Topic: Wireless and Mobile Networks

Topic: Wireless and Mobile Networks What you will learn IEEE 802.11 Mobile IPv6 IEEE 802.11e Centralized Architectures and CAPWAP A glance at VANETs 1/77 Elements of a wireless network Coverage area network infrastructure wireless hosts laptop, smartphone may be stationary (nonmobile) or mobile wireless does not always mean mobility 2/77

Elements of a wireless network (cont.) Coverage area network infrastructure wireless link used to connect a wireless host to a base station or to another wireless host also used as backbone link A MAC protocol coordinates link access various data rates, transmission range 3/77 Elements of a wireless network (cont.) transmission range Coverage area network infrastructure base station typically connected to a wired network relay - responsible for sending and receiving packets to and from a wireless host associated with it e.g., cell towers, 802.11 access points 4/77

Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into a wired network handoff: mobile changes base station handoff 5/77 Elements of a wireless network 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 6/77

Wireless network taxonomy infrastructure (e.g., APs) no infrastructure single hop Wireless hosts connect to a base station, which connects to a larger wired network, e.g., the Internet (WiFi, WiMAX, cellular networks) There is no base station, no connection to a larger wired network (Bluetooth) multiple hops There is a base station, which is wired to the larger network Wireless hosts may have to relay their communication through other wireless nodes to communicate via the base station Wireless Mesh Networks No base station. Hosts may have to relay messages in order to reach a destination (MANET, VANET) MANET: Mobile Ad hoc NETwork VANET: Vehicular Ad hoc NETwork 7/77 Some popular wireless network standards Key characteristics: coverage area and link rate Data rate (Mbps) 1300 802.11ac 600 802.11n 54 802.11a,g 5-11 802.11b 4 1 802.15.384.056 4G: LTE 802.11a,g point-to-point Enhanced 3G: HSPA 3G: UMTS/WCDMA, CDMA2000 2G: IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km transmission range 8/77

The Wireless Channel A radio channel between a transmitter unit u and a receiver unit v is established if and only if the power P r of the radio signal received by node v is above a certain threshold β P r > β β depends on the features of the wireless transceiver and on the communication data rate (the higher the data rate, the higher β) The received power P r depends on the transmission power P t and on the Path Loss PL(u,v), which models the radio signal degradation with distance P r = P t PL(u,v) The Free Space propagation model assumes the ideal propagation condition that there is only one clear Line-Of-Sight (LOS) path between the transmitter and receiver The path loss PL(u,v) is proportional to d 2, with d being the distance between u and v 9/77 Signal propagation Usually there is no clear line-of-sight path Receiving power additionally influenced by other mechanisms (frequency dependent), such as: Atmosphere (weather conditions) Shadowing (the higher the frequency, the more a signal behaves like light) Reflection Scattering Diffraction (at edges) 10/77

Multi-path propagation Signal can take many different paths between sender and receiver The signal reaches a receiver directly and phase shifted (delay spread) Distorted signal depending on the phases of the different parts Time dispersion: signal is dispersed over time Inter Symbol Interference (ISI): it limits the bandwidth of a radio channel If the receiver knows the delays of the different paths, it can compensate for the distorsion caused by the channel The sender can first transmit a training sequence known by the receiver The receiver then compares the received signal to the original training sequence and programs an equalizer used to compensate for the distortion 11/77 Effects of mobility Due to mobility, channel characteristics change over time and location If changes are too fast, the receiver cannot adapt fast enough the parameters of the equalizer and, thus, the error rate of transmission increases dramatically Quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away Slow changes in the average power received (long term fading) Typically, senders can compensate for long-term fading by increasing/decreasing sending power The received power always stays within certain limits 12/77

Antenna Diversity Due to multipath fading, the electromagnetic field distribution in two points that are few centimetres apart may be very different Antenna diversity technique can be used to try to solve the problem Access Point with two antennas RX1 RX2 13/77 Bit Error Rate-BER and Signal-to-Noise Ratio-SNR larger SNR easier to extract signal from noise SNR versus BER tradeoffs given physical layer: increase power -> increase SNR- >decrease BER given SNR: choose physical layer that meets BER requirement, giving the highest throughput The SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10 20 30 40 SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) 14/77

Motivation for a specialized MAC Can we apply media access methods from fixed networks? Example CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks The signal strength of the signal received is typically very small compared to the strength of the transmitted signal It is costly to build hardware that simultaneously transmits and listens in order to detect a collision Collision detection would not work due to the hidden terminal problem Since wireless networks do not use CD, once a station begins to transmit a frame, it transmit the frame entirely 15/77 Motivation - hidden and exposed terminals Hidden terminals: A and C cannot hear each other because of obstacles, signal attenuation A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C C A B A B C Exposed terminals B sends to A, C wants to send to another terminal (neither A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B 16/77

WiFi: IEEE 802.11 Wireless LANs Several standards: 802.11 legacy, 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac 802.11 802.11a 802.11b 802.11g Frequency ISM 2,4 GHz U-NII 5 GHz ISM 2,4 GHz ISM 2,4 GHz Data Rate 1, 2 6, 9, 12, 18, 24, 36, 48, 54 1, 2, 5.5, 11 1, 2, 5.5, 11, 6, 9, 12, 15, 24, 36, 48, 54 Physical layer FHSS, DSSS OFDM DSSS DSSS/OFDM Modulation BPSK, QPSK BPSK, QPSK, 16/QAM, 64/QAM BPSK, QPSK, CCK BPSK, QPSK, CCK, 16/QAM, 64/QAM Compatibility NONE NONE 802.11 802.11, 802.11b IEEE 802.11n uses Multiple-In Multiple Out (MIMO) technology In order to mitigate RF impairments and enhance transmission rate, multiple antennas on both the sending side and the receiving side transmit/receive different signals RF band: 2.4 GHz, 5 GHz Data rates from 54 Mbit/s to 600 Mbit/s (4x4 MIMO) IEEE 802.11ac, ratified in June 2013, uses frequencies in the 5GHz band The first generation of 802.11ac products will only support 3x3:3 MIMO design implementations, resulting in a 1.3 Gbps per radio data rate capability Eventually, data rates up to 6.9 Gbps (8x8 MIMO) should be supported 17/77 IEEE 802.11 - Infrastructure network STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA 2 802.11 LAN STA 3 Station (STA) Basic Service Set (BSS) group of stations using the same radio frequency BSS Identifier (BSSID): MAC address of the Access Point Access Point Portal bridge to other (wired) networks Distribution System interconnection network to form a logical network (EES: Extended Service Set) based on several BSS Each ESS is identified by a Service Set IDentifier (SSID) The SSID is included in beacons sent by the AP (see later) 18/77

IEEE 802.11 - Ad-hoc network STA 1 802.11 LAN IBSS 1 STA 2 IBSS 2 STA 3 Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency for an IBSS, the SSID is chosen by the client device that starts the network STA 5 STA 4 802.11 LAN 19/77 802.11b/g Channels and EIRP 20/77

802.11b/g Channels and EIRP (cont.) Maximum EIRP (Equivalent Isotropic Radiated Power) for IEEE802.11b 100 mw (20 dbm) IEEE802.11g 50 mw (17 dbm) When installing an access point, a network administrator assigns an SSID (displayed when you view available wireless networks on your notebook) and a channel number Sets of non-overlapping channels (in Europe) 1-6 - 11 2-7 - 12 3-8 - 13 Maximum transmission rate of 33 Mbps by installing three 802.11b APs at the same physical location (Co-location) 21/77 Joining a BSS with AP A STA willing to join a BSS must get in contact with the AP Passive scanning The station scans the channels for a Beacon frame that is periodically (normally, 100ms) broadcast by every AP Beacons include the Timestamp, the Beacon Interval, the SSID Actually, the beacons are not always periodic because a beacon is also deferred if the medium is busy (see later) Active scanning (the station tries to find an AP)! " #!" $ $ % & 22/77

Joining a BSS with AP (cont.) An STA selects one of the APs for association It may be required to authenticate itself The IEEE 802.11i (WPA2 for WiFi Alliance) standard secures WLANs % '()' ( * It provides for an extensible set of authentication mechanisms IEEE 802.1x + EAP (Extensible Authentication Protocol) Typically, username and password are employed The RADIUS server and protocol are de facto standard components for 802.11i AP: access point STA: client station wired network AS: Authentication server (RADIUS) EAPoL - Extensible Authentication Protocol over LAN 23/77 Joining a BSS with AP (cont.) Association process STA AP: Association Request frame AP STA: Association Response frame BBS 1 BBS 2 BBS 1 BBS 2 AP 1 1 1 1 AP 2 2 2 AP 1 AP 2 2 3 3 4 H1 H1 Passive scanning 2: Association Request frame 3: Association Response frame Active scanning (broadcast probe) The new AP informs the old AP via the DS IEEE 802.1f standardizes the exchange of information between APs to support roaming (handover, handoff) Only after the association is completed, a station can transmit and receive data frames 24/77

IEEE 802.11 MAC Protocol Performs the following functions: MAC Protocol Data Unit (MPDU) address MPDU (frame) format Error control Resource reservation Data segmentation and reassembly Three frame types: 1. Control: positive ACK, handshaking for accessing the channel (RTS, CTS) 2. Data Transfer: information to be transmitted over the channel 3. Management: synchronization, authentication, roaming, power management, 25/77 Data transfer services Asynchronous Data Service (mandatory) for delay-tolerant traffic exchange of data packets based on best-effort support of broadcast and multicast Two access methods, summarized as Distributed Coordination Function (DCF) Time-Bounded Service (optional) for real-time traffic implemented using a third access method, called Point Coordination Function (PCF) PCF is based on the polling of the stations and controlled by the AP (Point Coordinator) ) * The Point Coordinator splits the time in superframes ( + $,)+,* + )+* only asynchronous data service is offered 26/77

Time Slot Time is divided into slots A slot is the system time unit and its duration depends on the implementation of the physical layer. For example: 802.11g: 20 s 802.11a: 9 s Stations are synchronized with the AP in the infrastructure mode and among each other in the ad hoc mode the system is synchronous Synchronization is maintained through beacon frames (remember that beacons include the timestamp) 27/77 Inter-Frame Spaces (IFSs) The waiting time before medium access is controlled through different inter-frame spaces (priorities of medium access) SIFS (Short IFS) the highest priority level, for ACK, CTS, polling response PIFS (PCF IFS) = SIFS + 1 slot time medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) = SIFS + 2 slot times the lowest priority, for asynchronous data service EIFS (Extented IFS) = SIFS+ TxTime (ACK) + DIFS a node defers the transmission by a fixed duration whenever it detects an erroneous frame on the medium, for asynchronous data service DIFS medium busy DIFS PIFS SIFS contention next frame direct access if medium is free DIFS t 28/77

Basic DCF using CSMA/CA DIFS DIFS contention window (randomized back-off mechanism) medium busy next frame direct access if medium is free DIFS slot time CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance A station (wireless station or an AP) ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is idle for at least the duration of DIFS, the station can access the medium at once This allows for short access delay under light load if the medium is busy, the station has to wait for DIFS, then the station must additionally wait a random back-off time (multiple of slot-time) chosen within a Contention Window (CW) Collision Avoidance The station counts down. If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness!!!) t 29/77 Basic DCF- simple version DIFS DIFS bo e bo r DIFS bo e bo r DIFS bo e busy station 1 bo e busy station 2 station 3 busy bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time The figure describes the complete access mechanism for broadcast frames 30/77

IEEE 802.11 unicast data transfer An additional feature is provided for unicast data transfer The receiver of the data transmission answers with an acknowledgement (ACK) at once (after waiting for SIFS) if the packet was received correctly (CRC) If no ACK is returned, the sender automatically retransmits the frame sender DIFS data receiver SIFS ACK other stations waiting time DIFS contention data t 31/77 Exponential backoff For a retransmission a new random backoff time is chosen Retransmissions are not privileged The system tries to adapt to the current number of stations trying to send The number of slots is a random variable uniformly distributed in [0,CW-1] The contention window starts with a size of, e.g., CW min = 7 Each time a collision occurs, the contention window doubles up to a maximum of, e.g., CW max = 255 For i=1, CW1 = CWmin For i>1, CWi =2 CWi-1 with i>1 being the number of consecutive attempts for transmitting the MPDU For any i, CWi CWmax 32/77

DCF with RTS/CTS handshaking sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t After waiting for DIFS (plus a random backoff time if the medium was busy) the sender can issue a Request To send (RTS) The RTS packet includes the receiver address and the duration of the whole data transmission (in the duration field) Every station receiving this RTS has to set its Net Allocation Vector (NAV) in accordance with the duration field 33/77 DCF with RTS/CTS handshaking (cont.) After waiting for SIFS, the receiver of the data transmission answers with a Clear To Send (CTS) Based on duration field, every station receiving this CTS has to adjust its NAV Finally, the sender can send the DATA after SIFS After SIFS, the receiver of the DATA acknowledges if the transfer was correct RTS/CTS is a virtual reservation scheme (virtual carrier sensing) Designed to solve the hidden terminal problem Collisions can only occur at the beginning while the RTS is sent Using RTS/CTS can result in a non-negligible overhead An RTS threshold can determine when to use the additional mechanism (basically at larger frame size) and when to disable it (short frames) Area covered by RTS/CTS RTS/CTS works well, as all stations are within the transmission range of the AP RTS A CTS B C A is hidden for C C is hidden for A 34/77

802.11 frame duration of reserved transmission time (RTS/CTS) used to filter duplicates frame control 2 2 6 6 6 2 6 0-2312 4 duration address 1 address 2 address 3 seq control address 4 payload CRC 2 2 4 1 1 1 1 1 1 1 1 Protocol To From More Power More Type Subtype Retry WEP Rsvd version AP AP frag mgt data frame type (RTS, CTS, ACK, DATA) 35/77 802.11 frame: addressing (cont.) scenario to AP from address 1 address 2 address 3 address 4 AP ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address. It is the MAC address of the receiving AP within the DS TA: Transmitter Address. It is the MAC address of the transmitting AP within the DS 36/77

802.11 frame: addressing (cont.) H1 R1 router Internet R1 MAC addr H1 MAC addr 802.11 frame dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 physical receiver logical and physical sender logical receiver 37/77 Mobility in the same IP subnet H1 moves from BSS1 to BSS2. Remaining in same IP subnet, H1 keeps its IP address and all of its ongoing TCP connections switch: which AP is associated with H1? Backward-Learning: the switch will see the frame from H1 and remember which switch port can be used to reach H1 BBS1 H1 BBS2 38/77

Moving within the Internet: MIPv6 Mobile IPv6 allows devices to be reachable and maintain ongoing connections (e.g., FTP) while moving within the Internet topology Correspondent node Home Agent Subnet A Internet IPv6 IPv6 router Mobile node Subnet B 39/77 MIPv6 terminology Mobile Node (MN): a mobile node is a node that changes its location within the Internet topology Correspondent Node (CN): any node that communicates with the mobile node Home address: a stable address that belongs to the mobile node and is used by correspondent nodes to reach mobile nodes based on the 64-bit prefix assigned to the home link combined with the mobile node s interface identifier used also to allow a mobile node to be reachable by having a stable entry in the DNS Home link: a link to which the home address prefix is assigned Home Agent (HA): a router located on the home link that acts as a proxy for the mobile node while away from the home link. The home agent redirects packets addressed to a mobile node s home address to its current location (care-of address) using IP-in-IP tunneling Foreign link: any link (other than the home link) visited by a mobile node 40/77

MIPv6 terminology (cont.) Care-of Address (CoA): an address that is assigned to the mobile node when located in a foreign link formed based on stateless or stateful (DHCP) autoconfiguration Binding: the association of the mobile node s home address with a care-of address for a certain period of time The binding is refreshed (if the timer expires) or updated when the mobile node gets a new care-of address Binding cache: a cache containing a number of bindings for one or more mobile nodes A binding cache is maintained by both the correspondent node and the home agent Binding Update List: a list maintained by the mobile node containing all bindings that were sent to the mobile node s home agent and correspondent nodes maintained in order for the mobile node to know when a binding needs to be refreshed 41/77 Overview of MIPv6 While at home, the MN uses its permanent IP address (home address) When the mobile node moves from its home link to a foreign link, it first forms a CoA based on the prefix of the foreign link announced through the router advertisements Following address configuration, the mobile node informs its HA of such movement by sending a Binding Update (BU) message Correspondent node Home Agent Subnet A Internet IPv6 IPv6 router Binding Update Subnet B 42/77

Overview of MIPv6 (cont.) The home agent validates the message and, if the binding update is accepted, it creates an entry for the binding information in its binding cache (or updates an already existing one) Then, it acknowledges the binding update sent by the MN Correspondent node Home Agent Subnet A Internet IPv6 IPv6 router Binding Ack Subnet B 43/77 Overview of MIPv6 (cont.) The home agent sends a proxy neighbor advertisement (actually, more than once! Why?) addressed to the all-nodes multicast address on the link (FF02::1) The advertisement includes the mobile node s home address in the target address field and the home agent s link-layer address -. / 0 /$ The Home Agent flag is set in a router advertisement to indicate that the router sending this router advertisement is also functioning as a Mobile IPv6 home agent on this link The home agent also defends the mobile node s addresses (remember the DAD procedure related to IPv6) Upon receiving a packet addressed to the mobile node s home address, the home agent tunnels it to the mobile node s care-of address, which is included in the mobile node s binding cache entry (IPv6-in-IPv6) 44/77

Overview of MIPv6 (cont.) The tunnel entry point is the home agent (source address in the outer header), and the tunnel exit point is the mobile node s care-of address The tunnel is bidirectional The home agent may choose to secure the tunnel to the mobile node using an Authentication Header or ESP, depending on local policies within the home network Correspondent node Home Agent Subnet A Internet IPv6 IPv6 router tunnel Subnet B 45/77 Overview of MIPv6 (cont.) Routing packets through the home agent adds additional delays and uses more network bandwidth than direct communication Routing optimization is about routing packets between a mobile node and a correspondent node, using the shortest possible path The mobile node informs the correspondent node of its current location The correspondent node maintains a binding cache similar to the one maintained by the home agent. Correspondent node Home Agent Subnet A binding update and binding Ack Internet IPv6 IPv6 packets Subnet B 46/77

IEEE 802.11e Enhancement to 802.11 in order to support QoS A station using 802.11e is called QoS Enhanced Station (QSTA) An AP using 802.11e is called QoS Access Point (QAP) QSTA e QAP works within a QoS Basic Service Set (QBSS) The two coordination functions DCF e PCF are substituted by a single Hybrid Coordination Function (HCF), composed of EDCA (Enhanced Distributed Channel Access), contention based, conceived to provide some stochastic level of differentiation HCCA (HCF Coordinated Channel Access), polling based, provides collision free periods with guaranteed assignment and deterministic differentiation 47/77 802.11e: EDCA Evolution of CSMA/CA DCF, with the add-on of traffic classes to support QoS and differentiate traffic Designed to support frames with the same eight priority levels of 802.1p, but mapping them on only four Access Categories (ACs) Each 802.11e QSTA & QAP MUST have four separated AC queues Each AC queue is FIFO and behaves independently from the others as far as the CSMA/CA MAC protocol is concerned 48/77

802.11e: EDCA (cont.) ACs are differentiated based on their CSMA/CA parameters: IFS CWmin CWmax Backoff exponent Higher priority ACs are assigned parameters that result in shorter CWs so that a statistical advantage is gained in accessing the channel Protocol parameters become vectors AIFS[AC] CWmin[AC] CWmax[AC] bck[ac] CWnew[AC] = (CWold[AC] +1) bck[ac] -1 * Contention Window update 49/77 802.11e: EDCA (cont.) Arbitration InterFrame Space (AIFS) substitute the common DIFS Each AIFS is at least DIFS long Before entering the backoff procedure each Virtual Station will have to wait AIFS[AC], instead of DIFS 50/77

802.11e: EDCA (cont.) Each AC queue behaves like a different virtual station (independent sensing and backoff) If the backoff counters of two or more parallel ACs in the same QSTA reach zero at the same time, a scheduler inside the QSTA avoids the collision by granting the Transmission Opportunity (TXOP) to the AC with the highest priority TXOP is the time interval in which a STA may use the channel EDCA bursting advantage: better fairness among the same priority queues The lowest priority colliding behaves as if there were an external collision 51/77 Autonomous Architecture The Access Points (Fat AP) completely implement and terminate the 802.11 function so that frames on the wired LAN are 802.3 frames The AP is an addressable node Fat APs can provide router-like functions, such as DHCP server capabilities Management of the AP is done through SNMP or HTTP or CLI (Command Line Interface) Fat APs also have enhanced capabilities such as Access Control Lists (ACLs), QoS-related functions, such as enforcing IEEE 802.1p priority or DSCP (DiffServ) The downside of such APs is complexity, so that they have uses only in small network installations 52/77

Centralized Architecture APs are mounted in hard-to-reach areas, so that they can provide optimum radio connectivity for end stations Network managers prefer to install APs just once and not have to perform complex maintenance on them WLAN Controllers CAPWAP Protocol between WLAN Controller and AP for Configuration and Control Lightweight Access Points (APs) 53/77 Centralized Architecture (cont.) APs are connected to a WLAN controller (WLC) through a secure tunnel The Control and Provisioning of Wireless Access Points (CAPWAP) is the protocol used in order to communicate CAPWAP is responsible for discovery and selection of an WLC by the AP CAPWAP control packets are encrypted Wireless 802.11 frames are forwarded to the WLC encapsulated within CAPWAP data packets CAPWAP data packets encryption is possible, but this may result in severe throughput degradation 54/77

Centralized Architecture (cont.) Split MAC architecture: the implementation of the MAC functions is divided between the AP and the WLC APs are lightweight in the sense that they handle only a part of MAC functionalities Vendors differ in the type of MAC splitting between the AP and the WLC Typically, APs handle real-time MAC functions, such as beacon generation, probe transmission and response, control frame processing (RTS, CTS) and so on leave all the non real time MAC functionalities (authentication, association,...) to be processed by the WLC APs provide the wireless encryption while using the WLC for the actual key exchance APs provide additional functions such as DHCP relay for the station to obtain an IP address through DHCP APs can perform functions such as VLAN tagging based on the SSID that the client uses to associate with the AP (when the AP supports multiple SSIDs) 55/77 Centralized Architecture (cont.) The WLC manages the configurations and firmware of the controlled APs performs Radio Resource Management based on configuration and monitoring of the controlled APs Through CAPWAP control messages, the APs send statistics (number of transmit retries, number of erroneous frames, ) to the WLC For example, if two APs controlled by a WLC are interfering with each other, the WLC can send a signal to one of the APs to reduce its strength Handles QoS enforcement and ACL-based filtering Manages layer-2 and layer-3 mobility The extent of the several functions varies according to the vendor implementation On a per-building basis? On a per-campus basis 56/77

Ad Hoc Wireless Networks Collection of mobile hosts capable to form a temporary network without the support of any fixed infrastructure (infrastructureless, selforganizing, self-configuring) Network operations, such as routing and resource management, are performed in a distributed and cooperative manner Due to limited radio transmission range, multi-hop routing is usually used Each node can act as a host and as a router: a packet is forwarded from one node to another until it reaches the destination 57/77 Applications of Ad Hoc Wireless Networks Some interesting scenarios: Military applications Emergency operations Vehicular communications Underwater communications Acoustic communications 58/77

Ad Hoc Wireless Networks (cont.) Due to mobility associated with the nodes, network topology may experience continuous changes A B A B Different power levels among different nodes introduce asymmetric links Resources are typically constrained (bandwidth, battery power, etc.) 59/77 Main issues of Ad Hoc Wireless Networks Medium access scheme Routing Multicasting Transport layer protocol QoS provisioning Self-organization Security Energy management Addressing and service discovery Scalability 60/77

Effectiveness of RTS/CTS in Ad Hoc Networks The effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within the transmission range of receivers (so that they can receive CTS packet successfully) Some node out of the transmission range of the receiver may still interfere with the receiver Nodes within the interference range (R i ) of a receiver are called hidden nodes hidden node Area covered by RTS/CTS R tx tx d rx r hidden node Interference area not covered by RTS/CTS R i 61/77 Radio ranges 2010-2011 Three radio ranges related to a wireless radio: Transmission Range (R tx ): it represents the range within which a packet is successfully received if there is no interference from other radios. The transmission range is mainly determined by transmission power and radio propagation properties (i.e., attenuation). Carrier Sensing Range (R cs ): it is the range within which a transmitter triggers carrier sense detection. This range is mainly determined by the antenna sensitivity and by the transmission power. Interference Range (R i ): it defines the interference area A i = π R i2 around a receiver. All nodes located in this area are hidden nodes of the receiver. When the receiver is receiving a packet, if a hidden node starts a transmission, a collision will happen at the receiver. Transmission range and Carrier sensing range are fixed and affected by the properties of the wireless radios The interference range is not fixed, but related to the transmitter-receiver distance and can go far beyond the transmission range 62/77

Vehicular Ad Hoc Networks (VANETs) VANET Ad hoc network in which the nodes are vehicles IEEE 802.11p IEEE 802.11 amendment for the support of vehicular communications EDCA is the MAC protocol 63/77 Routing problem in VANETs The high speed of vehicles causes frequent topology changes It would be very difficult to keep routing tables updated Traffic on the wireless channel would increase because of the exchange of control messages DESTINATION SOURCE 64/77

Routing problem in VANETs The high speed of vehicles causes frequent topology changes It would be very difficult to keep routing tables updated Traffic on the wireless channel would increase because of the exchange of control messages Routeless Routing To borrow a technique thought for Intelligent Broadcasting 65/77 66/77

1. Only one node is the recipient of packets 2. All nodes that receive a given packet and are in the direction source-> destination (or last forwarder-> destination ) calculate the waiting time and participate in the contention process 3. The node whose timer first expires forwards the packet 4. Each node that overhears the transmission discards the packet 67/77 68/77

Implications for backhaul cabling The first generation of 802.11ac products will support data rate up to 1.3 Gbps The second generation of 802.11ac products is expected to support data rate up to 3.5 Gbps Operating at capacity, 802.11ac equipment is capable of far exceeding the performance provided by 1000BASE-T uplink Future offerings can be expected to utilize 10GBASE-T, or possibly dual or even quad 1000BASE-T uplinks, to support the multi-gigabit backhaul Category 6A (Class E A ) cabling should be used backhaul cabling OM3 or OM4 multimode fiber for backhaul should be considered where data rates higher than 10 Gbps are anticipated and for outdoor locations where distances are greater than 100m 69/77 Standards for access point cabling ISO/IEC TR-24704 proposes what it considered to be an optimal pattern for locating wireless access points The design is based on an array of tight-fitting hexagonal cells The coverage area of each cell is limited to a 12-meter radius TR-24704 recommends terminating the cable for each cell at an outlet located as close to the centre of the cell as possible 12 m 70/77

Standards for access point cabling (cont.) TIA TSB-162-A suggests a square grid of cabling areas, each about 18 meters wide In anticipation of the IEEE 802.11ac evolution, the revision of this standard recommends Category 6A cabling TIA-4966 recommends that AP density within large open indoor spaces be based on expected occupancy (see table) TC 71/77 Careful planning based on the RF environment, interference levels and sources, future capacity needs, cabling requirements and power needs Ideally, the cabling architecture and coverage analysis should work hand in hand to provide the maximum capacity and flexibility to meet both current and future end user needs When locating the APs, it is recommended that an RF survey be conducted to optimize the AP location(s) within a given cell Sample floor plan 72/77

An analysis of the RF environment is recommended In environments where capacity and other concerns are minimal, simply assessing RF propagation may be sufficient Some programs allow network planners to input the site s layout, conduct AP modeling, and compare RF simulations and surveys RF planning diagram with one access point deployed (grey area denotes limited or no coverage) 73/77 Signal weakness could be due to a number of variables, including the presence of RF-blocking materials such as cabinets or equipment, or structural impediments such as concrete walls Adding additional APs significantly improves coverage RF planning diagram using three APs 74/77

! " Along with ensuring adequate RF signal, the aggregate throughput should be considered The space has to be divided into grid cells, as recommended by TIA TSB-162-A or ISO/IEC TR 24704. Floor plan configured for a square-grid deployment (TIA TSB-162-A) 75/77! " AP positions and density can be adjusted to suit the occupancy RF planning diagram for high density AP deployment 76/77

# It is recommended to provide at least two cabling runs to each grid cell, as some grid cells may require two or more APs FD/HC Structured cabling diagram In addition to the RF environment and capacity planning, there are a number of other factors to consider in cabling and locating wireless access points (accessibility, power requirements, aesthetics,..) 77/77