Midterm Review
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
Access Networks DSL Cable networks Home networks Enterprise networks Wireless access networks
Network Core Packet Switching Circuit Switching Pros and Cons
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
Performance Metrics Packet Delay Packet Loss Throughput
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, 802.111 (WiFi), PPP physical: bits on the wire application transport network link physical
Physical Layer
Overview of Wireless Transmissions bit stream sender source coding channel coding modulation analog signal bit stream source decoding channel decoding receiver demodulation
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
What is Shannon Channel Capacity? What is Signal, Noise, and Interference? db and power conversion
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
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
Multiplexing Goal: multiple use of a shared medium Multiplexing in 4 dimensions space(s i ) time(t) frequency (f) code(c)
Modulation and Demodulation analog baseband digital signal data digital analog 101101001 modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier
Digital Modulation Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) BPSK (Binary Phase Shift Keying) QPSK (Quadrature Phase Shift Keying) 1 0 1 1 0 1 1 0 1 t t t Quadrature Amplitude Modulation (QAM) Q 0010 0011 φ a 0001 0000 I 1000
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)
802.11 and 802.15.4 MAC
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
Random Access Protocols Comparison among random access MAC protocols: Pure ALOHA Slotted ALOHA CSMA CSMA/CD
802.11: Infrastructure STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA 2 802.11 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
802.11: Ad hoc mode 802.11 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 4 802.11 LAN
802.11 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
Question Is 802.11 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 802.11 use? What are the back off mechanisms in DCF and MACAW?
Types 802.11 -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 2 2 6 6 6 6 Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 From DS More Data WEP Order CRC
802.11 -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
IEEE 802.15.4 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
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
Management Service Access to the PIB Association / disassociation GTS allocation Message pending Node notification Network scanning/start Network synchronization/search
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)