Chapter 7

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Chapter 7 Layer 2 Technologies

• Token Ring
• FDDI
• Ethernet and IEEE 802.3
• Layer 2 Devices
• Data Flow
• Ethernet 10BASE-T Troubleshooting

By Stacy Olson With help from Stephanie
Hutters Briefings
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Chapter 7 Layer 2 Technologies

• Token Ring
• Developed by IBM still used today
• Two frames
• Token
• Start Delimiter
• Access Control Byte
• Priority and Reservation Fields
• Token and Monitor bits
• End Delimiter

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Chapter 7 Layer 2 Technologies

• Data/Command Frame

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Chapter 7 Layer 2 Technologies

• Token Ring Token Passing
• Station can only transmit if it has the token
• Station passes token on if it has no data to
  transmit

• Station can hold the token
• for a maximum amount of
• time

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Chapter 7 Layer 2 Technologies

• Token Ring Characteristic
• Deterministic Taking Turns
• Maximum time to transmit is subject to exact
  calculation.
• Ideal for applications where predictability and
  dependability are paramount.

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Chapter 7 Layer 2 Technologies

• Token Ring Management Mechanisms
• Active Monitor
• One station acts as centralized source of timing
  information for other stations
• Can be any station
• Removes continuously circulating frames

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Chapter 7 Layer 2 Technologies

• Token Ring Management Mechanisms
• MSAU
• Multi Station Access Units
• Can see all information in a Token Ring Network
• Check for problems
• Selectively remove stations from the ring if
  needed

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Chapter 7 Layer 2 Technologies

• Token Ring Management Mechanisms
• Beaconing
• Detects and repairs network faults
• Sends a beacon frame, defining a failure domain
• Reporting station
• Nearest Active Upstream Neighbor (NAUN)
• Everything in between
• Initiates autoreconfiguration
• Nodes within the failure domain automatically
  perform diagnostics
• Attempt to reconfigure around the failure
• MSAUs use electrical reconfiguration to
  accomplish this

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Chapter 7 Layer 2 Technologies

• Token Ring Signaling
• Uses Manchester Encoding
• Combines data and clock into bit symbols, which
  are split into two halves, the polarity of the
  second half always being the reverse of the first
  half.
• 0 is high-to-low transition
• 1 is low-to-high transition

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Chapter 7 Layer 2 Technologies

• Token Ring Media and Physical Topologies
• Logical ring topology
• Physical star topology
• Stations are directly connected to MSAUs
• Patch cables connect MSAUs
• Lobe cables connect MSAUs to stations

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Overview of FDDI

• Fiber distributed data interface
• Fiddee
• Four specifications
• Media Access control
• Physical Layer Protocol
• Physical Layer Medium
• Station Management

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Media Access Control

• MAC
• Defines how the medium is accessed
• Frame format
• Token handling
• Addressing
• Algorithm for calculating a cyclic redundancy
  check and error recovery mechanisms

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Physical Layer Protocol

• PHY
• Defines data encoding/decoding procedures
• Clocking requirements
• Framing
• Other functions

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Physical Layer Medium

• Defines the characteristics of the transmission
  medium
• Fiber optic link
• Power levels
• Bit error rates
• Optical components
• Connectors

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Station Management

• Defines the FDDI station configuration
• Ring configuration
• Station insertion and removal
• Initialization
• Fault isolation and recovery
• Scheduling
• Collection of statistics

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• Preamble
• Prepares each station for the upcoming frame
• Start delimiter
• Frame Control
• Indicates the size of the address fields
• Indicates whether frame contains asynchronous or
  synchronous data
• Other control information

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• Destination address
• 6 bytes
• Unicast to one address
• Multicast to several addresses
• Broadcast to all addresses
• Source address
• Data
• Frame Check Sequence
• End Delimiter
• Frame Status

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FDDI Token
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FDDI MAC

• Token passing strategy
• Early token release
• New token can be released when the frame
  transmission has finished
• Deterministic
• Dual ring
• Ensures transmission, even if one ring is damaged
  or disabled
• Very reliable
• Real-time allocation of bandwidth
• Defines two types of traffic
• Synchronous
• Asynchronous

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Synchronous Traffic

• Consumes only a portion of the bandwidth
• Asynchronous traffic can consume the rest
• Synchronous bandwidth is allocated to those
  stations requiring continuous transmission, e.g.
  voice/video
• FDDI SMT specification defines a distributed
  bidding scheme to allocate FDDI bandwidth

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Asynchronous Traffic

• Bandwidth is allocated using an eight-level
  priority scheme
• Each station is assigned an asynchronous priority
  level
• FDDI also permits extended dialogues
• Stations may temporarily use all the asynchronous
  bandwidth
• FDDI priority mechanism can lock out stations
  that cannot use synchronous bandwidth, and have
  too low an asynchronous priority

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FDDI Signaling

• Uses an encoding scheme called 4B/5B
• Every four bits of data are sent as a 5 bit code
• Signal sources are LEDs or lasers

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FDDI Media

• Optical fiber is being installed at a rate of
  4000 miles per day in the United States
• Explosive growth worldwide

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Advantages of Optical Fiber

• Security
• Fiber does not emit electrical signals that can
  be tapped
• Reliability
• Fiber is immune to electrical interference
• Speed
• Optical fiber has much higher throughput
  potential than copper cable

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Types of Optical Fiber

• Modes are bundles of light rays entering the
  fiber at particular angles
• Single-mode
• Also known as mono-mode
• Only one mode propagates through fiber
• Higher bandwidth than multi-mode
• Longer cable runs than multi-mode
• Lasers generate light signals
• Used for inter-building connectivity

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Types of Optical Fiber

• Multi-mode
• Multiple modes propagate through fiber
• Different angles mean different distances to
  travel
• Transmissions arrive at different times
• Modal dispersion
• LEDs as light source
• Used for intra-building connectivity

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FDDI Rings

• FDDI specifies dual rings for physical
  connections
• Traffic on each ring travels in opposite
  directions
• Rings consist of two or more point-to-point
  connections between adjacent stations
• Primary ring is for data transmission
• Secondary ring is for back up

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Single-Attachment Stations

• SAS
• Class B
• Attach to one ring (primary)
• Attached through a concentrator
• Provides connection for multiple SASs
• Ensures that no one SAS can interrupt the ring

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Dual Attachment Stations

• DAS
• Class A
• Attach to both rings
• Has two ports to connect to the dual ring
• Both ports connect to both rings

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Chapter 7 Layer 2 Technologies

• Shortly after the 1980 IEEE 802.3 specification,
  Digital Equipment Corporation (DEC), Intel
  Corporation, and Xerox Corporation jointly
  developed and released an Ethernet specification.
  Version 2.0, that was substantially compatible
  with IEEE 802.3. Together, Ethernet and IEEE
  802.3 currently maintain the greatest market
  share of any LAN protocol.

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Chapter 7 Layer 2 Technologies

• Today, the term Ethernet is often used to refer
  to all carrier sense multiple access/collision
  detection (CSMA/CD) LANs that generally conform
  to Ethernet specifications, including IEEE 802.3.
  

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Chapter 7 Layer 2 Technologies
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Chapter 7 Layer 2 Technologies

• 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
• In the CSMA/CD access method, networking devices
  with data to transmit over the networking media
  work in a listen-before-transmit mode.

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NICs

• Provides ports for network connection
• Communicate with network via serial connection
• Communication with computer through parallel
  connection
• Resources required
• IRQ, I/O address, upper memory addresses

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Selection Factors for NICs

• Type of network
• Ethernet, Token Ring, FDDI
• Type of media
• Twisted pair, coax, fiber
• Type of system bus
• PCI, ISA

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NIC Operations

• Layer 1 Layer 2 device
• Primarily Layer 2
• Communicates with upper layers in the computer
• Logical Link Control (LLC)
• Has MAC address burned in
• Encapsulates data into frames
• Provides access to the media
• Also Layer 1
• Creates signals and interfaces with the media
• On-board transceiver

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Bridges

• Connects two network segments
• Can connect different layer 2 protocols
• Ethernet, Token Ring, FDDI
• Makes intelligent decisions about traffic
• Reduces unnecessary traffic
• Minimizes collisions
• Filters traffic based on MAC address
• Maintains address tables
• Rarely implemented today
• Conceptually important

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Bridge Operations

• Bridging occurs at the data link layer
• Controls data flow
• Handles transmission errors
• Provides physical addressing
• Manages access to the physical medium

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Bridge Operations

• Transparent to upper layers
• Best used in low traffic areas
• Can cause bottlenecks
• Must examine every packet
• Broadcasts
• Messages sent to all devices
• Destination MAC address unknown
• Bridge will always forward
• Can cause Broadcast Storm
• Network time outs, traffic slowdowns,
  unacceptable performance

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Switching Operation

• Microsegmentation
• Each switch port acts as a micro bridge (Layer 2
  device)
• Multiple traffic paths within the switch
• Virtual circuits
• Temporarily exist - only when needed
• Each data frame has a dedicated path
• No collisions
• Increases bandwidth availability
• Each host gets full bandwidth

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Advantages of Switches

• Much faster than bridges
• Hardware based, not software
• Support new uses
• e.g. virtual LANs
• Reduce collision domains

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Advantages of Switches

• Allows many users to communicate in parallel
• Creates virtual circuits
• Creates dedicated segments
• Collision free
• Maximizes bandwidth
• Cost effective
• Can simply replace hubs in same cable
  infrastructure
• Minimal disruption
• Flexible network management
• Software based configuration

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Broadcast Domains

• All hosts connected to the same switch are still
  in the same broadcast domain
• A broadcast from one node will be seen by all
  other nodes connected through the LAN switch

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Chapter 7 Layer 2 Technologies

• Two primary reasons for segmenting a LAN
• Isolate traffic between segments
• Achieve more bandwidth per user by creating
  smaller collision domains

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Chapter 7 Layer 2 Technologies

• Bridge Drawback
• Bridges increase the latency (delay) in a network
  by 10-30
• A bridge is considered a store-and-forward device
  slowing network transmissions, thus causing
  delay.

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Chapter 7 Layer 2 Technologies

• It is important to note that even though 100 of
  the bandwidth may be available, Ethernet networks
  perform best when kept under 30-40 of full
  capacity.
• Bandwidth usage that exceeds the recommended
  limitation results in increased collisions.

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Chapter 7 Layer 2 Technologies

• The Router is a layer 3 (Network) device, but
  operates at layers 1-3.
• Routers create the highest level of segmentation
  because of their ability to make exact
  determinations of where to send the data packet.
• Because routers perform more functions than
  bridges, they operate with a higher rate of
  latency.

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Chapter 7 Layer 2 Technologies
7.5.5 Identify Broadcast Domains and Collision
Domains
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Chapter 7 Layer 2 Technologies

• Good Luck
• on the
• Test!!