Introduction to Wireless Networking. Communication without Wires

Transcription

1 Introduction to Wireless Networking Communication without Wires

2 Overview Wireless and Mobile Network Architecture Wireless Link Characteristics IEEE Wireless LAN (WLAN) Cellular Networks

3 Wireless and Mobile Network Architecture

4 Wireless Is Everyone s Favorite Number of wireless (mobile) phone subscribers now exceeds number of wired phone subscribers (5-to)! Number of wireless Internet-connected devices exceeds number of people on Earth! Laptops, smartphones, tablets promise anytime untethered Internet access Two important (but different) challenges Wireless: Communication over wireless access link Mobility: Maintaining Internet connectivity for a mobile device that changes its point of attachment to the access network

5 Architecture of a wireless network network infrastructure

6 Devices network infrastructure Wireless Hosts Laptop, smartphone, tablet Run applications Limited battery life if mobile May be stationary (non-mobile) or mobile Wireless does not always mean mobility

7 Infrastructure network infrastructure Base Station Typically connected to wired network Or point to point wireless backhaul Relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers, access points

8 Radio Access Network Link network infrastructure Wireless Link Typically used to connect mobile(s) to base station Also used as backbone link Multiple access protocol coordinates link access Various data rates, transmission distance

9 Infrastructure Mode network infrastructure Infrastructure Mode Base station connects mobiles into wired network Handoff: mobile changes base station providing connection into wired network May or may not change IP subnet

10 Ad hoc Mode Ad hoc mode No base stations Nodes can only transmit to other nodes within link coverage Nodes organize themselves into a network: route among themselves

11 A Word about Ad hoc Routing Latencies in mobile ad hoc networks can get very high Each hop adds more latency Movement means routing tables need continual adjustment Peculiarities of wireless link (next up) make maintaining connectivity challenging Ad hoc networks have limited application One or two hop sensor networks Military applications (drones, etc.) Small sized fixed ad hoc meshes Maximum size of ad hoc network: 3 8 hops depending on application Practical size is 2 hops for mobile Fixed ad hoc backbones can extend The Internet

12 Wireless Link Characteristics

13 Bandwidth of selected wireless links ac Data rate (Mbps) n a,g b 4G: LTE a,g point-to-point 3G: UMTS/WCDMA/HSPDA, CDMA2000xEVDO 2.5G: GSM-Edge, CDMA2000 2G: IS-95, CDMA, GSM 5G peak Rate: Gbps! Indoor 10-30m cellular protocol in regulated spectrum IEEE WLAN in unregulated spectrum Outdoor m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km

14 Wireless Network Taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net, cellular relay no base station, no connection to larger Internet. May have to relay to reach other a given wireless node Signal propagation characteristics of wireless links makes maintaining connectivity a significant

15 Basic Sources of Signal Loss on Wireless Link Signal power decreases in proportion to the distance from the sender: For free space propagation: True of wired signals as well but not as steep Interference from other sources Devices using the same band Other devices communicating Microwave ovens for WLAN on 2.4 GHz band Wireless phones, garage door openers, baby monitors Radio frequency interference (RFI) inadvertently radiated Solar panel inverters and power supplies AC motors and wiring Controlling interference is the Number One task of radio protocols!

16 Signal Loss from Motion and the Urban Environment Multipath Interference Radio signal reflects off objects Arrives at a slightly different time Reduces signal strength Doppler Shift As the mobile device moves, the frequency of the signal changes slightly Changes channel response

17 Other Problems with Wireless Channels Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal strength C s signal strength Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B space Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B

18 Signal to Noise Ratio (SNR) and Bit Error Ratio (BER) SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs Given physical layer: increase power -> increase SNR->decrease BER Given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) High Level Modulation Scheme

19 Techniques for Controlling Interference Geographical Separation Divide the geographic area into cells and use some property of radio to isolate devices in a cell Frequency Spreading Code Channel Multiplexing Devices share the channel by: Either: Each device gets a fine grained access identifier (timeslot, spreading code, etc.) Or: Each device is responsible for arranging its transmission such that the channel is contention-free

20 Geographical Separation Break geographic coverage area into cells Power falls off quickly as distance Use channel multiplexing within and between cells to allow devices within a cell to share frequency Within a cell, devices are distinguished by their multiplexing identifier When the mobile moves to a new cell, change multiplexing identifier Handover When signal strength goes below a certain point, switch over new cell s base station Fine grained technique distinguishes

21 Access and Frequency Most cellular wireless protocols use different frequencies for uplink and downlink GSM, 3G, FD-LTE Exception: TD-LTE uses time division duplex to split the same channel between uplink and downlink For operators who don t have licenses for paired spectrum Wireless LAN protocols use different frequencies to reduce interference between cells , a, b, n, ac CSMA/CA uses to share channel between uplink and downlink GSM900 Uplink and Downlink European g channels, 2.4 GHz Band (22 MHz channels, 5 MHz center separation) Source: Source:

22 Channel Multiplexing Time Division Multiple Access (TDMA) Each device gets a specific time slot Code Division Multiple Access (CDMA) Each device gets a specific spreading code Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Check if someone is using channel first, random backoff if so Frequency Hopping Spread Spectrum (FHSS) Device changes frequency according to a prenegotiated random sequence Likelihood of interference is low Also slow frequency hopping Secondary technique Orthogonal Frequency Division Multiple Access (OFDMA) Break bandwidth down into separate subcarriers Assign each device a particular collection of subcarriers Particularly well suited to MIMO (Multiple Input Multiple Output) Best Bits per Hertz

23 MIMO Efficient Utilization of Multiple Antennas MIMO: Multiple Input Multiple Output Around 2005, realization that multiple antennas could increase throughput Use multipath propagation to your advantage! Maybe even signal steering to follow mobile Base station has lots of antennas Device only has a couple Use of MIMO approximately doubles throughput! MIMO incorporated into all recent wireless standards HSPA, LTE, n, ab Source:

24 IEEE Wireless LAN (WLAN)

25 Wireless LAN Standard * * * Notes: * indicates uses MIMO DSSS = Direct Sequence Spread Spectrum, SC = Single Carrier all use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) for multiple access all have base-station and ad-hoc network versions Source:

26 LAN architecture BSS 1 Internet hub, switch or router BSS 2 wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only

27 802.11: Channels, association Example b/g: 2.4GHz-2.485GHz spectrum in US Divided into 11 channels in US, 13 in Europe, 14 in Japan Access Point (AP) admin chooses channel for AP Interference with neighboring AP Possible if both on same channel Even if not on same channel because only one group of 3 channels do not overlap For a, 24 channels do not overlap Host must associate with an AP Host scans channels, listening for beacon frames containing AP s name (Service Set IDentifier, SSID) and MAC address Selects AP to associate with May perform authentication Runs DHCP to get IP address in AP s subnet If AP on same subnet as previous, no need to change address

28 802.11: passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 AP AP 2 AP AP 2 H1 Passive Scanning: (1)Beacon frames sent from APs (2)Association Request frame sent: H1 to selected AP (3)Association Response frame sent from H1 Active Scanning: (1)Probe Request frame broadcast from H1 (2)Probe Response frames sent from APs (3)Association Request frame sent: H1 to selected AP

29 IEEE : Multiple Access Avoid collisions: 2+ nodes transmitting at same time : CSMA - sense before transmitting Don t collide with ongoing transmission by other node : no collision detection Difficult to receive (sense collisions) when transmitting due to weak received signals (fading) Can t sense all collisions in any case: hidden terminal, fading Goal is to avoid collisions: CSMA/C(ollision)A(voidance) A B C C A B A s signal strength C s signal strength space

30 IEEE MAC Protocol: CSMA/CA sender 1 If sense channel idle for DIFS then transmit entire frame (no CD) 2 If sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver - If frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS: Distributed Interframe Spacing SIFS: Short Interframe Spacing DIFS sender data ACK receiver SIFS

31 Avoiding Collisions for Long Data Frames Basic idea: Allow sender to reserve channel rather than random access of data frames Avoids collisions for long data frames Sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame avoid data frame collisions completely other stations defer transmissions using small reservation packets!

32 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) RTS(A) reservation collision RTS(B) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A)

33 Frame: Addressing frame control duration address 1 address 2 address 3 seq control address 4 payload CRC Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to Address 2: MAC addresswhich AP is attached of wireless host or AP transmitting this frame Address 4: used only in ad hoc mode

34 Frame: Addressing H1 R1 router Internet R1 MAC addr H1 MAC addr dest. address source address frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame

35 Frame: More duration of reserved transmission time (RTS/CTS) frame seq # (for RDT) frame control duration address 1 address 2 address 3 seq control address 4 payload CRC Protocol version Type Subtype To AP From AP More frag 1 1 Power Retry mgt More data WEP Rsvd frame type (RTS, CTS, ACK, data)

36 802.11: Mobility within same Subnet H1 remains in same IP subnet: IP address can remain same If the subnet changes then some IP mobility technique must be used switch: which AP is associated with H1? MAC learning: switch will see frame from H1 and remember which switch port can be used to reach H1 BBS 1 H1 BBS 2

37 802.11: advanced capabilities Rate adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point BER SNR(dB) 1. SNR decreases, BER increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER

38 802.11: advanced capabilities Power Management Node-to-AP: I am going to sleep until next beacon frame AP knows not to transmit frames to this node node wakes up before next beacon frame Beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame

39 Cellular Networks

40 The Gs :Most Commonly Deployed The G Approximat e Deployment Date Protocol Modulation/ Access Maximum Data Rate Maximum Latency 1G 1980 s US,AUS AMPS SE,NO,FI,CH,NE,RU NMT UK,DE- TACS, C-450 JP TZ-80x, JTACS, Many others Analog, later TDMA for US (IS36) Predigital: none US IS36: 48.6 kbps 1-5 s 2G 1990 for voice 2000 for data EU,rest of the world ex. JP and US: GSM US: IS-95 Japan: PDC A few others GSM/GPRS: TDMA IS-95: CDMA PDC: TDMA GPRS: kbps IS-95: kbps PDC: kbps 1-5 s 3G 2000 World : UMTS, EDGE US, Korea, few others: IS-2000 EDGE: TDMA UMTS: WCDMA IS-2000: CDMA EDGE: kbps UMTS: Mbps IS-2000: 153 kbps4.7 Mbps ms 4G 2010 World: LTE US, Korea, few others: WiMax LTE: OFDMA downlink SC-FDMA uplink WiMax: OFDMA LTE-Advanced: 500 MbpsGbps WiMax: 5628 Mbps ms 5G Planned for 2022 Unknown Probably some version of LTE Likely 10 Gbps <1 ms

41 Comparing WiFi with Cellular Cellular Standardized by 3GPP Licensed Spectrum Varies depending on G Mobile data rate up to 100 Mbps for trains Cell size to 10 s of km Expensive RBS Pico/microcell BS available Radio resource management co-ordinates radio use between cells Extensive access network with mobility management Support for policy, QoS, charging Inherited from legacy telephone network Standardized by IEEE Unlicensed Spectrum ISM Bands: 2.4 GHz, 5 GHz, 60 GHz Fixed data rate up to 1 Gbps walking speed Cell size to 100 m Cheap APs WiFi No radio resource management between APs Access network is Ethernet w. L2 mobility only No support for policy and charging QoS support in 802.1

42 Evolved Packet Core : Access Network for Cellular Link types EPC supports: 2-4G radio access networks Through 3G UTRAN Packet Core for 2-3G 4G Packet Core is new WiFi, WiMax in trusted and untrusted forms trusted means it uses the operator s AAA system IS-2000 US CDMA 2K standard Wired Broadband Probably not much deployment Radio Resource Management Cells co-ordinate use of spectrum between themselves Share information about: Spreading codes for CDMA/WCDMA Subcarriers for OFDMA Reduces interference, improves reception quality

43 What Cellular Has That WiFi Doesn t Mobility management Control plane GTP-C officially GPRS Tunnelling Protocol but long ago left GSM behind Data plane GTP-U PMIP Proxy Mobile IP Dual Stack Mobile IP Policy, QoS, Charging Policy control points For WiFi and WiMax Ability to establish different QoS at radio, MAC, and IP level Charging and billing control points

44 Detailed EPC Architecture 2&3G WiFi & WiMax We focuswired here Broadband IS-2000 Source: Olsson, et. al., EPC and 4G Packet Networks, Academic Press, 2013

45 Basic EPC For LTE Offline Charging System (OFCS) Online Charging System (OCS) Operator WAN/Internet Gz Gy Rx S10 Home Subscriber Serivce (HSS) S6a Mobility Management Entity (MME) S1-MME User Equipment (UE) X2 S11 Serving Gateway (SGW) enode B SGi Packet Data Network Gateway (PGW) S5/S8 S1-U LTE Radio Access Gx Gxc Policy Control And Rating Function (PCRF) We focus on mobility management S9

46 EPC Mobility Management: More than Just IP Mobility Terminals that are not actively communicating are notified about incoming traffic Allows terminals to go into power saving Idle Mode, paged when traffic comes in Terminal can initiate traffic towards services on the Internet and other users This is IP Ongoing sessions are maintained as Mobility the Management user moves within or between access technologies Through anchored IP mobility management This is Idle Mode packet delivery

47 IP Mobility Management Basics If an end host moves, its IP address must remain the same to maintain session connectivity IP addresses are bound into a topology Choice #1: Use /32 host routes, change routing in routers & switches as the host moves Nobody does this Requires too much bandwidth on control plane and makes all the RIBs/FIBs too big Choice #2: Anchor host IP address back in the network, tunnel packets to host, move wireless end as host moves Everybody does this Minimizes control update messages and confines large RIB size to the anchor EPC uses bearers to tunnel packets as host moves

48 Bearers 3GPP s term for a virtual network slice dedicated to a single IP address But there can be multiple TCP & UDP flows running over a single bearer Comes with a traffic classification for IP network and radio access QoS Every UE gets a default bearer with Best Effort traffic classification If a UE accesses a media service like voice calling, an enhanced bearer with Enhanced QoS is brought up. Single direction only Uplink or downlink Radio Channel GTP-U Source:

49 GTP-U GTP-U is the tunnel protocol for EPC Like VxLAN or GRE UPD protocol: The GTP-U port is 2152 Encapsulation Header Fields Version - Version number, 1 PT Protocol type, differentiates GTP (1) from GTP (0) Sp reserved, set to 0 E set to 1 if there is an extension header S set to 1 if there is a sequence number PN set to 1 if there is an N-PDU Message type indicates type of GTP message Length length of payload Tunnel Endpoint IDentifier (TEID) Identifies the endpoint of the tunnel Sequence number Number increasing in value N-PDU used by the SGW for Radio Access Network handover Extension Header Type Type of the next extension header Source:

50 Paging for LTE: Packet Delivery to Idle Mode UE For xy PGW Operator WAN/Internet xy where are you? xy where are you? SGW Broadcast on Paging Channel for xy! Packet for xy and he s asleep! MME xy xy where are you? xy where are you? Hmm. He last reported in in the Blue Tracking Area Please reactivate my default bearer!

51 Idle Mode Packet Delivery UE goes into Idle Mode when not in use Still listening to control channels but not transmitting Saves power Tracking area Collection of cells in which network is tracking a UE as it moves Independent of L2 LAN and L3 subnet Size and configuration depends on radio reception and geographic area enodebs advertise tracking area ID in their control channel messages When a UE moves from one tracking area to another, updates network about its location Paging When a packet arrives at the PGW for an idle mode UE, a locating procedure is instituted in the UE s last reported tracking area Utilizes a special paging channel to which all idle mode UEs listen

52 Paging for LTE: Packet Delivery to Idle Mode UE For xy Huh? Oh, a packet Zzzz! must have arrived! xy where are you? xy where are you? PGW SGW Broadcast on Paging Channel for xy! Operator WAN/Internet Packet for xy and he s asleep! MME xy xy where are you? xy where are you? Hmm. He last reported in in the Blue Tracking Area Please reactivate my default bearer!

53 Paging for LTE:Tracking Area Update (TAU) Blue Tracking Area (TA) enodeb: TA Green UE: Whoops! Need to enodeb: TA Blue change UE: No need to change I ve changed to Green! TAU update succeeded Sure, Can you here send it is. me UE xy s Done. context? Old MME HSS Cancel xy s context! MME xy Green Done. Here s Cancel Tracking xy s xy s context subscriber in Area data, Old (TA) MME! Note: Paging systems are Radio Access Technology (RAT) dependent, 2 & 3G are slightly different

54 Summary Wireless is a particularly challenging medium Interference Multi-path Clever coding tricks have increased channel capacity steadily in the last 20 years From less than 100Kbps to over 6 Gbps is a cheap and popular link technology Uses CSMA/CA for controlling device access With RTS/CTS as a secondary technique Variety of different modulation techniques FHSS, DSSS, OFDM, SC Connection into wired link is through Ethernet MAC learning handles mobility management at link layer

55 Acknowledgements Kurose and Ross, Computer Networking: A Top Down Approach, University of Massachusetts

56 Additional Slides

57 Wireless Channel Model The Wireless Channel C S S = sent signal amplitude R = received signal amplitude N = noise I = interference R = F(S,N,I) C = G(S,N,B) R C = channel capacity B = channel bandwidth

58 Statistical Model for Channel Response: Rayleigh Fading Rayleigh distribution used to model channel in urban environments Lots of scattering Lots of multipath where: r = envelope amplitude of received signal 2σ2 = predetection mean power of multipath signal Source:

59 Information Theory: Shannon-Hartley Theorem Maximum rate at which information can be transmitted over a channel of a specific bandwidth in the presence of noise Establishes the channel capacity Where C = channel capacity in bits / second B = bandwidth of channel in hertz S = average signal power in watts N = the noise power in watts If channel rate R < C, then intelligent coding techniques can improve the rate

60 Code Division Multiple Access (CDMA) Unique code assigned to each user; i.e., code set partitioning All users share same frequency in each cell, but each user has own chipping sequence (i.e., code) to encode data Allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) Encoded signal = (original data) X (chipping sequence) Decoding: inner-product of encoded signal and chipping sequence 6-60

61 CDMA encode/decode sender data bits code d1 = d0 = slot 1 slot 0 channel output Zi,m Zi,m= di.cm slot 1 channel output slot 0 channel output received input receiver code slot 1 slot 0 Di = Σ Zi,m.cm m=1 M M d1 = slot 1 channel output d0 = 1 slot 0 channel output

62 CDMA: two-sender interference Sender 1 channel sums together transmissions by sender 1 and 2 Sender 2 using same code as sender 1, receiver recovers sender 1 s original data from summed channel data!