1. Review: MAC

2. Data Link Layer5-2 Link Layer: Introduction 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 physically adjacent node over a link

3. Data Link Layer5-3 Link layer: context 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 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

4. Data Link Layer5-4 Link Layer Services framing, link access:  encapsulate datagram into frame, adding header, trailer  channel access if shared medium  “MAC” addresses used in frame headers to identify source, dest different from IP address! reliable delivery between adjacent nodes  we learned how to do this already (chapter 3)!  seldom used on low bit-error link (fiber, some twisted pair)  wireless links: high error rates Q: why both link-level and end-end reliability?

5. Data Link Layer5-5 Link Layer Services (more) 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

6. Data Link Layer5-6 Multiple Access Links and Protocols Two types of “links”: point-to-point  PPP for dial-up access  point-to-point link between Ethernet switch and host broadcast (shared wire or medium)  old-fashioned Ethernet  upstream HFC  802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical)

7. Data Link Layer5-7 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 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

8. Data Link Layer5-8 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized:  no special node to coordinate transmissions  no synchronization of clocks, slots 4. simple

9. Data Link Layer5-9 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning  divide channel into smaller “pieces” (time slots, frequency, code)  allocate piece to node for exclusive 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

10. Data Link Layer5-10 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 1 3 4 1 3 4 6-slot frame

11. Data Link Layer5-11 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 pkt, frequency bands 2,5,6 idle frequency bands time FDM cable

12. Data Link Layer5-12 Random Access 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 (e.g., via delayed retransmissions) Examples of random access MAC protocols:  slotted ALOHA  ALOHA  CSMA, CSMA/CD, CSMA/CA

13. Data Link Layer5-13 Slotted ALOHA Assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot  if no collision: node can send new frame in next slot  if collision: node retransmits frame in each subsequent slot with prob. p until success

14. Data Link Layer5-14 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

15. Data Link Layer5-15 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives  transmit immediately collision probability increases:  frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]

16. Data Link Layer5-16 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don’t interrupt others!

17. Data Link Layer5-17 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

18. Data Link Layer5-18 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA  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: received signal strength overwhelmed by local transmission strength human analogy: the polite conversationalist

19. Data Link Layer5-19 CSMA/CD collision detection

20. Data Link Layer5-20 “Taking Turns” MAC protocols channel partitioning MAC protocols:  share channel efficiently and fairly at high load  inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! random access MAC protocols  efficient at low load: single node can fully utilize channel  high load: collision overhead “taking turns” protocols look for best of both worlds!

21. Data Link Layer5-21 “Taking Turns” MAC protocols Polling: master node “invites” slave nodes to transmit in turn typically used with “dumb” slave devices concerns:  polling overhead  latency  single point of failure (master) master slaves poll data

22. Data Link Layer5-22 “Taking Turns” MAC protocols Token passing:  control token passed from one node to next sequentially.  token message  concerns:  token overhead  latency  single point of failure (token) T data (nothing to send) T

23. Data Link Layer5-23 Summary of MAC protocols channel partitioning, by time, frequency or code  Time Division, Frequency Division random access (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 taking turns  polling from central site, token passing  Bluetooth, FDDI, IBM Token Ring

24. Introduction: Wireless/Mobile

25. Computers for the next decades? Computers are integrated  small, cheap, portable, replaceable - no more separate devices Technology is in the background  computer are aware of their environment and adapt (“location awareness”)  computer recognize the location of the user and react appropriately (e.g., call forwarding, fax forwarding, “context awareness”)) Advances in technology  more computing power in smaller devices  flat, lightweight displays with low power consumption  new user interfaces due to small dimensions  more bandwidth per cubic meter  multiple wireless interfaces: wireless LANs, wireless WANs, regional wireless telecommunication networks etc. („overlay networks“)

26. Mobile communication Two aspects of mobility:  user mobility: users communicate (wireless) “anytime, anywhere, with anyone”  device portability: devices can be connected anytime, anywhere to the network Wireless vs. mobile Examples  stationary computer  notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA) The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks:  local area networks: standardization of IEEE 802.11, ETSI (HIPERLAN)  Internet: Mobile IP extension of the internet protocol IP  wide area networks: e.g., internetworking of GSM and ISDN

27. Applications I Vehicles  transmission of news, road condition, weather, music via DAB  personal communication using GSM  position via GPS  local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy  vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance Emergencies  early transmission of patient data to the hospital, current status, first diagnosis  replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.  crisis, war,...

28. Typical application: road traffic ad hoc UMTS, WLAN, DAB, DVB, GSM, cdma2000, TETRA,... Personal Travel Assistant, PDA, Laptop, GSM, UMTS, WLAN, Bluetooth,...

29. Mobile and wireless services – Always Best Connected UMTS 2 Mbit/s UMTS, GSM 384 kbit/s LAN 100 Mbit/s, WLAN 54 Mbit/s UMTS, GSM 115 kbit/s GSM 115 kbit/s, WLAN 11 Mbit/s GSM/GPRS 53 kbit/s Bluetooth 500 kbit/s GSM/EDGE 384 kbit/s, DSL/WLAN 3 Mbit/s DSL/ WLAN 3 Mbit/s

30. Applications II Travelling salesmen  direct access to customer files stored in a central location  consistent databases for all agents  mobile office Replacement of fixed networks  remote sensors, e.g., weather, earth activities  flexibility for trade shows  LANs in historic buildings Entertainment, education,...  outdoor Internet access  intelligent travel guide with up-to-date location dependent information  ad-hoc networks for multi user games History Info

31. Location dependent services Location aware services  what services, e.g., printer, fax, phone, server etc. exist in the local environment Follow-on services  automatic call-forwarding, transmission of the actual workspace to the current location Information services  „push“: e.g., current special offers in the supermarket  „pull“: e.g., where is the Black Forrest Cherry Cake? Support services  caches, intermediate results, state information etc. „follow“ the mobile device through the fixed network Privacy  who should gain knowledge about the location

32. Mobile devicesperformance Pager receive only tiny displays simple text messages Mobile phones voice, data simple graphical displays PDA graphical displays character recognition simplified WWW Palmtop tiny keyboard simple versions of standard applications Laptop/Notebook fully functional standard applications Sensors, embedded controllers www.scatterweb.net

33. Effects of device portability Power consumption  limited computing power, low quality displays, small disks due to limited battery capacity  CPU: power consumption ~ CV 2 f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally Loss of data  higher probability, has to be included in advance into the design (e.g., defects, theft) Limited user interfaces  compromise between size of fingers and portability  integration of character/voice recognition, abstract symbols Limited memory  limited value of mass memories with moving parts  flash-memory or ? as alternative

34. Wireless networks in comparison to fixed networks Higher loss-rates due to interference  emissions of, e.g., engines, lightning Restrictive regulations of frequencies  frequencies have to be coordinated, useful frequencies are almost all occupied Low transmission rates  local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS Higher delays, higher jitter  connection setup time with GSM in the second range, several hundred milliseconds for other wireless systems Lower security, simpler active attacking  radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones Always shared medium  secure access mechanisms important

35. Areas of research in mobile communication Wireless Communication  transmission quality (bandwidth, error rate, delay)  modulation, coding, interference  media access, regulations ... Mobility  location dependent services  location transparency  quality of service support (delay, jitter, security) ... Portability  power consumption  limited computing power, sizes of display,...  usability ...

36. Simple reference model used here Application Transport Network Data Link Physical Medium Data Link Physical Application Transport Network Data Link Physical Data Link Physical Network Radio

37. Influence of mobile communication to the layer model  service location  new applications, multimedia  adaptive applications  congestion and flow control  quality of service  addressing, routing, device location  hand-over  authentication  media access  multiplexing  media access control  encryption  modulation  interference  attenuation  frequency Application layer Transport layer Network layer Data link layer Physical layer

38. Overlay Networks - the global goal regional metropolitan area campus-based in-house vertical handover horizontal handover integration of heterogeneous fixed and mobile networks with varying transmission characteristics