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Last class <ul><li>Random Access Protocols </li></ul><ul><ul><li>Slotted Aloha </li></ul></ul><ul><ul><li>Aloha </li></ul>...
Slotted ALOHA (Efficiency 0.36) <ul><li>Pros </li></ul><ul><li>single active node can continuously transmit at full rate o...
Pure (unslotted) ALOHA <ul><li>unslotted Aloha: simpler, no synchronization </li></ul><ul><li>when frame first arrives </l...
CSMA/CD collision detection
“Taking Turns” MAC protocols <ul><li>Polling:   </li></ul><ul><li>master node “invites” slave nodes to transmit in turn </...
ARP: Address Resolution Protocol <ul><li>Each IP node (Host, Router) on LAN has  ARP  table </li></ul><ul><li>ARP Table: I...
Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Mobile and wireless networks, CDMA </li></...
Ethernet Frame Structure <ul><li>Sending adapter encapsulates IP datagram (or other network layer protocol packet) in  Eth...
Ethernet uses CSMA/CD <ul><li>No slots </li></ul><ul><li>adapter doesn’t transmit if it senses that some other adapter is ...
Ethernet CSMA/CD algorithm <ul><li>1. Adaptor receives datagram from net layer & creates frame </li></ul><ul><li>2. If ada...
Ethernet’s CSMA/CD (more) <ul><li>Jam Signal:  make sure all other transmitters are aware of collision; 48 bits </li></ul>...
CSMA/CD efficiency <ul><li>T prop  = max prop between 2 nodes in LAN </li></ul><ul><li>t trans  = time to transmit max-siz...
10BaseT and 100BaseT <ul><li>10/100 Mbps rate; latter called “fast ethernet” </li></ul><ul><li>T  stands for Twisted Pair ...
Gbit Ethernet <ul><li>uses standard Ethernet frame format </li></ul><ul><li>allows for point-to-point links and shared bro...
Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Wireless links, characteristics, CDMA </li...
Hubs <ul><li>Hubs are essentially physical-layer repeaters: </li></ul><ul><ul><li>bits coming from one link go out all oth...
Interconnecting with hubs  <ul><li>Pros: </li></ul><ul><li>Enables interdepartmental communication </li></ul><ul><li>Exten...
Switch <ul><li>Link layer device </li></ul><ul><ul><li>stores and forwards Ethernet frames </li></ul></ul><ul><ul><li>exam...
Forwarding <ul><li>How to determine onto which LAN segment to forward frame? </li></ul><ul><li>Looks like a routing proble...
Self learning <ul><li>A switch has a  switch table </li></ul><ul><li>entry in switch table:  </li></ul><ul><ul><li>(MAC Ad...
Filtering/Forwarding <ul><li>When switch receives a frame: </li></ul><ul><li>index switch table using MAC dest address </l...
Switch example <ul><li>Suppose C sends frame to D </li></ul><ul><li>Switch receives frame from from C </li></ul><ul><ul><l...
Switch example <ul><li>Suppose D replies back with frame to C.  </li></ul><ul><li>Switch receives frame from from D </li><...
Switch: traffic isolation <ul><li>switch installation breaks subnet into LAN segments </li></ul><ul><li>switch  filters  p...
Switches: dedicated access <ul><li>Switch with many interfaces </li></ul><ul><li>Hosts have direct connection to switch </...
More on Switches <ul><li>cut-through switching:  frame forwarded from input to output port without first collecting entire...
Institutional network hub hub hub switch to external network router IP subnet mail server web server
Switches vs. Routers <ul><li>both store-and-forward devices </li></ul><ul><ul><li>routers: network layer devices (examine ...
Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Mobile and wireless networks, CDMA </li></...
Wireless and Mobile Networks <ul><li>Background:   </li></ul><ul><li># wireless (mobile) phone subscribers now exceeds # w...
Elements of a wireless network network  infrastructure <ul><li>wireless hosts </li></ul><ul><li>laptop, PDA, IP phone </li...
Elements of a wireless network network  infrastructure <ul><li>base station </li></ul><ul><li>typically connected to wired...
Elements of a wireless network <ul><li>wireless link </li></ul><ul><li>typically used to connect mobile(s) to base station...
Elements of a wireless network network  infrastructure <ul><li>infrastructure mode </li></ul><ul><li>base station connects...
Elements of a wireless network <ul><li>Ad hoc mode </li></ul><ul><li>no base stations </li></ul><ul><li>nodes can only tra...
Wireless Link Characteristics <ul><li>Differences from wired link …. </li></ul><ul><ul><li>decreased signal strength:  rad...
Wireless network characteristics <ul><li>Multiple wireless senders and receivers create additional problems (beyond multip...
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Chapter 6 slides, Computer Networking, 3rd edition

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Transcript of "Chapter 6 slides, Computer Networking, 3rd edition"

  1. 1. Last class <ul><li>Random Access Protocols </li></ul><ul><ul><li>Slotted Aloha </li></ul></ul><ul><ul><li>Aloha </li></ul></ul><ul><ul><li>CSMA/CD </li></ul></ul><ul><ul><li>“Taking Turns” Protocols </li></ul></ul><ul><li>Link-Layer Addressing </li></ul><ul><li>Today </li></ul><ul><li>Ethernet, Hubs and Switches </li></ul><ul><li>Mobile and wireless networks, CDMA </li></ul><ul><li>IEEE 802.11 wireless LANs </li></ul>
  2. 2. Slotted ALOHA (Efficiency 0.36) <ul><li>Pros </li></ul><ul><li>single active node can continuously transmit at full rate of channel </li></ul><ul><li>highly decentralized: only slots in nodes need to be in sync </li></ul><ul><li>simple </li></ul><ul><li>Cons </li></ul><ul><li>collisions, wasting slots </li></ul><ul><li>idle slots </li></ul><ul><li>clock synchronization </li></ul>
  3. 3. Pure (unslotted) ALOHA <ul><li>unslotted Aloha: simpler, no synchronization </li></ul><ul><li>when frame first arrives </li></ul><ul><ul><li>transmit immediately </li></ul></ul><ul><li>collision probability increases: </li></ul><ul><ul><li>frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1] </li></ul></ul>
  4. 4. CSMA/CD collision detection
  5. 5. “Taking Turns” MAC protocols <ul><li>Polling: </li></ul><ul><li>master node “invites” slave nodes to transmit in turn </li></ul><ul><li>concerns: </li></ul><ul><ul><li>polling overhead </li></ul></ul><ul><ul><li>latency </li></ul></ul><ul><ul><li>single point of failure (master) </li></ul></ul><ul><li>Token passing: </li></ul><ul><li>control token passed from one node to next sequentially. </li></ul><ul><li>token message </li></ul><ul><li>concerns: </li></ul><ul><ul><li>token overhead </li></ul></ul><ul><ul><li>latency </li></ul></ul><ul><ul><li>single point of failure (token) </li></ul></ul>
  6. 6. ARP: Address Resolution Protocol <ul><li>Each IP node (Host, Router) on LAN has ARP table </li></ul><ul><li>ARP Table: IP/MAC address mappings for some LAN nodes </li></ul><ul><li>< IP address; MAC address; TTL> </li></ul><ul><ul><li>TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) </li></ul></ul>1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 237.196.7.23 237.196.7.78 237.196.7.14 237.196.7.88 Question: how to determine MAC address of B knowing B’s IP address?
  7. 7. Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Mobile and wireless networks, CDMA </li></ul><ul><li>IEEE 802.11 wireless LANs </li></ul>
  8. 8. Ethernet Frame Structure <ul><li>Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame </li></ul><ul><li>Preamble: </li></ul><ul><li>7 bytes with pattern 10101010 followed by one byte with pattern 10101011 </li></ul><ul><li>used to synchronize receiver, sender clock rates </li></ul>
  9. 9. Ethernet uses CSMA/CD <ul><li>No slots </li></ul><ul><li>adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense </li></ul><ul><li>transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection </li></ul><ul><li>Before attempting a retransmission, adapter waits a random time, that is, random access </li></ul>
  10. 10. Ethernet CSMA/CD algorithm <ul><li>1. Adaptor receives datagram from net layer & creates frame </li></ul><ul><li>2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits </li></ul><ul><li>3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! </li></ul><ul><li>4. If adapter detects another transmission while transmitting, aborts and sends jam signal (48 bits) </li></ul><ul><li>5. After aborting, adapter enters exponential backoff : after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. </li></ul><ul><li>Adapter waits K · 512 bit times and returns to Step 2 </li></ul>
  11. 11. Ethernet’s CSMA/CD (more) <ul><li>Jam Signal: make sure all other transmitters are aware of collision; 48 bits </li></ul><ul><li>Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec </li></ul><ul><li>Exponential Backoff: </li></ul><ul><li>Goal : adapt retransmission attempts to estimated current load </li></ul><ul><ul><li>heavy load: random wait will be longer </li></ul></ul><ul><li>first collision: choose K from {0,1}; delay is K · 512 bit transmission times </li></ul><ul><li>after second collision: choose K from {0,1,2,3}… </li></ul><ul><li>after ten collisions, choose K from {0,1,2,3,4,…,1023} </li></ul>
  12. 12. CSMA/CD efficiency <ul><li>T prop = max prop between 2 nodes in LAN </li></ul><ul><li>t trans = time to transmit max-size frame </li></ul><ul><li>Efficiency goes to 1 as t prop goes to 0 </li></ul><ul><li>Goes to 1 as t trans goes to infinity </li></ul><ul><li>Much better than ALOHA, but still decentralized, simple, and cheap </li></ul>
  13. 13. 10BaseT and 100BaseT <ul><li>10/100 Mbps rate; latter called “fast ethernet” </li></ul><ul><li>T stands for Twisted Pair </li></ul><ul><li>Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub </li></ul>twisted pair hub
  14. 14. Gbit Ethernet <ul><li>uses standard Ethernet frame format </li></ul><ul><li>allows for point-to-point links and shared broadcast channels </li></ul><ul><li>in shared mode, CSMA/CD is used; short distances between nodes required for efficiency </li></ul><ul><li>uses hubs, called here “Buffered Distributors” </li></ul><ul><li>Full-Duplex at 1 Gbps for point-to-point links </li></ul><ul><li>10 Gbps now ! </li></ul>
  15. 15. Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Wireless links, characteristics, CDMA </li></ul><ul><li>IEEE 802.11 wireless LANs </li></ul>
  16. 16. Hubs <ul><li>Hubs are essentially physical-layer repeaters: </li></ul><ul><ul><li>bits coming from one link go out all other links </li></ul></ul><ul><ul><li>at the same rate </li></ul></ul><ul><ul><li>no frame buffering </li></ul></ul><ul><ul><li>no CSMA/CD at hub: adapters detect collisions </li></ul></ul><ul><ul><li>provides net management functionality </li></ul></ul><ul><ul><ul><li>can disconnect a malfunctioning adapter </li></ul></ul></ul>twisted pair hub
  17. 17. Interconnecting with hubs <ul><li>Pros: </li></ul><ul><li>Enables interdepartmental communication </li></ul><ul><li>Extends max distance btw. nodes </li></ul><ul><li>If a hub malfunctions, the backbone hub can disconnect it </li></ul><ul><li>Cons: </li></ul><ul><li>Collision domains are transferred into one large, common domain </li></ul><ul><li>Cannot interconnect 10BaseT and 100BaseT hubs </li></ul>hub hub hub hub
  18. 18. Switch <ul><li>Link layer device </li></ul><ul><ul><li>stores and forwards Ethernet frames </li></ul></ul><ul><ul><li>examines frame header and selectively forwards frame based on MAC dest address </li></ul></ul><ul><ul><li>when frame is to be forwarded on segment, uses CSMA/CD to access segment </li></ul></ul><ul><li>transparent </li></ul><ul><ul><li>hosts are unaware of presence of switches </li></ul></ul><ul><li>plug-and-play, self-learning </li></ul><ul><ul><li>switches do not need to be configured </li></ul></ul>
  19. 19. Forwarding <ul><li>How to determine onto which LAN segment to forward frame? </li></ul><ul><li>Looks like a routing problem... </li></ul>1 2 3 hub hub hub switch
  20. 20. Self learning <ul><li>A switch has a switch table </li></ul><ul><li>entry in switch table: </li></ul><ul><ul><li>(MAC Address, Interface, Time Stamp) </li></ul></ul><ul><ul><li>stale entries in table dropped (TTL can be 60 min) </li></ul></ul><ul><li>switch learns which hosts can be reached through which interfaces </li></ul><ul><ul><li>when frame received, switch “learns” location of sender: incoming LAN segment </li></ul></ul><ul><ul><li>records sender/location pair in switch table </li></ul></ul>
  21. 21. Filtering/Forwarding <ul><li>When switch receives a frame: </li></ul><ul><li>index switch table using MAC dest address </li></ul><ul><li>if entry found for destination then{ </li></ul><ul><li>if dest on segment from which frame arrived then drop the frame </li></ul><ul><li>else forward the frame on interface indicated </li></ul><ul><li>} </li></ul><ul><li>else flood </li></ul>forward on all but the interface on which the frame arrived
  22. 22. Switch example <ul><li>Suppose C sends frame to D </li></ul><ul><li>Switch receives frame from from C </li></ul><ul><ul><li>notes in bridge table that C is on interface 1 </li></ul></ul><ul><ul><li>because D is not in table, switch forwards frame into interfaces 2 and 3 </li></ul></ul><ul><li>frame received by D </li></ul>hub hub hub switch A B C D E F G H I address interface A B E G 1 1 2 3 1 2 3
  23. 23. Switch example <ul><li>Suppose D replies back with frame to C. </li></ul><ul><li>Switch receives frame from from D </li></ul><ul><ul><li>notes in bridge table that D is on interface 2 </li></ul></ul><ul><ul><li>because C is in table, switch forwards frame only to interface 1 </li></ul></ul><ul><li>frame received by C </li></ul>hub hub hub switch A B C D E F G H I address interface A B E G C 1 1 2 3 1
  24. 24. Switch: traffic isolation <ul><li>switch installation breaks subnet into LAN segments </li></ul><ul><li>switch filters packets: </li></ul><ul><ul><li>same-LAN-segment frames not usually forwarded onto other LAN segments </li></ul></ul><ul><ul><li>segments become separate collision domains </li></ul></ul>collision domain collision domain collision domain hub hub hub switch
  25. 25. Switches: dedicated access <ul><li>Switch with many interfaces </li></ul><ul><li>Hosts have direct connection to switch </li></ul><ul><li>No collisions; full duplex </li></ul><ul><li>Switching: A-to-A’ and B-to-B’ simultaneously, no collisions </li></ul>switch A A’ B B’ C C’
  26. 26. More on Switches <ul><li>cut-through switching: frame forwarded from input to output port without first collecting entire frame </li></ul><ul><ul><li>slight reduction in latency </li></ul></ul><ul><li>combinations of shared/dedicated, 10/100/1000 Mbps interfaces </li></ul>
  27. 27. Institutional network hub hub hub switch to external network router IP subnet mail server web server
  28. 28. Switches vs. Routers <ul><li>both store-and-forward devices </li></ul><ul><ul><li>routers: network layer devices (examine network layer headers) </li></ul></ul><ul><ul><li>switches are link layer devices </li></ul></ul><ul><li>routers maintain routing tables, implement routing algorithms </li></ul><ul><li>switches maintain switch tables, implement filtering, learning algorithms </li></ul>
  29. 29. Overview <ul><li>Ethernet </li></ul><ul><li>Hubs and Switches </li></ul><ul><li>Mobile and wireless networks, CDMA </li></ul><ul><li>IEEE 802.11 wireless LANs </li></ul>
  30. 30. Wireless and Mobile Networks <ul><li>Background: </li></ul><ul><li># wireless (mobile) phone subscribers now exceeds # wired phone subscribers! </li></ul><ul><li>computer nets: laptops, palmtops, PDAs </li></ul><ul><li>two important (but different) challenges </li></ul><ul><ul><li>communication over wireless link </li></ul></ul><ul><ul><li>handling mobile user who changes point of attachment to network </li></ul></ul>
  31. 31. Elements of a wireless network network infrastructure <ul><li>wireless hosts </li></ul><ul><li>laptop, PDA, IP phone </li></ul><ul><li>run applications </li></ul><ul><li>may be stationary (non-mobile) or mobile </li></ul><ul><ul><li>wireless does not always mean mobility </li></ul></ul>
  32. 32. Elements of a wireless network network infrastructure <ul><li>base station </li></ul><ul><li>typically connected to wired network </li></ul><ul><li>relay - responsible for sending packets between wired network and wireless host(s) in its “area” </li></ul><ul><ul><li>e.g., cell towers 802.11 access points </li></ul></ul>
  33. 33. Elements of a wireless network <ul><li>wireless link </li></ul><ul><li>typically used to connect mobile(s) to base station </li></ul><ul><li>multiple access protocol coordinates link access </li></ul><ul><li>various data rates, transmission distance </li></ul>network infrastructure
  34. 34. Elements of a wireless network network infrastructure <ul><li>infrastructure mode </li></ul><ul><li>base station connects mobiles into wired network </li></ul><ul><li>handoff: mobile changes base station providing connection into wired network </li></ul>
  35. 35. Elements of a wireless network <ul><li>Ad hoc mode </li></ul><ul><li>no base stations </li></ul><ul><li>nodes can only transmit to other nodes within link coverage </li></ul><ul><li>nodes organize themselves into a network: route among themselves </li></ul>
  36. 36. Wireless Link Characteristics <ul><li>Differences from wired link …. </li></ul><ul><ul><li>decreased signal strength: radio signal attenuates as it propagates through matter (path loss) </li></ul></ul><ul><ul><li>interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); </li></ul></ul><ul><ul><li>multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times </li></ul></ul><ul><li>… . make communication across (even a point to point) wireless link much more “difficult” </li></ul>
  37. 37. Wireless network characteristics <ul><li>Multiple wireless senders and receivers create additional problems (beyond multiple access): </li></ul><ul><li>Hidden terminal problem </li></ul><ul><li>B, A hear each other </li></ul><ul><li>B, C hear each other </li></ul><ul><li>A, C can not hear each other </li></ul><ul><li>means A, C unaware of their interference at B </li></ul><ul><li>Signal fading: </li></ul><ul><li>B, A hear each other </li></ul><ul><li>B, C hear each other </li></ul><ul><li>A, C can not hear each other interferring at B </li></ul>A B C A B C A’s signal strength space C’s signal strength
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