Transcript Layer 2 Technologies

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Semester 1
CHAPTER 7
Connecting The Internet Generation
Wael Yousif
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Content
• Token-ring.
• FDDI LAN.
• Ethernet and IEEE 802.3.
• Layer 2 devices and effects on data flow.
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BASIC OF TOKEN-RING
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Variants
IBM developed the first
Token Ring network in
the 1970s. It is still
IBM's primary LAN
technology, and is
second only to Ethernet
(IEEE 802.3) in terms of
LAN implementation.
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Ring topology
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Data passing
• When a station has information to
transmit, it seizes the token and sends
data frame to the next station.
• When frame reaches the destination
station, the data is copied for processing.
• Frame continues to circle the ring until it
returns to the sending station.
• Sending station removes the frame from
the ring, verifies receipt, and releases the
token.
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Token-Ring frame format
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Start delimiter and End delimiter
• Start delimiter.
– Alert for the arrival of a token.
• End Delimiter
– Completes the token or data/command
frame.
– Contains damage indicator.
– Last of logical sequence.
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Access control
P P P T M R R R
• P: Priority bits
• T: Token bit
• M: Monitor bit
• R: Reservation bits
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Priority and reservation bits
• B'000' Normal User Priority
• B'001' Normal User Priority
• B'010' Normal User Priority
• B'011' Normal User priority
• B'100' Bridge/Router
• B'101' Reserved IBM
• B'110' Reserved IBM
• B'111' Station Management
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Priority management
• Using the priority field and the reservation
field.
• Stations with a higher priority can reserve
the token for the next network pass.
• Stations that raise a token's priority level
must reinstate the previous priority after
their transmission has been completed.
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Frame control
• Only present in data/command frames.
• Indicates whether frame contains data
or control information.
• If control, this byte specifies type of
control information.
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Destination and Source addresses
•
•
•
•
Universal Address.
Local Administered Address.
Broadcast Address
Functional Address (0x0C0000 00XXXX)
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Data
• Length limited by the maximum time a
station may hold the token.
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Frame checksum
• Frame Check Sequence.
• Source fills field with calculated
value dependent on frame contents.
• Destination recalculates to check
data integrity.
• Frame is discarded if damaged.
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Frame status
• Address recognized / frame copied indicator.
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Management mechanisms
• Active Monitor
– One station acts as centralized source of
timing information for other stations.
– Removes continuously circulating frames by
set monitor bit to 1.
– Start a token, when token have been lost.
• Beaconing
– Detects and repairs network faults.
– Initiates auto-reconfiguration.
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Physical topology
• Physical topology : Star.
• Logical topology : Ring.
• IBM Token Ring network
stations are connected to
MSAU (Multi-Station
Access Unit).
• Many MSAU can be wired
together to form one large
ring.
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Multi-MSAU
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Physical connection
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BASIC OF FDDI
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Characteristics
• Fiber Distributed Data Interface.
• FDDI is popular as a campus backbone
technology.
100 Mbps
Token passing
Dual-ring
Fiber Optic Cable
Total fiber length of 200Km
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FDDI dual-ring (PR and SR)
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Fiber-optic modes
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FDDI Connections
• Class A: connect
directly with PR – SR.
– DAC: Dual Attachment
Concentrator
– DAS: Dual Attachment
Station
• Class B: connect via
FDDI concentrator.
– SAS: Single
Attachment Station
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Operation mechanisms
• Connection Establishment
– Station connect to neighbors to form the ring.
– Negotiate the length of the link.
• Ring Initialization
– Station claim the right to generate a token.
• Steady-state Operation
– Token passing
• Ring Maintenance
– Detects and repairs token or network faults.
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FDDI topology
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ETHERNET AND IEEE 802.3
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Ethernet introduction
• Ethernet is the most widely used local
area network (LAN) technology.
• Ethernet was designed to carry data at
high speeds for very limited distances.
• Ethernet is well suited to applications
where a local communication medium
must carry sporadic, occasionally heavy
traffic at high peak data rates.
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Datalink and Physical layers
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Comparing Ethernet and IEEE 802.3
• Specify similar technologies.
• Broadcast network.
• Using CSMA/CD algorithm.
• Hardware implementation.
• Differences:
– Ethernet provides services corresponding to
physical and datalink layer.
– IEEE 802.3 specifies the physical layer and
the channel-access portion of the data link
layer but does not define a LLC protocol.
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Ethernet family: 1000Base-SX-LX
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Ethernet family: 1000Base-T
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Ethernet family: 100Base-TX
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Ethernet family: 10Base-T
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Ethernet family
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Ethernet frame format
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Preamble
• Note that a frame is
Ethernet or IEEE 802.3.
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Start of frame delimiter (SOF)
• The IEEE 802.3: synchronize the
frame-reception portions of all
stations on the LAN.
• Be explicitly specified in Ethernet.
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Source and destination addresses
•
•
•
•
MAC addresses.
Unicast.
Multicast (D)
Broadcast (D)
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Type (Ethernet)
• Specifies the upper-layer protocol
to receive the data after Ethernet
processing is completed
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Length (IEEE 802.3)
• The length indicates the number of
bytes of data that follows this field
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Data (Ethernet)
• the data contained in the frame is
sent to an upper-layer protocol
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Data (IEEE 802.3)
• Data send to LLC layer, including
LLC header and upper-layer data
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Frame check sequence (FCS)
• This sequence contains a 4 byte CRC
value that is created by the sender
and is recalculated by the receiver to
check for damaged frames
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Media Access Control (MAC)
• Shared-media broadcast technology.
• Ethernet’s MAC performs three functions:
1. transmitting and receiving data packets
2. decoding data packets and checking them
for valid addresses before passing them to
the upper layers of the OSI model
3. detecting errors within data packets or on
the network
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Broadcast technology
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Broadcast address
FF-FF-FF-FF-FF-FF
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CSMA/CD
• When a station wishes to transmit, it checks the
network to determine whether another station is
transmitting.
• If network is free, the station proceeds with the
transmission.
• While sending, the station monitors the network
to ensure that no other station is transmitting.
• If a transmitting node recognizes a collision, it
transmits a jam signal so that all other nodes
recognize collision.
• All transmitting nodes then stop sending for a
backoff time (randomly 0 .. 2n - 1 of 51.2ms).
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CSMA/CD (cont.)
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CSMA/CD Algorithm
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Ethernet star topology
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Ethernet star topology (cont.)
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TIA/EIA-568-A HC Standard
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TIA/EIA-568-A: Distance limit
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LAYER 2 DEVICES
AND
EFFECTS ON DATAFLOW
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Layer 2 Devices
• NIC (Network Interface Card)
– Connect your computer with network.
– Provide MAC addresses to each connection.
– Implement CSMA/CD algorithm.
• Bridge
– Forward or filter frame by MAC address.
• Switch
– Multi-port bridge.
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NIC
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NIC (cont.)
• Provides ports for network connection.
• When selecting a network card, consider:
1. Type of network:
•
•
•
Ethernet
Token Ring
FDDI
2. Type of media
•
•
•
Twisted-pair
Coaxial
Fiber-optic
3. Type of system bus
•
•
PCI
ISA
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NIC: Layer 2 functions
• Logical link control (LLC): communicates
with upper layers in the computer
• Naming: provides a unique MAC address
identifier
• Framing: part of the encapsulation
process, packaging the bits for transport
• Media Access Control (MAC): provides
structured access to shared access media
• Signaling: creates signals and interface
with the media
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Bridge
• Connects network segments.
• Make intelligent decisions about whether
to pass signals on to the next segment.
• Improve network performance by
eliminating unnecessary traffic and
minimizing the chances of collisions.
• Divides traffic into segments and filters
traffic based on MAC address.
• Often pass frames b/w networks operating
under different Layer 2 protocols.
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Bridge (cont.)
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Bridge (cont.): Filter
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Bridge (cont.): Forward
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LAN Switch
• Switches connect LAN segments.
• LAN switches are considered multi-port
bridges with no collision domain.
• Use a MAC table to determine the segment
on which a frame needs to be transmitted.
• Switches often replace shared hubs and
work with existing cable infrastructures.
• Higher speeds than bridges.
• Support new functionality, such as VLAN.
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LAN Switch (cont.)
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LAN Switch: MAC table
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LAN Switch: Micro-segmentation
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Benefits of LAN Switch
• No collision domain, because of microsegmentation.
• Low latency levels and a high rate of
speed for frame forwarding
• Increases the bandwidth available on a
network
• Is performed in hardware instead of in
software, it is significantly faster.
• BUT: All hosts connected to the switch are
still in the same broadcast domain.
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Why segment LANs?
• Isolate traffic between segments.
• Achieve more bandwidth per user by
creating smaller collision domains.
• LANs are segmented by devices like
bridges, switches, and routers.
• Extend the effective length of a LAN,
permitting the attachment of distant
stations.
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Segmentation with bridges
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Segmentation with bridges (cont.)
• Bridges increase the latency (delay) in a
network by 10-30%.
• A bridge is considered a store-andforward device because it must receive
the entire frame and compute the cyclic
redundancy check (CRC) before
forwarding can take place.
• The time it takes to perform these tasks
can slow network transmissions, thus
causing delay.
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Segmentation with switches
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Segmentation with switches (cont.)
• Allows a LAN topology to work faster and
more efficiently.
• Uses bandwidth so efficiently, the
available bandwidth can reach to 100%.
• Ease bandwidth shortages and network
bottlenecks (such as client-server).
• A computer connected directly to an
Ethernet switch is its own collision
domain and accesses the full 10Mbps.
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Segmentation with routers
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Segmentation with routers (cont.)
• Routers operates at the network layer
• Routers bases all of its forwarding
decisions on the Layer 3 protocol
address.
• Routers ability to make exact
determinations of where to send the data
packet.
• Router operate with a higher rate of
latency.
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Teaching topology
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Basic 10BaseT troubleshooting
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