5a-1

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18 Ethernet, Hubs, Bridges, Switches

• Last Modified
• 10/24/2016 41852 PM

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Ethernet

• dominant LAN technology
• First widely used LAN technology
• Kept up with speed race 10, 100, 1000 Mbps

Metcalfes Ethernet sketch
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Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates

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Ethernet Frame Structure (more)

• Addresses 6 bytes, frame is received by all
  adapters on a LAN and dropped if address does not
  match
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

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Ethernet Unreliable, connectionless

• connectionless No handshaking between sending
  and receiving NICs
• unreliable receiving NIC doesnt send acks or
  nacks to sending NIC
• stream of datagrams passed to network layer can
  have gaps (missing datagrams)
• gaps will be filled if app is using TCP
• otherwise, app will see gaps
• Ethernets MAC protocol unslotted CSMA/CD

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Ethernet uses CSMA/CD

• A sense channel, if idle
• then
• transmit and monitor the channel
• If detect another transmission
• then
• abort and send jam signal
• update collisions
• delay as required by exponential backoff
  algorithm
• goto A
• else done with the frame set collisions to
  zero
• else wait until ongoing transmission is over and
  goto A

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Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

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Manchester encoding

• used in 10BaseT
• each bit has a transition
• allows clocks in sending and receiving nodes to
  synchronize to each other
• no need for a centralized, global clock among
  nodes!
• Hey, this is physical-layer stuff!

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Ethernet Technologies 10Base2

• 10 10Mbps 2 under 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface
  to its other interfaces physical layer device
  only!

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10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Hub to which nodes are connected by twisted pair,
  thus star topology
• CSMA/CD implemented at hub

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10BaseT and 100BaseT (more)

• Max distance from node to Hub is 100 meters
• Hub can disconnect jabbering adapter
• Hub can gather monitoring information, statistics
  for display to LAN administrators

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Gbit Ethernet

• use standard Ethernet frame format
• allows for point-to-point links and shared
  broadcast channels
• in shared mode, CSMA/CD is used short distances
  between nodes to be efficient
• uses hubs, called here Buffered Distributors
• Full-Duplex at 1 Gbps for point-to-point links

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Repeaters

• Physical Layer devices operating at bit levels
  repeat received bits on one interface to all
  other interfaces
• Extend the range of a signal by amplifying
• Useful when want to connect devices beyond the
  IEEE 802.3 specifications for distance limitation
  of 328 feet or 100 meters
• Examples outdoor installations, mine shafts,
  remote locations, etc.

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Hubs

• Also physical layer device, but may have some
  management
• Hubs can be arranged in a hierarchy (or
  multi-tier design), with backbone hub at its top
• Hubs do not isolate collision domains node may
  collide with any node residing at any segment in
  LAN
• Hub Advantages
• Simple, inexpensive device
• Multi-tier provides graceful degradation
  portions of the LAN continue to operate if one
  hub malfunctions
• Extends maximum distance between node pairs (100m
  per Hub)

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Hubs

• physical-layer (dumb) repeaters
• bits coming in one link go out all other links at
  same rate
• all nodes connected to hub can collide with one
  another
• no frame buffering
• no CSMA/CD at hub host NICs detect collisions

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Hub limitations

• Single collision domain results in no increase in
  max throughput
• multi-tier throughput same as single segment
  throughput
• Also less secure hear traffic from/to everyone
  on the hub
• Individual LAN restrictions pose limits on number
  of nodes in same collision domain and on total
  allowed geographical coverage
• Difficult to connect different Ethernet types,
  but can have dual speed hubs (e.g., 10BaseT and
  100baseT)

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Switch

• link-layer device smarter than hubs, take active
  role
• store, forward Ethernet frames
• examine incoming frames MAC address, selectively
  forward frame to one-or-more outgoing links when
  frame is to be forwarded on segment, uses CSMA/CD
  to access segment
• transparent
• hosts are unaware of presence of switches
• plug-and-play, self-learning
• switches do not need to be configured

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Switch allows multiple simultaneous
transmissions

• Switch isolates collision domains
• Hosts have dedicated, direct connection to switch
• A-to-A and B-to-B simultaneously, without
  collisions
• not possible with dumb hub
• Does not forward out all interfaces
• Buffers frames
• Ethernet protocol used on each incoming link, but
  no collisions full duplex
• each link is its own collision domain

switch with six interfaces (1,2,3,4,5,6)
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• Collision domain
• When I speak, who else can I prevent from
  speaking at the same time
• Hub one collision domain Switch collision
  domain per port
• Broadcast domain
• When I deliberately send a broadcast address, who
  all hears it
• VLANs separate broadcast domains

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Managed vs Unmanaged

• Switches more likely than hubs or repeaters to
  have sophisticated management features
• Log in remotely and configure, get
  reports/statistics etc.
• More control over what each port or group of
  ports can do (e.g. establish groups of ports into
  virtual LANs or VLANs that further divide the
  broadcast domain)

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Switches (more)

• Switch advantages
• Isolates collision domains resulting in higher
  total max throughput and more security
• Can connect different type Ethernet since it is a
  store and forward device ( dual speed hub is
  compromise between full switch and hub that does
  this)

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Switch frame filtering, forwarding

• Switches filter packets
• same-LAN -segment frames not forwarded onto other
  LAN segments
• Forwarding
• how to know which LAN segment on which to forward
  frame?
• looks like a routing problem?

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Switch self-learning
A

• switch learns which hosts can be reached through
  which interfaces
• when frame received, switch learns location of
  sender incoming LAN segment
• records sender/location pair in switch table

C
B
1
2
3
6
4
5
C
B
A
Switch table (initially empty)
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Switch frame filtering/forwarding

• When frame received
• 1. record link associated with sending host
• 2. index switch table using MAC dest address
• 3. if entry found for destination then
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
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Self-learning, forwarding example
A
C
B

• frame destination unknown

1
2
3
flood
6
4
5

• destination A location known

C
selective send
B
A
Switch table (initially empty)
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• Generally on a switch only see traffic to/from
  your machine and broadcast traffic
• Can attack switch by sending many MACs and
  overflowing its storage of which MACs on which
  port gt will begin to act like hub ( flooding
  each packet out every port)

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Interconnecting switches

• switches can be connected together

S1
A
C
B

• Q sending from A to G - how does S1 know to
  forward frame destined to F via S4 and S3?
• A self learning! (works exactly the same as in
  single-switch case!)

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Switches vs. Routers
application transport network link physical

• both store-and-forward devices
• routers Layer 3 or network-layer devices
  (examine network-layer headers)
• switches are Layer 2 or link-layer devices
  (examine link-layer headers)
• routers maintain routing tables, implement
  routing algorithms
• switches maintain switch tables, implement
  filtering, learning algorithms

switch
application transport network link physical
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Switch Pros and Cons

• Switch operation is simpler requiring less
  processing bandwidth
• - Topologies are restricted with bridges a
  spanning tree must be built to avoid cycles
• - Switch do not offer protection from broadcast
  storms (endless broadcasting by a host will be
  forwarded by a bridge)

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Routers Pros and Cons

• arbitrary topologies can be supported, cycling
  is limited by TTL counters (and good routing
  protocols)
• provide firewall protection against broadcast
  storms
• - require IP address configuration (not plug and
  play)
• - require higher processing bandwidth

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Network Diagrams
Shared
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Sample Icons

• Icons for in network diagrams

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Summary

• Layer 3 Devices (Network Layer)
• Router
• Layer 2 Devices (Link Layer)
• Bridge
• Switch
• Layer 1 Devices (Physical Layer)
• Repeaters
• Hubs

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Outtakes
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Institutional network
mail server
to external network
web server
router
IP subnet
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Switch Learning example

• Suppose C sends frame to D and D replies back
  with frame to C

• C sends frame, switch has no info about D, so
  floods to both LANs
• switch notes that C is on port 1
• frame ignored on upper LAN
• frame received by D

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Switch Learning example

• D generates reply to C, sends
• switch sees frame from D
• switch notes that D is on interface 2
• switch knows C on interface 1, so selectively
  forwards frame out via interface 1

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Spanning Tree

• for increased reliability, desirable to have
  redundant, alternate paths from source to dest
• with multiple simultaneous paths, cycles result -
  bridges may multiply and forward frame forever
• solution organize bridges in a spanning tree by
  disabling subset of interfaces

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Spanning Tree Algorithm
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• VLAN tagging

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Interconnection Without Backbone

• Not recommended for two reasons
• - single point of failure at Computer Science hub
• - all traffic between EE and SE must path over CS
  segment

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Backbone Switch
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Ethernet Switches

• Sophisticated bridges
• Switches usually switch in hardware, bridges in
  software
• large number of interfaces
• Like bridges, layer 2 (frame) forwarding,
  filtering using LAN addresses
• Can support combinations of shared/dedicated,
  10/100/1000 Mbps interfaces

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Switching

• Switching A-to-B and A-to-B simultaneously, no
  collisions
• cut-through switching frame forwarded from input
  to output port without awaiting for assembly of
  entire frame
• slight reduction in latency
• Store and forward switching entire frame
  received before transmission out an output port
• Fragment-free switching compromise, before send
  out the output port receive enough of the packet
  to do some error checking (ex. detect and drop
  partial frames)

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Ethernet Limitations

• Q Why not just one big Ethernet?
• Limited amount of supportable traffic on single
  LAN, all stations must share bandwidth
• limited length 802.3 specifies maximum cable
  length
• large collision domain (can collide with many
  stations)