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1 Midterm Review

2 Internet Structure network edge: Hosts: clients and servers Server often in data centers access networks, physical media:wired, wireless communication links network core: interconnected routers network of networks mobile network institutional network home network global ISP regional ISP

3 Access Networks DSL Cable networks Home networks Enterprise networks Wireless access networks

4 Network Core Packet Switching Circuit Switching Pros and Cons

5 Internet structure: network of networks Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP access ISP access ISP access ISP access ISP access ISP access ISP access ISP access ISP at center: small # of well-connected large networks tier-1 commercial ISPs(e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider network (e.g, Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs

6 Performance Metrics Packet Delay Packet Loss Throughput

7 Internet protocol stack application:supporting network applications FTP, SMTP, HTTP transport: process data transfer TCP, UDP network:routing of datagrams from source to destination IP, routing protocols link:data transfer between neighboring network elements Ethernet, (WiFi), PPP physical: bits on the wire application transport network link physical

8 Physical Layer

9 Overview of Wireless Transmissions bit stream sender source coding channel coding modulation analog signal bit stream source decoding channel decoding receiver demodulation

10 Frequencies for Communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 µm 3 THz 1 µm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Frequency and wave length: λ = c/f, wave length λ, speed of light c 3x10 8 m/s, frequency f

11 What is Shannon Channel Capacity? What is Signal, Noise, and Interference? db and power conversion

12 Signal Propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) Receiving power additionally influenced by shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges shadowing reflection refraction scattering diffraction

13 Path Loss Free space model Two-ray ground reflection model P ( d) = Log-normal shadowing Indoor model PGG t t rht d L r 4 P ( d)[ dbm] = r 2 h P ( d) = 2 r 2 PG t tgrλ 2 (4π ) d L r 2 P ( d)[ dbm] 10n log( t PL ( d)[ db] = PL( d) + d d 0 nw * WAF ) C * WAF X σ d PL ( d)[ db] = PL( d0) + 10n log( ) + d 0 nw nw < X σ C C

14 Multiplexing Goal: multiple use of a shared medium Multiplexing in 4 dimensions space(s i ) time(t) frequency (f) code(c)

15 Modulation and Demodulation analog baseband digital signal data digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier

16 Digital Modulation Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) BPSK (Binary Phase Shift Keying) QPSK (Quadrature Phase Shift Keying) t t t Quadrature Amplitude Modulation (QAM) Q φ a I 1000

17 Spread spectrum technology Principles power interference spread signal power detection at receiver signal spread interference f f DSSS (Direct Sequence Spread Spectrum) FHSS (Frequency Hopping Spread Spectrum)

18 and MAC

19 MAC Protocols: a taxonomy Channel Partitioning divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use Examples TDMA: partition time slots FDMA: partition frequency Random Access allow collisions recover from collisions

20 Random Access Protocols Comparison among random access MAC protocols: Pure ALOHA Slotted ALOHA CSMA CSMA/CD

21 802.11: Infrastructure STA 1 ESS LAN BSS 1 Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA LAN STA 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Access Point station integrated into the wireless LAN and the distribution system Basic Service Set (BSS) group of stations using the same AP Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS

22 802.11: Ad hoc mode LAN Direct communication within a limited range STA 1 IBSS 1 STA 2 STA 3 Station (STA): terminal with access mechanisms to the wireless medium IBSS 2 STA 5 Independent Basic Service Set (IBSS): group of stations using the same network STA LAN

23 MAC Layer Distributed and centralized access methods DCF CSMA/CA (mandatory) collision avoidance via randomized back-off mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem PCF (optional) access point polls terminals according to a list

24 Question Is a MAC protocol or PHY protocol? How does Pure Aloha access the medium? How does slotted Aloha access the medium? What are the major issues with Aloha? What is CSMA? What MAC protocol does Ethernet use? Why is collision detection hard in wireless networks? What MAC protocol does use? What are the back off mechanisms in DCF and MACAW?

25 Types Frame format control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses Sender, receiver, BSS identifier Miscellaneous bytes sending time, checksum, frame control, data Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt Address Data 4 bits From DS More Data WEP Order CRC

26 MAC management Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network Synchronization timing Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements MIB - Management Information Base managing, read, write

27 IEEE MAC overview Star networks: devices are associated with coordinators Forming a PAN, identified by a PAN identifier Coordinator Bookkeeping of devices, address assignment, generate beacons Talks to devices and peer coordinators Beacon-mode superframe structure GTS assigned to devices upon request b 27

28 Data Service Data transfer to neighboring devices Acknowledged or unacknowledged Direct or indirect Using GTS service Maximum data length (MSDU) amaxmacframesize (102 bytes) Slide 28

29 Management Service Access to the PIB Association / disassociation GTS allocation Message pending Node notification Network scanning/start Network synchronization/search

30 Channel Access Mechanism In non beacon-enabled networks - unslotted CSMA/CA channel access mechanism In beacon-enabled networks - slotted CSMA/CA channel access mechanism Based on a basic time unit called BackoffPeriod (BP) = aunitbackoffperiod = 80 bits (0.32 ms)

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