1.1 Introduction

Upon the completion of this chapter you will be able to:

  • Identify device connectivity options.
  • Describe the purpose and functions of the physical layer in the network.
  • Describe basic principles of the physical layer standards.
  • Identify the basic characteristics of copper cabling.
  • Build a UTP cable used in Ethernet networks.
  • Describe fiber-optic cabling and its main advantages over other media.
  • Describe wireless media.
  • Select the appropriate media for a given requirement and connect devices.
  • Describe the purpose and function of the data link layer in preparing communication for transmission on specific media.
  • Describe the Layer 2 frame structure and identify generic fields.
  • Identify several sources for the protocols and standards used by the data link layer.
  • Compare the functions of logical topologies and physical topologies.
  • Describe the basic characteristics of media access control methods on WAN topologies.
  • Describe the basic characteristics of media access control methods on LAN topologies.
  • Describe the characteristics and functions of the data link frame.

4.0.1 Introduction >4.0.1.2 Activity - Managing the Medium

The figure on this page shows two learners connected to the same media and there are 2 packets on the line at the same time. One packet is yellow and the other is red. This demonstrates how computers share the same media.

The description given for this figure is "Data link protocols govern how to format a frame for use with different media."

Objectives

Describe the purpose and function of the data link layer in preparing communication for transmission on specific media.
In this activity, you will discuss how communication with a single data-link layer domain can be performed immediately by addressing the intended node directly. You will also consider the increasing difficulty of communication if multiple nodes in a single domain need to communicate.

4.1. Physical Layer

4.1.1 Getting it Connected >4.1.1.1 Connecting to the network

Image 1 on this page shows the back of a home wireless router highlighting the four Ethernet ports (the lan ports), one Ethernet port (Internet connection) and the top of the router (the built in wireless antenna).

Image 2 on this page shows the wireless router and a laptop. The laptop is connected to one of the four lan ports.

4.1.1 Getting it Connected >4.1.1.2 Network Interface Cards

The figure on this page shows the interior of a house with several rooms on two floors. There are many wireless networking devices throughout the home. There is a range extender on the first floor that allows devices that are farther away from the wireless router to connect with a good signal.

4.1.2 Purpose of the physical Layer >4.1.2.1 The physical layer

The figure on this page shows two OSI models side by side. One for the source node, on the left and one for the destination node, on the right:

  • Between the upper three layers is a box with the words Application Data in it.
  • Between the transport layers are 3 boxes with the word Data in each one. This shows how the Application Data is segmented.
  • Between the network layers is the Data from the transport layer with a header attached to the front if it.
  • Between the data link layers is the Data and Header from the network layer with a frame header attached to the front and a frame trailer attached to the end.
  • Between the physical layers is a string of ones and zeros that are the bits that make up the frame.
  • Below the models is a digital signal representing how the bits are sent to the destination over the media.
  • Between each layer on the left, the source, there is an arrow pointing down to the next layer as the data flows down the OSI model.
  • Between each layer on the right, the destination, there is an arrow pointing up to the next layer as the data flows back up the OSI model.

The description given for this figure is "Encapsulation and De-encapsulation"

4.1.2 Purpose of the physical Layer >4.1.2.2 Physical layer media

The figure on this page shows examples of signals for different types of media: copper, fiber and wireless:

  • The copper media uses electrical signals.
  • The fiber media uses light pulses.
  • The wireless media uses microwave signals.

4.1.2 Purpose of the physical Layer >4.1.2.3 Physical Layer standards

Figure 1 on this page shows the OSI model:

  • Layer 7 to the upper sub-layer of layer 2 are implemented in software and governed by the IETF
  • The lower sub-layer of layer 2 and layer 1 are implemented in hardware and governed by several organizations, the ISO, EIA/TIA, ITU-T, ANSI (or ansi) and the IEEE pronounced I triple E.

Figure 2 shows a table that describes what each of the standards organizations oversees:

Standard OrganizationNetworking Standards
ISO* ISO 8877: Officially adopted RJ connectors(e.g. RJ-11 , RJ-45)
* ISO 11801: Network cabling standard similar to EIA/TIA 5 6 8
EIA/TIA* TIA-5 6 8-C: Telecommunications cabling standards used by nearly all voice, video and data networks.
* TIA-5 6 9-B: Commercial Building Standards for Telecommunications pathways and spaces.
* TIA-5 9 8-C:Fibre Optic colour coding
* TIA-9 4 2:Telecommunications Infrastructure Standard for data centers
ANSI5 6 8-C: RJ-45 pin outs. Co-developed with EIA/TIA
ITU-TG9 9 2:ADSL
IEEE*8 0 2.3:Ethernet
* 8 0 2.11 :Wireless LAN(W LAN) & Mesh (Wi-Fi certification)
* 8 0 2.15:Bluetooth

4.1.2 Purpose of the physical Layer >4.1.2.4 Lab - Identifying Network devices and Cabling

See Lab Descriptions

4.1.3 Fundamental Principles of Layer 1 >4.1.3.1 Physical Layer Fundamental Principles

Figure 1 on this page is a table with columns for media types, physical components, frame encoding techniques and signalling methods for copper cable:

MediaPhysical ComponentsFrame Encoding TechniqueSignalling Method
Copper cable* UTP
* Coaxial
* Connectors
* NIC's
* Ports
* Interface
* Manchester Encoding
* Non-Return to Zero (NRZ) techniques
* 4 B/5 B codes are used with Multi-Level transition Level 3(M T-3) signalling
* 8 B/10 B
* PAM5
* Changes in the electromagnetic field
* Intensity of the electromagnetic field
* Phase of the electromagnetic wave

Figure 2 on this page illustrates how AM and FM techniques are used to send a signal as described in the page notes.

4.1.3 Fundamental Principles of Layer 1 >4.1.3.2 Bandwidth

The table on this page displays units of bandwidth, their abbreviations and the equivalence in bits per second:

Units of bandwidthAbbreviationEquivalence
Bits per secondb p s1 b p s = fundamental unit of bandwidth
Kilobits per secondk b p s1 k b p s = 1,000 b p s = 10^3 b p s
Megabits per secondM b p s1 M b p s = 1,000,000 b p s = 10^6 b p s
Gigabits per secondG b p s1 G b p s = 1,000,000,000 b p s = 10^9 b p s
Terabits per secondT b p s1 T b p s = 1,000,000,000,000 b p s = 10^12 b p s

4.1.3 Fundamental Principles of Layer 1 >4.1.3.3 Throughput

The image on this page shows a speedometer that measures bandwidth from zero to one hundred megabits per second. There are two boxes, one for download speed which shows 80.07 megabits per second and one for upload which shows 8.78 megabits per second.

4.1.3 Fundamental Principles of Layer 1 >4.1.3.4 Types of Physical Media

The image on this page shows the back of a Cisco 1941 router with the various interfaces highlighted and labelled:

  • FastEthernet Switch Ports
  • SHDSL Interface
  • Management Ports
  • Gigabit Ethernet Interfaces
  • USB Type A Connectors

4.1.3 Fundamental Principles of Layer 1 >4.1.3.5 Activity - Physical Layer Terminology

Figure 1 on this page lists five Physical Layer terms:

  • Synchronous
  • Signalling method
  • Frame encoding
  • Asynchronous
  • Physical components

The figure also lists the five Physical Layer descriptions:

  • Hardware devices, media , and connectors which transmit and carry bit signals.
  • How 1s and 0s are represented on the media -varies , depending on encoding scheme.
  • Evenly spaced time duration for signals.
  • A method of converting stream of data bits into grouping of bits-predefined.
  • Arbitrarily spaced time duration for signals.

The learner is asked to match each of the Physical Layer terms with the corresponding Physical Layer description.

Figure 2 on this page lists another five Physical Layer terms:

  • Bandwidth
  • Pulse-coded modulation
  • Goodput
  • Throughput
  • Frequency modulation

The figure also lists another five Physical Layer descriptions:

  • How much usable data is transferred over a given amount of time.
  • Amount of data that is allowed by the medium to flow during a given set of time.
  • The actual measure of data bits over a given period of time.
  • A technique to convert voice analog to digital signals.
  • Transmission method where the carrier frequency varies according to the signals sent.

The learner is asked to match each of the Physical Layer terms with the corresponding Physical Layer description.

4.2 Network Media

4.2.1 Copper Cabling >4.2.1.1 Characteristics of Copper Media

The animation on this page shows several signal graphs. They include a pure digital signal, interference, the combination of the two and the signal that the computer will read. The computer reads a signal that is a combination of the original pure digital signal and the interference.

4.2.1 Copper Cabling >4.2.1.2 Copper Media

The images on this page show three different copper cables:

  • Unshielded Twisted-Pair (UTP)
    which has four pairs of twisted wires.
  • Shielded Twisted-Pair (STP)
    which has the same four pairs of twisted wires and a covering or shielding.
  • Coaxial Cable
    which is a single copper wire inside an insulating material surrounded by a shielding.

4.2.1 Copper Cabling >4.2.1.3 Unshielded Twisted-Pair Cable

The figure on this page shows an enlarged diagram of a UTP cable highlighting the different components:

  • Outer Jacket: Protects the copper wire from physical damage
  • Twisted-Pair: Protects the signal from interference
  • Colour-Coded Plastic Insulation: Electrically isolates wires from each other and identifies each pair

4.2.1 Copper Cabling >4.2.1.4 Shielded Twisted-Pair

The figure on this page shows an enlarged diagram of a STP cable highlighting the different components:

  • Jacket
  • Braided or Foil Shield (around all the twisted-pairs)
  • Foil Shields (around each individual set of twisted-pairs)
  • Twisted-Pairs

4.2.1 Copper Cabling >4.2.1.5 Coaxial Cable

The figure on this page shows an enlarged diagram of a Coaxial cable highlighting the different components:

  • Outer Jacket
  • Braided Copper Shielding
  • Plastic Insulation
  • Copper Conductor

The figure also displays images of Coaxial Connectors:

  • BNC
  • N-type
  • F-type

4.2.1 Copper Cabling >4.2.1.6 Copper media Safety

The images on this page displays proper cabling practices to avoid potential fire and electrical hazards:

  • Image 1 shows a worker in a closet with electrical panels and network panels.
    The description for this image is "The separation of data and electrical power cabling must comply with safety codes."
  • Image 2 shows the front of a switch with cables neatly attached.
    The description for this image is "Cables must be connected correctly
  • Image 3 shows two workers inspecting network equipment and cabling.
    The description for this image is "Installations must be inspected for damage."
  • Image 4 shows an electrical panel showing a breaker and indicator lights for grounded and protected.
    The description for this image is "Equipment must be grounded correctly."

4.2.1 Copper Cabling >4.2.1.7 Activity - Copper Media Characteristics

Column 1 in the table on this page lists the following UTP, STP and Coaxial copper media characteristics:

  1. The new ethernet 10GB standard uses this form of copper media
  2. Attaches antennas to wireless devices - can be bundled with fibre optic cabling for two-way data transmission
  3. Counters EMI and RFI by using shielding techniques and special connectors
  4. Most common network media
  5. Terminates with B C, N type and F type connectors

Columns 2-4 of the table have column headings of:

  • UTP
  • STP
  • Coaxial

The learner is asked to match the characteristic to the media type.

4.2.2 UTP Cabling >4.2.2.1 Properties of UTP cabling

The image on this page shows a UTP cable with some of the outer jacket removed, exposing the twisted-pairs.

4.2.2 U T P Cabling >4.2.2.2 UTP Cabling Standards

The figure on this page illustrates 3 categories of UTP cable:

  • Category 3 Cable (UTP):
    • Used for voice communication
    • Most often used for phone lines
  • Category 5 and 5e cable (UTP):
    • Used for data transmission
    • Cat5 supports 100 Mb/s and can support 1000 Mb/s ,but it is not recommended
    • Cat5e supports 1000Mb/s
  • Category 6 cable (UTP):
    • Used for data transmission
    • An added separator is between each pair of wires allowing it to function at higher speeds
    • Supports 1000 Mb/s - 10 Gb/s,though 10 Gb/s, though 10Gb/s is not recommended

4.2.2 U T P Cabling >4.2.2.3 UTP Connectors

The YouTube video in figure 1 on this page , titled " CCNA - Network Cable" displays a UTP cable terminated with an RJ 45 connector. The video has no audio.

Figure 2 on this page shows two pictures of RJ 45 plugs (male) and two pictures of RJ 45 sockets (female).

Figure 3 on this page shows two pictures of RJ 45 plugs:

  1. One where the outer jacket of the cable is not in the connector and the individual wires are sloppy.
    Wires are exposed, untwisted, and not entirely covered by the sheath.
  2. The other is neat and properly attached.
    Wires are untwisted to the extent necessary to attach the connector.

4.2.2 U T P Cabling >4.2.2.4 Types of UTP Cable

The figure on this page shows two RJ 45 wiring standards, the T5 6 8 A and the T5 6 8 B. Below is a table showing the cable types, their application and the standards:

Cable TypeStandardApplication
Ethernet Straight-throughBoth ends T5 6 8 A or both ends T5 6 8 BConnects a network host to a network device such as a switch or hub .
Ethernet CrossoverOne end T5 6 8 A,other end T5 6 8 B* Connects two network hosts
* Connects two network intermediary devices (switch to switch ,or router to router )
RolloverCisco proprietaryConnects a workstation serial port to a router console port , using an adapter

4.2.2 UTP Cabling >4.2.2.5 Testing UTP Cables

The image on this page shows a worker using a cable tester in a network wiring closet.

4.2.2 UTP Cabling >4.2.2.6 Activity- Cable Pinouts

The learner is asked to to correctly align the wire colours to build a UTP 5 6 8B, straight-through cable pinout.

4.2.2 UTP Cabling >4.2.2.7 Lab - Building an Ethernet Crossover Cable

See Lab Descriptions.

4.2.3 Fiber Optic Cabling >4.2.3.1 Properties of Fiber Optic Cabling

The image on this page shows a 10G BASE-LR fibre connection.

4.2.3 Fiber Optic Cabling >4.2.3.2 Fiber Media Cable Design

The image on this page shows a cross section of a fiber optic cable. The optic fiber is shown in 5 concentric circles representing:

  • The jacket on the outside:
    Added to protect the fibre against abrasion, solvents, and other contaminants. This outer jacket composition can vary depending on the cable usage.
  • Strengthening material:
    Surrounds the buffer, prevents the fibre cable from being stretched when it is being pulled. The material used is often the same material used to produce bullet proof vests.
  • Buffer:
    Used to help shield the core and cladding from damage
  • Cladding:
    Made from slightly different chemicals than those used to create the core. It tends to act like a mirror by reflecting light back into the core of the fiber. This keeps light in the core as it travels down the fiber.
  • Core in the centre:
    The core is actually the light transmission element at the centre of the optical fiber. This core is typically silica, or glass. Light pulses travel through the fiber core.

4.2.3 Fiber Optic Cabling >4.2.3.3 Types of Fiber Media

Figure 1 on this page shows a diagram of single mode fiber. The laser creates a straight beam of light:

  • Glass core = 9 microns
  • Glass cladding 125 microns diameter
  • Outside of cladding is polymeric coating

The characteristics of single mode fiber are:

  • Small core
  • Less dispersion
  • Suited for long distance applications
  • Uses lasers as the light source
  • Commonly used with campus backbones for distances of several thousand meters.

Figure 2 on this page shows a diagram of multi mode fiber. The LED's make beams of light that reflect off the inside of the core:

  • Glass core = 50/62.5 microns
  • Glass cladding 125 microns diameter
  • Outside of cladding is coating

The characteristics of multi mode fiber are:

  • Larger core than single mode cable.
  • Allows greater dispersion and therefore, loss of signal
  • Suited for long distance applications , but shorter than single mode
  • Used L E D's as the light source .
  • Commonly used with LANS or distances of a couple hundred meters within a campus network.

4.2.3 Fiber Optic Cabling >4.2.3.4 Network Fiber Connectors

Figure 1 on this page shows four pictures of fiber connectors:

  • S T Connectors(Straight-Tip Connectors)
  • S C Connectors(Subscriber Connector)
  • L C Connector(Lucent Connector)
  • Duplex Multimode L C Connectors

Figure 2 on this page shows four pictures of fiber patch chords for each of the four connectors:

  • S C-S C Multimode patch cord
  • L C-L C Single-mode Patch cord
  • S T-L C Multimode patch cord
  • S C-S T Single-mode Patch cord

4.2.3 Fiber Optic Cabling >4.2.3.5 Testing Fiber Cables

The image on this page shows a Fiber cable testing tool called an Optical Time Domain Reflectometer (OTDR).

4.2.3 Fiber Optic Cabling >4.2.3.6 Fiber versus Copper

The table on this page compares the characteristics of UTP cable versus Fiber Optic cable:

Implementation issuesUTP cablingFiber-optic Cabling
Bandwidth supported10 Mb/s - 10 Gb/s10 Mb/s - 100 Gb/s
DistanceRelatively short (1-100 meters)Relatively high (1 - 100,000 meters)
Immunity to EMI and RF IlowHigh (Completely immune)
Immunity to electrical hazardsLowHigh (Completely immune)
Media and connector hostsLowestHighest
Installation skills requiredLowestHighest
Safety precautionsLowestHighest

4.2.3 Fiber Optic Cabling >4.2.3.7 Activity - Fiber Optics Terminology

Column 1 in the table on this page lists the following descriptions of fiber-optic media:

  1. Can help data travel approximately 1.24 miles or 2 km/2000m
  2. Uses light emitting diodes (LED's) as a data light source transmitter
  3. Uses lasers in a single stream as a data light source transmitter
  4. Used to connect long-distance telephony and cable TV applications
  5. Can travel approximately 62.5 miles or 100 km/100000 m
  6. Used within a campus network

Columns 2 and 3 of the table have column headings of:

  • Multimode
  • Single-mode

The learner is asked to match the description to the fiber-optic cable type.

4.2.4 Wireless Media >4.2.4.1 Properties of Wireless Media

The figure on this page depicts many different wireless devices such as:

  • Mobile phone
  • Television
  • PC
  • Laptop
  • Mouse

4.2.4 Wireless Media >4.2.4.2 Types of Wireless Media

The figure on this page shows the standards for standard wireless:

  • Wi-Fi
    • IEEE 8 0 2.11 standards
    • Commonly referred to as Wi-Fi
    • Uses CSMA/CA
    • Variations include:
      • 8 0 2.11 a: 54 Mb/s, 5 GHz
      • 8 0 2.11 b: 11 Mb/s, 2.4 GHz
      • 8 0 2.11 g: 54 Mb/s, 2.4 GHz
      • 8 0 2.11 n: 600 Mb/s , 2.4 , and 5 GHz
      • 8 0 2.11 a.c: 1 Gb/s , 5 GHz
      • 8 0 2.11 a.d: 7 Gb/s , 2.4 GHz, 5 GHz and 60 GHz
  • Bluetooth
    • IEEE 8 0 2.15 standard
    • Supports speeds up to 3 Mb/s
    • Provide device pairing over distances from 1 to 100 meters
  • Wi Max
    • IEEE 8 0 2.16 standard
    • Provides speeds up to 1 Gb/s
    • Uses a point-to-multipoint topology to provide wireless broadband access

4.2.4 Wireless Media >4.2.4.3 Wireless LAN

The image on this page shows a Cisco 8 0 2.11 a c Wireless Router.

4.2.4 Wireless Media >4.2.4.4 8 0 2.11 Wi-Fi Standards

The table on this page lists the maximum speed, frequency and backwards compatibility for the different 8 0 2.11 standards:

StandardMaximum SpeedFrequencyBackward Compatible
8 0 2.11 a54 Mb/s5 GHzNo
8 0 2.11 b11 Mb/s2.4 GHzNo
8 0 2.11 g54 Mb/s2.4 GHz8 0 2.11 b
8 0 2.11 n600 Mb/s2.4 GHz and 5GHz8 0 2.11 a/b/g
8 0 2.11 a.c1.3Gb/s (1300 Mb/s)5 GHz8 0 2.11 a/n
8 0 2.11 a.d7 Gb/s (7000 Mb/s)2.4 GHz, 5 GHz and 60 GHz8 0 2.11 a/b/g/n/ac

4.2.4 Wireless Media >4.2.4.5 Packet Tracer – Connecting a Wired and Wireless LAN

Objectives:

Part 1: Connect to the Cloud
Part 2: Connect Router0
Part 3: Connect Remaining Devices
Part 4: Examine the Physical Topology

4.2.4 Wireless Media >4.2.4.6 Lab – Viewing Wired and Wireless NIC Information

See Lab Descriptions.

4.3 Data link layer Protocols

4.3.1 Purpose of the data Link Layer >4.3.1.1 The Data Link Layer

The figure on this page shows a learner at a computer. Below is the OSI model with layer two highlighted. There is a line, representing a connection, going from the learner to a router connected to the Internet.

The description given for this figure is "The data link layer prepares network data for the physical network."

4.3.1 Purpose of the data Link Layer >4.3.1.2 Data Link Sublayers

The figure on this page shows layer 1, the Physical layer, layer 2, the Data Link layer, and layer 3, the Network layer, of the OSI model. Layer 2 is divided into its two sub-layers, the upper sub-layer is LLC and the lower is MAC. The figure also shows that 8 0 2.3 Ethernet, 8 0 2.11 Wi-Fi and 8 0 2.15 Bluetooth, span layer 1 and the lower sub-layer (MAC) of layer 2.

4.3.1 Purpose of the data Link Layer >4.3.1.3 Media Access Control

The animation on this page illustrates how IP Packets travel on different media as they move from source to destination. The media shown includes copper wire, fiber, satellite wireless and home router wireless.

The packets from the PC in Paris travel to the router over copper wire. They then travel through optic fiber to the transmitting satellite dish. The packets are then sent as radio signals to the satellite and then to the receiving satellite dish. They then travel through optic fiber to the router in Japan and then over wireless to the laptop.

Data link layer protocols govern how to format a frame for use on different media. Different protocols may be in use for different media.

At each hop along the path, an intermediary device accepts the frame from one medium, de-encapsulates the frame and then forwards the packets in a new frame. The headers of each frame are formatted for the specific medium that it will cross.

4.3.1 Purpose of the data Link Layer >4.3.1.4 Providing Access to Media

The animation on this page illustrates how the frame header and trailer change depending on the media.

The packet travels through an Ethernet connection to the router encapsulated with LAN Header and LAN trailer. The packet is de-encapsulated and then re-encapsulated with WAN header and WAN Trailer to travels through a serial connection.

The data link layer is responsible for controlling the transfer of frames across the media.

4.3.2 Layer 2 Frame Structure >4.3.2.1 Formatting Data for Transmission

Figure shows the OSI model. Layers 5, 6 and 7 are shaded yellow. These are the Application layer, the Presentation layer, and the Session layer, of the TCP/IP model. Layer 4 is shaded green. This is the Transport layer of the TCP/IP model. Layer 3 is shaded purple. This is the Network layer of the TCP/IP model. Layers 1 and 2 are shaded pink. These are the Data Link layer and Physical layer of the TCP/IP model. There is a figure representing a frame next to the Data Link layer of the OSI model.

The components of a frame are:

Frame HeaderPacket / Segment / DataTrailer

4.3.2 Layer 2 Frame Structure >4.3.2.2 Creating a Frame

The figure on this page shows a frame with header, data and trailer:

HeaderPacket
(Data)
Trailer

The header is expanded to show the header fields:

Frame
Start
AddressingTypeControl

The trailer is expanded to show the trailer fields:

Error DetectionFrame Stop

4.3.2 Layer 2 Frame Structure >4.3.2.3 Activity Generic Frame Fields

[Figure 1 on this page shows graphical representation of the frame. It also lists the Layer 2 fields:

  • Data
  • Type
  • Header
  • Trailer
  • Control
  • Packet (Data)
  • Addressing
  • Frame Stop
  • Error Detection
  • Frame Start

The learner is asked to drag each of the fields to their relevant position in the frame.

Figure 2 on this page lists the five generic frame fields:

  • Data
  • Control
  • Addressing
  • Type
  • Frame stop indicator flag

The learner is asked to match each of the fields to their appropriate description as follows:

  • Contains the IP header, transport layer PDU, and data
  • Identifies the Layer 3 protocol used by the LLC
  • Marks the end of the frame
  • Identifies source and destination hosts by MAC address
  • Specifies special flow control services

4.3.3 Layer 2 Standards >4.3.3.1 Data Link Layer Standards

The table on this page highlights various standard organizations and some of their more important data link layer protocols.

Standard OrganizationNetworking Standards
IEEE* 8 0 2.2: Logical Link Control (L L C)
* 8 0 2.3: Ethernet
* 8 0 2.4: Token bus
* 8 0 2.5: Token ring
* 8 0 2.11: Wireless LAN (W LAN) & Mesh(Wi-Fi certification)
* 8 0 2.15: Bluetooth
* 8 0 2.16: WiMax
ITU-T* G.9 9 2: ADSL
* G.8 1 0 0 – G.8 1 9 9: MPLS over Transport aspects
* Q.9 2 1: ISDN
* Q.9 2 2: Frame Relay
ISO* HDLC (High Level Data Link Control)
* ISO.9 3 1 4:FDDI Media Access Control (MAC)
ANSIX3T 9.5 and X3T 12: Fiber Distributed Data Interface (FDDI)

4.3.3 Layer 2 Standards >4.3.3.2 Activity – Data Link Layer Standard Organizations

The table on this page has 4 columns. The column headings are the four Data-Link Layer Standards Organisations:

  • IEEE
  • ITU-T
  • ISO
  • ANSI

The learner is asked to match the following data-link protocols to their standards organisation:

  • HDLC
  • FDDI MAC
  • 8 0 2.3 Ethernet
  • ADSL
  • ISDN
  • 8 0 2.15 Bluetooth
  • 8 0 2.11 Wireless & Wi-Fi
  • FDDI

4.4 Media Access Control

4.4.1 Topologies >4.4.1.1 Controlling Access to the Media

The figure on this page shows three computers all connected to the same media. Each computer has a callout that says "We need rules for how to share the media".

4.4.1 Topologies >4.4.1.2 Physical and Logical Topologies

Figure 1 on this page shows a floor plan with the location of each network drop and where it is on the floor plan. This is called a Physical Topology.

This physical topology shows an Admin office and three Classrooms. The Admin office consists of two PC's, and a printer connected to an admin hub which is connected to a switch. The Admin office also has three servers, a Mail server, a Web server, and a File server. These servers are also connected to the switch which is connected to a router.

Each classroom consists of three PC's connected to a Classroom hub which are connected to an Ethernet switch. The Ethernet switch is connected to the same router as the Admin office.

Figure 2 on this page shows a router that has two interfaces each connected to a LAN. Each LAN consists of rooms with computers and IP addresses. This is called a Logical Topology.

The same scenario as in the physical topology is here represented in a logical topology. In logical representation an Ethernet is represented as a pipe.

The Admin office is represented by an Admin Group with IP addresses of 192.168.2.4, 192.168.2.5, and 192.168.2.6, and a Department Server with IP addresses of Mail server 192.168.2.1, Web server 192.168.2.2 , File server 192.168.2.3. The Admin Group and Department Server are both connected to the Ethernet pipe whose I P address is 192.168.2.0. The Ethernet is connected to the router.

The classrooms are represented as Classroom 1 with IP addresses 192.168.1.1, 192.168.1.2, and 192.168.1.3, Classroom 2 with IP addresses 192.168.1.4, 192.168.1.5, and 192.168.1.6, and Classroom 3 with IP addresses 192.168.1.7, 192.168.1.8, and 192.168.1.9. The three classrooms are connected to the Ethernet pipe whose IP address is 192.168.1.0. The Ethernet is connected to the router.

4.4.2 WAN Topologies >4.4.2.1 Common Physical WAN topologies.

The figures on this page represent different types of WAN topologies:

  • Point to point, which is a simple connection between two routers
  • Hub and spoke, which is a single router that has point to point connections to several other routers
  • Full mesh, which is several routers each connected with a point to point connection to several other routers.

4.4.2 WAN Topologies >4.4.2.2 Physical Point-to-Point Topology

The figure on this page shows two routers connected through the Internet with a point to point connection. The point to point connection is limited to two nodes.

4.4.2 WAN Topologies >4.4.2.3 Logical Point-to-Point Topology

Figure 1 on this page shows the logical point-to-point connection between two routers through a cloud which is empty.

Figure 2 represents the logical point-to-point topology of the same two routers showing the physical devices in between the two routers. Adding intermediate physical connections may not change the logical topology. The logical point-to-point connection is the same.

4.4.2 WAN Topologies >4.4.2.4 Half and Full Duplex

Figure 1 on this page shows a Point-to-point connection. Two computers are connected with an Ethernet cable. Both computers have callouts displaying "Only you and I communicate on this line. We can talk anytime."

Figure 2 on this page is an animation illustrating half-duplex communication. A server is connected to a switch. First the server waits to receive data from the switch and then it sends data to the switch.

Figure 3 on this page is an animation illustrating full-duplex communication. The server and switch can both send and receive data at the same time.

4.4.3 LAN Topologies >4.4.3.1 Physical LAN Topologies

The figures on this page represent four physical topologies:

  • Star topology :The computers are connected to a single centre point
  • Extended star topology :Two star topologies connected together
  • Bus topology : Several computers connected to the same media
  • Ring topology: Several computers connected to the same media that runs in a circle.

4.4.3 LAN Topologies >4.4.3.2 Logical Topology for Shared Media

Figure 1 on this page represents contention based access. It shows three computers connected to the same media. Each computer has a call out displaying "I try to send when I am ready".

Figure 2 on this page represents controlled access. It shows three computers connected to the same media. Each has a call out. One says "I have nothing to send". Another says "It is my turn to send. I will send now". The third says "I have a packet to send, but it is not my turn. I will wait".

4.4.3 LAN Topologies >4.4.3.3 Contention-Based Access

The figure on this page shows three computers connected to the same media. Each computer has a callout that says "I try to send when I am ready".

The following table lists the characteristics of contention based technologies:

CharacteristicsContention-Based Technologies
* Stations can transmit at anytime
* Collisions exist
* There are mechanisms to resolve contention for the media
* CSMA/CD for 8 0 2.3 Ethernet networks
* CSMA/CD 8 0 2.11 wireless networks

4.4.3 LAN Topologies >4.4.3.4 Logical Multi-Access Topology

The animation on this page demonstrates how computers send frames on a multi-access topology. There are five computers A, B, C, D, and E, connected to the media:

  1. Host A has three callouts:
    1. "I need to transmit to E."
    2. "I check for other transmissions."
    3. "No other transmissions are detected."
    4. "Transmitting..."

Host A sends the data, which is seen by all hosts, to Host E where it is processed. After Host A sends the data, Host B wants to send data to Host D:

  1. Host B has the following callouts:
    1. "I need to transmit to D."
    2. "I check for other transmissions."
    3. "Transmission detected. I'll wait..."
  2. After the Data from Host A is received by Host E, Host B has the following callouts:
    1. "No other transmissions are detected."
    2. "Transmitting..."

Host B sends the data, which is seen by all hosts, to Host D where it is processed.

4.4.3 LAN Topologies >4.4.3.5 Controlled Access

The figure on this page shows three computers connected to the same media. Each has a call out. One says "I have nothing to send", the second says "It is my turn to send. I will send now" and the third says "I have a packet to send, but it is not my turn. I'll wait". This figure demonstrates Controlled Access as opposed to contention based access to the line.

The following table lists the characteristics of controlled access technologies:

CharacteristicsControlled Access Technologies
* Only one station transmit at a time
* Devices wishing to transmit must wait their turn
* No collisions
* May use a token passing method
* Token Ring (IEEE 8 0 2.5)
* Fiber Distributed Data Interface (FDDI)

4.4.3 LAN Topologies >4.4.3.6 Ring Topology

The animation on this page illustrates how ring topologies work. When Host A wishes to send to Host D, the packet is placed on the line and travels around the ring. As it passes each host, the host checks to see if it is for that host. If not, it ignores it:

  1. Host A has the following callout:
    1. "I need to transmit to D."
  2. Hosts B and C have the following callouts:
    1. "Is this frame for me?"
    2. "No."
  3. Host D has the following callouts:
    1. "Is this frame for me?"
    2. "Yes."

4.4.3 LAN Topologies >4.4.3.7 Activity – Logical and Physical Topologies

The table on this page lists the following characteristics of data link layer media access control methods in column 1:

  1. CSMA/CD
  2. Star
  3. Contention-based access
  4. Bus
  5. CSMA/CA
  6. Controlled access
  7. Point-to-Point
  8. Ring
  9. Hb and Spoke
  10. Mesh

The column headings for columns 2 and 3 are:

  • Physical Topology
  • Logical Topology

The learner is asked to classify each media access control method as a Physical or lLgical Topology characteristic.

4.4.4 Data Link Frame >4.4.4.1 The Frame

Figure 1 on this page shows two routers connected by a satellite network connection. Greater effort needed to ensure delivery = higher overheads = slower transmission rates. In a fragile environment, more controls are needed to ensure delivery. The header and trailer fields are larger as more control information is needed.

Figure 2 on this page shows a corporate LAN and demonstrates that in a protected environment, fewer controls are needed to ensure delivery. Less effort needed to ensure delivery = lower overheads = faster transmission rates. In a protected environment, we can count on the frame arriving at its destination. Fewer controls are needed, resulting in smaller fields and smaller frames.

4.4.4 Data Link Frame >4.4.4.2 The Header

The figure on this page shows a graphic of a frame including the header, the data and the trailer. The header is separated into its individual fields:

  • Start Frame, which tells other devices on the network that a frame is coming along the medium
  • Address, which stores the source and destination data link addresses
  • Type/Length, which is an optional field used by some protocols to state either what type of data is coming or possibly the length of the frame

4.4.4 Data Link Frame >4.4.4.3 Layer 2 Address

The figure on this page shows two logical topologies.

The multi-access topology consists of five computers connected in a single line. In this topology there are many possible destinations, therefore, Data link layer addresses are required.

IThe point-to-point topology consists of 2 connected routers. In this topology there is only one possible destination so there is no need for Data link layer addresses.

4.4.4 Data Link Frame >4.4.4.4 The Trailer

The figure on this page shows a graphic of a frame including the header, the data and the trailer. The header is separated into its individual fields as well as the trailer. The trailer fields are:

  • F.C.S., which is used for error checking .The source calculates a number based on the frame’s data and places that number in the FCS field. The destination then recalculates the data to see if the

FCS matches. If they don’t match, the destination deletes the frame

  • Stop frame, also called the Frame Trailer which is an optional field that is used when the length of the frame is not specified in the Type/Length field. It indicates the end of the frame when transmitted.

4.4.4 Data Link Frame >4.4.4.5 LAN and WAN Frames

The animation on this page shows several routers connecting two end devices. The layer two frame changes from hop to hop depending on the physical media:

  1. 8 0 2.11 Wireless Frame from laptop to Router1
  2. PPP Frame from Router1 to Router2
  3. HDLC from Router2 to Router3
  4. Frame Relay from Router3 through cloud to Router4
  5. Ethernet frame from Router4 to switch and then to PC

4.4.4 Data Link Frame >4.4.4.6 Ethernet Frame

The figure on this page shows an Ethernet frame matching the field name with its size:

Field namePreambleDestinationSourceTypeDataFrame
Check
Sequence
Size8 bytes6 bytes6 bytes2 bytes46 – 1500
bytes
4 bytes

'* Preamble': Used for synchronization; also contains a delimiter to mark the end of the timing information.

  • Destination Address: 48 bit MAC address for the destination node.
  • Source Address: 48 bit MAC address for the source node.
  • Type: Value to indicate which upper layer protocol will receive the data after the Ethernet process is complete.
  • Data or payload : This is the PDU, typically an I.P.v.4 packet, that is to be transported over the media.
  • Frame Check Sequence (FCS) : A value used to check for damaged frames.

4.4.4 Data Link Frame >4.4.4.7 PPP Frame

The figure on this page shows a Point to Point frame matching the field name with its size:

Field nameFlagAddressControlProtocolDataFCS
Size1 byte1 byte1 byte2 bytesvariable2 or 4
bytes
  • Flag:A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 0.1.1.1.1.1.1.0
  • Address: A single byte that contains the standard PPP broadcast address. PPP does not assign individual station addresses.
  • Control: A single byte that contains the binary sequence 0.0.0.0.0.0.1.1 , which calls for transmission of learner data in an unsequenced frame.
  • Protocol: Two bytes that identify the protocol encapsulated in the data field of the frame.The most up-to-data values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC)
  • Data: Zero or more bytes that contain the datagram for the protocol specified in the protocol field.
    *
    Frame Check Sequence (FCS)
    ':Normally 16 bits (2 bytes).By prior agreement , consenting

PPP implementations can use a 32-bit( 4 byte) FCS for improved error detection.

4.4.4 Data Link Frame >4.4.4.8 802.11 Wireless LAN Protocol

The figure on this page shows a wireless frame with its component fields. The frame control field and the sequence control fields are further sub divided into their sub fields.

802.11 Wireless LAN Protocol:

!Field
name
Frame controlDuration/I DDASARASequence controlTAFrame bodyFCS
!Size
(Octets)
22666260 – 23124

Sequence Control:

!Field nameFragment NumberSequence Number
!Size (Bits)412

Frame control:

!Field nameProtocol VersionTypeSubtypeTo DSFrom DSMore FlagRetryPwr MgtMore DataWEPOrder
!Size (Bits)22411111111

4.4.4 Data Link Frame >4.4.4.9 Activity – Frame Fields

Figure 1 on this page represents a blank Ethernet frame displaying the sizes for each field. The sizes, in bytes, for each field from left to right are:

  • 8
  • 6
  • 6
  • 2
  • 46 - 1500
  • 4

There is also a list of fields names:

  • Frame Check Sequence
  • Destination
  • Type
  • Data
  • Preamble
  • Source

The learner is asked to move each field name to the appropriate place in order to build an Ethernet frame.

Figure 2 on this page is a blank PPP frame displaying the sizes for each field.The sizes, in bytes, for each field from left to right are:

  • 1
  • 1
  • 1`
  • 2
  • Variable
  • 2 or 4

There is also a list of fields names:

  • Control
  • Flag
  • Protocol
  • Data
  • Address
  • FCS

The learner is asked to move each field name to the appropriate place in order to build a PPP frame.

Figure 3 on this page is a blank 8 0 2.11 Wireless frame displaying the sizes for each field.The sizes, in octets, for each field from left to right are:

  • 2
  • 2
  • 6
  • 6
  • 6
  • 2
  • 6
  • 0 - 2312
  • 4

There is also a list of fields names:

  • Sequence Control
  • TA
  • Frame Body
  • FCS
  • Frame Control
  • SA
  • Duration/I D
  • DA
  • RA

The learner is asked to move each field name to the appropriate place in order to build an 8 0 2.11 wireless frame.

4.5 Summary

4.5.1 Summary >4.5.1.1 Class Activity – Linked In!

The figure on this page shows a computer in Paris and one in Japan. The connection will include the LAN connection to the router and a satellite connection between Japan and Paris.

The description given for this image is " The Network Access Layer combines the type of data link and signalling method to deliver data packets securely and seamlessly."

Objectives

Connect devices using wired and wireless media.
In this activity, the learner is asked to map their network cabling or technology type ideas to a physical topology. Because this chapter focuses on the network access layer, they should be able to prepare a basic physical model. No logical (IP addressing) schemes are required for this activity.

4.5.1 Summary >4.5.1.2 Summary

The figure on this page shows a computer in Paris and one in Japan. This diagram demonstrates the different physical connections from hop to hop including copper Ethernet, fiber optics and satellite wireless.

Data link layer protocols govern how to format a frame for use on different media.

Different protocols may be in use for different media.

At each hop along the path, an intermediary device accepts frames from one medium, de-encapsulates the frame and then forwards the packets in a new frame. The headers of each frame are formatted for the specific medium that it will cross.

End of Chapter 4: Network Access.

Next - Chapter 5: Ethernet.

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Page last modified on July 31, 2014, at 01:49 AM