The Data-Link Layer: Ethernet, ARP and LANs

1
The Data-Link LayerEthernet, ARP and LANs

• Based on slides from the Computer Networking A
  Top Down Approach Featuring the Internet by
  Kurose and Ross

2
The Data Link Layer

• Our goals
• Understand principles behind data link layer
  services
• Sharing a broadcast channel multiple access.
• Link layer addressing.
• Interconnection of different LAN segments.
• Instantiation and implementation of various link
  layer technologies

3
Link Layer Introduction

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• layer-2 packet is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
4
Link layer context

• transportation analogy
• trip from Princeton to Lausanne
• limo Princeton to JFK
• plane JFK to Geneva
• train Geneva to Lausanne
• tourist datagram
• transport segment communication link
• transportation mode link layer protocol
• travel agent routing algorithm

• Datagram transferred by different link protocols
  over different links
• e.g., Ethernet on first link, frame relay on
  intermediate links, 802.11 on last link
• Each link protocol provides different services
• e.g., may or may not provide rdt over link

5
Physical Media

• physical link
• Transmitted data bit propagates across link.
• guided media
• signals propagate in solid media (e.g. copper,
  fiber).
• unguided media
• signals propagate freely (e.g. radio bands).
• The physical Layer defines the representation
  of bits.
• Its also provides protection by both detecting
  and correcting corrupted bits (See how it works).

6
Physical Media (Examples)

• Twisted Pair (TP)
• Two insulated copper wires. May be shielded or
  not.
• Category 3
• Traditional phone wires. Supports 10-Mbps
  Ethernet.
• Category 5
• Supports 100Mbps Ethernet (Fast-Ethernet).

7
Physical Media (Examples)

• Coaxial cable
• Two concentric shielded wires.
• Baseband
• single channel on cable.
• broadband
• multiple channel on cable.
• common uses for 10-Mbps Ethernet and TV cables.

8
Physical Media (Examples)

• Fiber
• Glass fiber carrying light pulses.
• Single mode or multi mode.
• High point-to-point speed.
• Very low error rate (caused by low fibers
  attenuation).
• Secured.
• Common used for 100-Mbps Ethernet and 1000-Mbps
  Ethernet (Gigabit Ethernet).

9
Physical Media (Examples)

• Many more
• Radio bands (WiFi, high bit error rate).
• Microwave (Requires hosts in light of sight).
• Satellite (very slow RTT).

10
Link Layer Services

• Recall that we're actually considering the MAC
  layer in the IEEE 802 model.
• Framing (Frame structure)
• encapsulate datagram into frame, adding header,
  trailer
• Link Access (The protocol)
• Addressing
• Introduces MAC addresses used in frame headers
  to identify hosts (actually NICs) who are part of
  the network.
• Different for IP addresses!
• Channel Access
• Defines the set of rules which allows the hosts
  to use the (possibly shared) medium.

11
Link Layer Services

• Flow Control
• pacing between adjacent sending and receiving
  nodes
• Error Detection
• errors caused by signal attenuation, noise.
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission
• Half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time
• RDT
• Offers some reliability between the hosts (Why is
  this redundant?).

12
Link Layer Services
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

13
Link Types

• Broadcast
• Traditional Ethernet and its predecessors.
• Upstream HFC.
• 802.11 wireless LAN.
• Well focus on Broadcast media.

• Three types of links
• point-to-point (P2P)
• PPP for dial-up access.
• Point-to-point link between Ethernet switch and
  host.
• Switched Networks
• ATM (used for WAN).

14
Multiple Access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes
  interference
• collision if node receives two or more signals at
  the same time
• Multiple ACcess (MAC) Protocol
• distributed algorithm that determines how nodes
  share channel, i.e., determine when node can
  transmit
• communication about channel sharing must use
  channel itself!
• no out-of-band channel for coordination

15
Human Analogy

• Rules for party conversation
• "Give everyone a chance to speak."
• "Don't speak until you are spoken to." 
• "Don't monopolize the conversation." 
• "Raise your hand if you have a question." 
• "Don't interrupt when someone is speaking." 
• "Don't fall asleep when someone else is talking

16
MAC Protocols measures

• Assume a shared medium with a channel rate of R
  bpsec.
• Efficient
• When one node wants to transmit it ca send at
  rate R.
• Fair
• When N users want to transmit, each can send at
  average rate R/N.
• Decentralized
• No special node uses to coordinate transmission
  (no leader).
• No synchronization of clocks or slots.
• Fault tolerant.
• Simple
• Should be very fast and implemented in NICs
  firmware.

17
MAC Protocol Types

• Three broad classes
• Channel Partitioning
• Divide channel into smaller pieces (Time-Slots,
  Frequancy-Bands or by code).
• allocate piece to a node for exclusive its use.
• Random Access
• Channel not divided, allow collisions.
• Recover from collisions.
• Taking turns
• Nodes take turns, but nodes with more to send can
  take longer turns.
• Might uses a leader to coordinate the turns.

18
Channel Partitioning MAC protocols TDMA

• TDMA (Time Division Multiple Access)
• Access the channel in "rounds.
• Each station gets fixed length slot (length
  packets trans time) in each round. Each slot
  called a Time-Slot.
• Unused slots go idle.
• Example 6-station LAN, 1,3,4 have packtes,
  slots 2,5,6 idle

19
Channel Partitioning MAC protocols FDMA

• FDMA (Frequency Division Multiple Access)
• Channel spectrum divided into frequency bands.
• Each station assigned fixed frequency band.
• Unused transmission time in frequency bands go
  idle.
• example 6-station LAN, 1,3,4 have packets,
  frequency bands 2,5,6 idle

time
frequency bands
20
TDM / FDM summary

• FDM Enables the division of a channel with
  capacity C bits per seconds to N sub-channels,
  each gets a different frequency range and
  capacity of C/N.
• TDM The division of channel to N sub-channels,
  each gets C/N capacity, by giving the entire
  channel to each of the N stations for 1/N of the
  time.
• ? The division makes each sub channel less busy,
  but the overall waiting time is bigger by a
  factor of N compared to having one channel
  (Little theorem).

21
Channel Partitioning MAC protocols CDMA

• Code Division Multiple Access (CDMA)
• used in several wireless broadcast channels
  (cellular, satellite, etc) standards
• unique code assigned to each user i.e., code
  set partitioning
• all users share same frequency, but each user has
  own chipping sequence (i.e., code) to encode
  data
• encoded signal (original data) X (chipping
  sequence)
• decoding inner-product of encoded signal and
  chipping sequence
• allows multiple users to coexist and transmit
  simultaneously with minimal interference (if
  codes are orthogonal)

22
Random Access MAC Protocols

• From here on, well focus on Random Access MAC
  protocols.
• When node has packet to send
• Transmit at full channel data rate R.
• No a priori coordination among nodes
• Two or more transmitting nodes ? collision.
• Random access MAC protocol specifies
• How to detect collisions
• How to recover from collisions.
• Examples of random access MAC protocols
• ALOHA.
• slotted ALOHA.
• CSMA/CD (Ethernet).
• CSMA/CA (Wireless).

23
Aloha Protocol

• Invented at the 70s in Hawaii.
• Intended for Radio networks, but suitable for
  every network where the station can listen to the
  channel while broadcasting, and determine whether
  others also transmit.
• Basic idea every station may transmit when it
  wants to. If collision is detected between
  frames, back off and try again later.
• If two frames are broadcast at the same time on
  the channel, a collision occurs and the both need
  to retransmit.

24
Aloha Protocol

• All hosts
• Transmit on one frequency (fT).
• Receive on other frequency (fR).
• There is a central node which repeats whatever it
  receives from fT frequency on the other fR
  frequency.
• The central node used as a repeater.
• Collisions are detected by the hosts
• Receiving corrupted data (host knows what should
  be received).

25
Aloha Protocol

1. Accept a new frame arrives
2. Transmit immediately and listen If a collision
   occurred, wait a random time, and repeat to
   stage 2. Otherwise, go back to stage 1 to
   handle a new frame.

26
Aloha Protocol

• Simple.
• Robust against failure of a host.
• Distributed (excluding the central node, which
  uses as a repeater).
• High load implicates low utilization of the
  channel and high delays.

27
Aloha - efficiency
Efficiency is the long-run fraction of
successful transmissions when there are many
nodes, each with many frames to send

• Only 18...
• Suppose there are n stations, and the probability
  that a station starts transmitting in a time unit
  is p.
• Then The probability that exactly one node
  transmits in a time unit is

28
Aloha - efficiency
Efficiency is the long-run fraction of
successful transmissions when there are many
nodes, each with many frames to send

• Only 18...
• Maximize the utilization function by
  differentiation yields maximum point at
  with utilization of .
  
• Trivial improvement
• Why be vulnerable for 2 time units?
• Synchronize, and use slotted time. May transmit
  only at integer times

29
Slotted Aloha

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

30
Slotted Aloha

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized only slots in nodes need to
  be in sync
• simple

• Cons
• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less
  than time to transmit packet
• clock synchronization

31
Slotted Aloha - efficiency

• For max efficiency with N nodes, find p that
  maximizes Np(1-p)N-1
• For many nodes, take limit of Np(1-p)N-1 as N
  goes to infinity, gives 1/e .37

Efficiency is the long-run fraction of
successful slots when there are many nodes, each
with many frames to send

• Suppose N nodes with many frames to send, each
  transmits in slot with probability p
• prob that node 1 has success in a slot
  p(1-p)N-1
• prob that any node has a success Np(1-p)N-1

At best channel used for useful transmissions
37 of time!
32
Aloha - Summary

• Very popular at the beginning of time (i.e., 70s
  to 80s).
• Very simple to handle.
• Lots and lots of basic probabilities calculations
  for students!
• Major problem Nodes dont check whats going on
  in the channel, each acting on its own. No
  manners!

33
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• Protocol
• Listen to the channel
• If channel sensed idle, transmit entire frame
• If channel sensed busy, defer transmission by.
• 1-Persistent CSMAWait until channel is quiet
  and transmit immediately. If collision occurs,
  wait a random time and listen again (go to 1).
• Non-Persistent CSMAWait a random time and
  listen again (go to 1).
• They differ only by the treatment of 1st
  transmission.
• CSMA human analogy dont interrupt others!

34
CSMA/CD (Collision Detection)

• CSMA/CD carrier sensing, deferral as in CSMA
• When transmitting, try to sense if there is a
  collision.
• collisions detected within short time.
• colliding transmissions aborted, reducing channel
  wastage.
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals.
• difficult in wireless LANs receiver shut off
  while transmitting.
• human analogy the polite conversationalist

35
CSMA/CD Minimum Packet Size
36
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access

37
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send acks
  or nacks to sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps

38
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time .1 microsec for 10 Mbps Ethernet for
  K1023, wait time is about 50 msec

• 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?
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

See/interact with Java applet on AWL Web
site highly recommended !
39
802.3 CSMA/CD (Ethernet) Algorithm
40
Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle and then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2

41
Ethernet Minimum Packet Size
42
Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division, Code Division
• Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• CSMA/CA used in 802.11 (Wireless).

43
LAN technologies

• Data link layer so far
• MAC protocols. The random protocol approach.
• Next LAN technologies
• Addressing
• Ethernet
• Hubs, bridges and switches

44
MAC Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM, but can be modified.

45
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
46
LAN Address (more)

• MAC address allocation administered by IEEE.
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address ? portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on IP subnet to which node is attached

47
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
237.196.7.88
48
ARP protocol Same LAN (network)

• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

• A wants to send datagram to B, and Bs MAC
  address not in As ARP table.
• A broadcasts ARP query packet, containing B's IP
  address
• Dest MAC address FF-FF-FF-FF-FF-FF
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)

49
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
50

• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As adapter sends frame
• Rs adapter receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs MAC address
• R creates frame containing A-to-B IP datagram
  sends to B

A
R
B
51
Ethernet

• dominant wired LAN technology, developed at the
  70s
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simple, cheap.
• Kept up with speed race 10 Mbps 10 Gbps

Metcalfes Ethernet sketch
52
Ethernet topology Through the Years

• Now star topology prevails
• Connection choices hub or switch (more later)
• Fast Ethernet 100 Mb/s
• Gigabit Ethernet 1Gbps

• Classic Ethernet

• Shared Bus with CSMA/CD
• Bus maximal length 500 m.
• Transmission rate 10Mb/s.

Through the years the only common is The Frame
53
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 what is the length, in clock ticks, of one
  bit.

Type/ length length or type of frame
54
Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (eg ARP packet), it passes data in frame to
  net-layer protocol
• otherwise, adapter discards frame
• MAC addresses, also called Physical addresses
• 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. Before CRC there is
  a padding field for the CRC to pad to 64 bytes.

55
Ethernet Technology 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!

56
Ethernet technology 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Nodes connect to a hub star topology 100 m
  max distance between nodes and hub

57
Ethernet technology 100BaseT

• Problem must keep minimal packet size when
  bandwidth increases.
• With fixed cable length and propagation speed,
  must
• increase minimal size proportionally to bandwidth
  increase!
• E.g. 100Mb/s, 1500m of cable prop remains 6µs,
  minimal
• size becomes 1200 bits.
• Solutions
• Cable length limited to 100m.
• Prevent collisions by Ethernet Switches
  (later).
• Max distance from node to Hub is 100 meters

58
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 efficiency.
• uses hubs, called here Buffered Distributors
• Full-Duplex at 1 Gbps for point-to-point links.
• 10 Gbpsec now!

59
Hubs

• Q Why not just one big LAN?
• Limited amount of supportable traffic on single
  LAN, all stations must share bandwidth
• limited length 802.3 (Ethernet) specifies
  maximum cable length
• large collision domain (can collide with many
  stations)
• limited number of stations 802.5 (token ring)
  have token passing delays at each station

60
Hubs (Multiport repeaters, Bus in a box)

• Physical Layer devices essentially repeaters
  operating at bit levels repeat received bits on
  one interface to all other interfaces
• Cant interconnect 10BaseT 100BaseT (because
  segments dont share the same rate).
• Hubs can be arranged in a hierarchy (or
  multi-tier design), with backbone hub at its top

61
Hubs (Multiport repeaters, Bus in a box)

• Each connected LAN referred to as LAN segment
• Hubs do not isolate collision domains node may
  collide with any node residing at any segment in
  LAN
• Extends max distance between nodes, but all the
  segments become one large collision domain.
• 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)

62
Bridges

• Link Layer devices operate on Ethernet frames,
  examining frame header and selectively forwarding
  frame based on its destination
• Bridge isolates collision domains since it
  buffers frames
• When frame is to be forwarded on segment, bridge
  uses CSMA/CD to access segment and transmit
• Store and forward element. So different types of
  Ethernet types can be connected.
• Transparent no need for any change to hosts LAN
  adapters
• Forwarding is selective do not always flood. All
  connected segments can work independently in
  parallel!

63
Bridge Filtering

• bridges learn which hosts can be reached through
  which interfaces maintain filtering tables
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender location in filtering table
• filtering table entry
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in Filtering Table dropped (TTL can
  be 60 minutes)

64
Bridge Operation

• bridge procedure(in_MAC, in_port,out_MAC)
• lookup in filtering table (out_MAC) receive
  out_port
• if (out_port not valid) / no entry found for
  destination /
• then flood / forward on all but the
  interface on which
  the frame arrived/
• if (in_port out_port) /destination is on LAN
  on which frame was received /
• then drop the frame
• Otherwise (out_port is valid) /entry found for
  destination /
• then forward the frame on interface indicated

65
Bridge Learning example

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

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

66
Bridge Learning example
C 1

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

67
What will happen with loops?Incorrect learning
68
What will happen with loops?Frame looping
C
2
2
C,??
C,??
1
1
A
69
What will happen with loops?Frame looping
B
2
2
B,2
B,1
1
1
A
70
Introducing Spanning Tree

• Allow a path between every LAN without causing
  loops (loop-free environment)
• Bridges communicate with special configuration
  messages (BPDUs)
• Standardized by IEEE 802.1D
• Note redundant paths are good, active redundant
  paths are bad (they cause loops)

71
How to Construct a Spanning Tree?

• Bridges run a distributed spanning tree
• Algorithm
• Select what ports (and bridges) should actively
  forward frames
• Finding the root flooding
• Building a tree Bellman-Ford Algorithm
• Can combine efficiently
• Standardized in IEEE 802.1 specification

72
Spanning Tree Requirements

• Each bridge is assigned a unique identifier
• A broadcast address for bridges on a LAN
• A unique port identifier for all ports on all
  bridges
• MAC address
• Bridge id port number

73
Spanning Tree ConceptsRoot Bridge

• The bridge with the lowest bridge ID value is
  elected the root bridge
• One root bridge chosen among all bridges
• Every other bridge calculates a path to the root
  bridge

74
Spanning Tree ConceptsPath Cost

• A cost associated with each port on each bridge
• default is 1
• The cost associated with transmission onto the
  LAN connected to the port
• Can be manually or automatically assigned
• Can be used to alter the path to the root bridge

75
Spanning Tree ConceptsRoot Port

• The port on each bridge that is on the path
  towards the root bridge
• The root port is part of the lowest cost path
  towards the root bridge
• If port costs are equal on a bridge, the port
  with the lowest ID becomes root port

76
Spanning Tree ConceptsRoot Path Cost

• The minimum cost path to the root bridge
• The cost starts at the root bridge
• Each bridge computes root path cost independently
  based on their view of the network

77
Spanning Tree Concepts Designated Bridge

• Only one bridge on a LAN at one time is chosen
  the designated bridge
• This bridge provides the minimum cost path to the
  root bridge for the LAN
• Only the designated bridge passes frames towards
  the root bridge

78
Example Spanning Tree
B8
B3
B5

• Protocol operation
• Picks a root
• For each LAN, picks a designated bridgethat is
  closest to the root.
• All bridges on a LANsend packets towards the
  root via the designated bridge.

B7
B2
B1
B6
B4
79
Example Spanning Tree
B8
Spanning Tree
B3
B5
B1
root port
B7
B2
B2
B4
B5
B7
B1
Root
B8
Designated Bridge
B6
B4
80
Spanning Tree AlgorithmAn Overview

• 1. Determine the root bridge among all bridges
• 2. Each bridge determines its root port
• The port in the direction of the root bridge
• 3. Determine the designated bridge on each LAN
• The bridge which accepts frames to forward
  towards the root bridge
• The frames are sent on the root port of the
  designated bridge

81
Spanning Tree AlgorithmSelecting Root Bridge

• Initially, each bridge considers itself to be the
  root bridge
• Bridges send BDPU frames to its attached LANs
• The bridge and port ID of the sending bridge
• The bridge and port ID of the bridge the sending
  bridge considers root
• The root path cost for the sending bridge
• Best one wins
• (lowest root ID/cost/priority)

82
Spanning Tree AlgorithmSelecting Root Ports

• Each bridge selects one of its ports which has
  the minimal cost to the root bridge
• In case of a tie, the lowest uplink (transmitter)
  bridge ID is used
• In case of another tie, the lowest port ID is used

83
Spanning Tree AlgorithmSelect Designated Bridges

• Initially, each bridge considers itself to be the
  designated bridge
• Bridges send BDPU frames to its attached LANs
• The bridge and port ID of the sending bridge
• The bridge and port ID of the bridge the sending
  bridge considers root
• The root path cost for the sending bridge
• 3. Best one wins
• (lowest ID/cost/priority)

84
Forwarding/Blocking State

• Root and designated bridges will forward frames
  to and from their attached LANs
• All other ports are in the blocking state

85
Ethernet Switches

• layer 2 (frame) forwarding, filtering using LAN
  addresses
• Switching A-to-B and A-to-B simultaneously, no
  collisions
• large number of interfaces
• often individual hosts, star-connected into
  switch
• Ethernet, but no collisions!
• Confused with Ethernet bridges

86
Ethernet Switches

• cut-through switching frame forwarded from
  input to output port without awaiting for
  assembly of entire frame
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces
• Offers VLANS (Virtual LANs).
• Nowadays routers are actually combined with
  Ethernet switches.

87
Ethernet Switches (more)
Dedicated
Shared
88
Summary comparison
89
Road-Map and Keywords

• IEEE 802 Model compared to the OSI.
• LLC, MAC.
• Physical Media
• Coax, Twisted Pairs, Fibers.
• Link Types
• Point-to-point, Broadcast, Switched.
• Different MAC protocol approaches
• Channel Partitioning, Random Access, Taking
  Turns.
• Portioning MAC protocols
• TDMA, FDMA, CDMA.
• Random Access MAC protocols
• Aloha, Slotted Aloha.
• LAN technology Ethernet Protocol
• MAC Addresses, Frame Structure, ARP,
• LAN interconnect
• Hubs Bridges and Ethernet Switches.

90
IEEE 802 Model Compared to the OSI

• The Data-Link and Physical layers in the OSI
  model are divided to other layers according to
  the IEEE 802 model

IEEE 802.1 Higher Levels Interface
Higher Layers
IEEE 802.2 Logical Link Control (LLC)
Data-Link Layer
IEEE 802.3 CSMA/CD Medium Access Control
IEEE 802.11 Wireless Medium Access Control
IEEE 802.5 Token Ring Medium Access Control
CSMA/CD Medium
Wireless Medium
Token Ring Medium
Physical Layer
OSI
IEEE 802
91
IEEE 802 Model Compared to the OSI

• The LLC provides common interface for common LAN
  functionality.
• There are various media which offer different
  methods for communication (OSI so called Physical
  layer).
• Each LAN technology uses different MAC (Medium
  Access Control) method to use its corresponding
  medias.
• What kind of medias do we have?
• What kind of corresponding MAC protocols do we
  have?