# Outline / Wireless Networks and Applications Lecture 9: Wireless LANs Aloha and 802 Wireless. Regular Ethernet CSMA/CD

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1 Page 1 Outline / Wireless Networks and Applications Lecture 9: Wireless LANs Aloha and 802 Wireless Peter Steenkiste Data link fundamentals» And what changes in wireless Aloha Ethernet Wireless-specific challenges and wireless standards Spring Semester Peter A. Steenkiste, CMU 1 Peter A. Steenkiste, CMU 2 Regular Ethernet CSMA/CD Carrier Sense Multiple Access/ Collision Detection (CSMA/CD) Multiple Access: multiple hosts are competing for access to the channel Carrier-Sense: make sure the channel is idle before sending listen before you send Collision Detection: collisions are detected by listening on the medium and comparing the received and transmitted signals Collisions results in 1) aborting the colliding transmissions and 2) retransmission of the packets Exponential backoff is used to reduce the chance of repeat collisions» Also effectively reduces congestion Peter A. Steenkiste, CMU 3 Discard Packet Packet? Sense Carrier Send attempts < 16 attempts == 16 Detect Collision Jam channel b=calcbackoff() ; wait(b); attempts++; Peter A. Steenkiste, CMU 4 No Yes

2 Ethernet Backoff Calculation How to Handle Transmission When Line is Sensed Busy Challenge: how do we avoid that two nodes retransmit at the same time collision Exponentially increasing random delay» Infer number senders from # of collisions» More senders increase wait time First collision: choose K from {0,1}; delay is K x 512 bit transmission times After second collision: choose K from {0,1,2,3} After ten or more collisions, choose K from {0,1,2,3,4,,1023} p-persistent scheme:» Transmit with probability p once the channel goes idle» Delay the transmission by t prop with the probability (1-p) 1-persistent scheme: p = 1» E.g. Ethernet nonpersistent scheme:» Reschedule transmission for a later time based on a retransmission delay distribution (e.g. exp backoff)» Senses the channel at that time» Repeat the process When is each solution most appropriate? Peter A. Steenkiste, CMU 5 Peter A. Steenkiste, CMU 6 Collisions Dealing with Collisions A B C Time Collisions will happen: nodes can start to transmit simultaneously» Vulnerability window depends on length of wire Recovery requires that both transmitters can detect the collision reliably» Clearly a problem as shown on previous slide How can we guarantee detection?» Packets must be long enough and,» Wires must be short enough» This guarantees that ALL nodes will see both packets simultaneously, i.e., see the collision» Not really relevant to wireless Peter A. Steenkiste, CMU 7 Peter A. Steenkiste, CMU 8 Page 2

3 So What about Wireless? Summary Depends on many factors, but high level: Random access solutions are a good fit for data in the unlicensed spectrum» Lower control complexity, especially for contention-based protocols (e.g., Ethernet)» There may not always be a centralized controller» May need to support multi-hop» Also used in many unlicensed bands Cellular uses scheduled access» Need to be able to guarantee performance» Have control over spectrum simplifies scheduled access» More on this later in the course Wireless uses the same types of protocols as wired networks» But it is inherently a multiple access technology Some fundamental differences between wired and wireless may result in different design choices» Higher error rates» Must support variable bit rate communication» Signal propagation and radios are different Peter A. Steenkiste, CMU 10 Peter A. Steenkiste, CMU 11 Outline Wireless Ethernet is a Good Idea, but Data link fundamentals» And what changes in wireless Aloha Ethernet Wireless-specific challenges and wireless standards Peter A. Steenkiste, CMU 12 Attenuation varies with media» Also depends strongly on distance, frequency Wired media have exponential dependence» Received power at d meters proportional to 10 -kd» Attenuation in db = k d, where k is db/meter Wireless media has logarithmic dependence» Received power at d meters proportional to d -n» Attenuation in db = n log d, where n is path loss exponent; n=2 in free space» Signal level maintained for much longer distances? But we are ignoring the constants!» Wireless attenuation at 2.4 GHz: db» In practice numbers can be much lower for wired Peter A. Steenkiste, CMU 13 Page 3

4 Implications for Wireless Ethernet Hidden Terminal Problem Collision detection is not practical» Ratio of transmitted signal power to received power is too high at the transmitter» Transmitter cannot detect competing transmitters (is deaf while transmitting)» So how do you detect collisions? Not all nodes can hear each other» Ethernet nodes can hear each other by design» Listen before you talk often fails» Hidden terminals, exposed terminals,» Capture effects Made worse by fading» Changes over time! S1 CTS RTS R1 CTS Lack signal between S1 and S2 and cause collision at R1 Severity of the problem depends on the sensitivity of the carrier sense mechanism» Clear Channel Assessment (CCA) threshold S2 R2 Peter A. Steenkiste, CMU 14 Peter A. Steenkiste, CMU 15 Exposed Terminal Problem Capture Effect R R1 S1 S1 S2 Carrier sense prevents two senders from sending simultaneously although they do not reach each other s receiver Severity again depends on CCA threshold» Higher CCA reduces occurrence of exposed terminals, but can create hidden terminal scenarios Peter A. Steenkiste, CMU 16 S2 R2 Sender S2 will almost always win if there is a collision at receiver R. Can lead to extreme unfairness and even starvation. Solution is power control» Very difficult to manage in a non-provisioned environment! Peter A. Steenkiste, CMU 17 Page 4

5 Page 5 Wireless Packet Networking Problems Summary Some nodes suffer from more interference than others» Node density» Traffic volume sent by neighboring nodes Leads to unequal throughput Similar to wired network: some flows traverse tight bottleneck while others do not Peter A. Steenkiste, CMU 18 Wireless signal propagation creates problems for wireless Ethernet» Collision Detection is not possible» Hidden and exposed terminals» Capture effect Aloha was the first wireless data communication protocol» Simple: send whenever you want to» Has low latency but low capacity Peter A. Steenkiste, CMU 19 Outline History Data link fundamentals» And what changes in wireless Ethernet Aloha Wireless-specific challenges and wireless standards» 802 protocol overview» Wireless LANs » Personal Area Networks Peter A. Steenkiste, CMU 20 Aloha wireless data network Car phones» Big and heavy portable phones» Limited battery life time» But introduced people to mobile networking» Later turned into truly portable cell phones Wireless LANs» Originally in the 900 MHz band» Later evolved into the standard» Later joined by the and standards Cellular data networking» Data networking over the cell phone» Many standards throughput is the challenge Peter A. Steenkiste, CMU 21

7 Outline IEEE Overview 802 protocol overview Wireless LANs » Overview of » MAC, frame format, operations» management» *» Deployment example Personal Area Networks Adopted in 1997 with goal of providing» Access to services in wired networks» High throughput» Highly reliable data delivery» Continuous network connection, e.g. while mobile The protocol defines» MAC sublayer» MAC management protocols and services» Several physical (PHY) layers: IR, FHSS, DSSS, OFDM Wi-Fi Alliance is industry group that certifies interoperability of products Peter A. Steenkiste, CMU 26 Peter A. Steenkiste, CMU 27 Infrastructure and Ad Hoc Mode Architecture Infrastructure mode: stations communicate with one or more access points which are connected to the wired infrastructure» What is deployed in practice AP ESS Existing Wired LAN AP Two modes of operation:» Distributed Control Functions - DCF Our Focus» Point Control Functions PCF» PCF is rarely used - inefficient Alternative is ad hoc mode: multi-hop, assumes no infrastructure» Rarely used, e.g. military» Hot research topic! Infrastructure Network Ad Hoc Network BSS BSS: Basic Service Set ESS: Extended Service Set BSS BSS BSS Ad Hoc Network Peter A. Steenkiste, CMU 28 Peter A. Steenkiste, CMU 29 Page 7

8 Terminology for DCF Outline Stations and access points BSS - Basic Service Set» One access point that provides access to wired infrastructure» Infrastructure BSS ESS - Extended Service Set» A set of infrastructure BSSs that work together» APs are connected to the same infrastructure» Tracking of mobility DS Distribution System» AP communicates with each other» Thin layer between LLC and MAC sublayers 802 protocol overview Wireless LANs » Overview of » MAC, frame format, operations» management» *» Deployment example Personal Area Networks Peter A. Steenkiste, CMU 30 Peter A. Steenkiste, CMU 31 How Does WiFi Differ from Wired Ethernet? Features of MAC protocol Signal strength drops off quickly with distance» Path loss exponent is highly dependent on context Should expect higher error rates» Solutions? Makes it impossible to detect collisions» Difference between signal strength at sender and receiver is too big» Solutions? Senders cannot reliably detect competing senders resulting in hidden terminal problems» Solutions? Supports MAC functionality» Addressing» CSMA/CA Error detection (FCS) Error correction (ACK frame) Flow control: stop-and-wait Fragmentation (More Frag) Collision Avoidance (RTS-CTS) Peter A. Steenkiste, CMU 32 Peter A. Steenkiste, CMU 33 Page 8

9 Page 9 Carrier Sense Multiple Access DCF mode transmission without RTS/CTS Before transmitting a packet, sense carrier If it is idle, send» After waiting for one DCF inter frame spacing (DIFS) If it is busy, then» Wait for medium to be idle for a DIFS (DCF IFS) period» Go through exponential backoff, then send (non-persistent solution)» Want to avoid that several stations waiting to transmit automatically collide» Cost of back off is high and expect a lot of contention Wait for ack» If there is one, you are done» If there isn t one, assume there was a collision, retransmit using exponential backoff DIFS source destination other Data NAV SIFS Must defer access Ack DIFS CW Random backoff Peter A. Steenkiste, CMU 34 Peter A. Steenkiste, CMU 35 Exponential Backoff Collision Avoidance Force stations to wait for random amount of time to reduce the chance of collision» Backoff period increases exponential after each collision» Similar to Ethernet If the medium is sensed it is busy:» Wait for medium to be idle for a DIFS (DCF IFS) period» Pick random number in contention window (CW) = backoff counter» Decrement backoff timer until it reaches 0 But freeze counter whenever medium becomes busy» When counter reaches 0, transmit frame» If two stations have their timers reach 0; collision will occur; After every failed retransmission attempt:» increase the contention window exponentially» 2 i 1 starting with CW min up to CW max e.g., 7, 15, 31, Peter A. Steenkiste, CMU 36 Difficult to detect collisions in a radio environment» While transmitting, a station cannot distinguish incoming weak signals from noise its own signal is too strong Why do collisions happen?» Near simultaneous transmissions Period of vulnerability: propagation delay» Hidden node situation: two transmitters cannot hear each other and their transmission overlap at a receiver S1 CTS RTS Data Peter A. Steenkiste, CMU 37 R1 CTS S2

10 Page 10 Request-to-Send and Clear-to-Send Virtual Carrier Sense Before sending a packet, first send a station first sends a RTS» Collisions can still occur but chance is relatively small since RTS packets are short The receiving station responds with a CTS» Tells the sender that it is ok to proceed RTS and CTS use shorter IFS to guarantee access» Effectively priority over data packets First introduced in the Multiple Access with Collision Avoidance (MACA) protocol» Fixed problems observed in Aloha Peter A. Steenkiste, CMU 38 RTS and CTS notify nodes within range of sender and receiver of upcoming transmission Stations that hear either the RTS or the CTS remember that the medium will be busy for the duration of the transmission» Based on a Duration ID in the RTS and CTS» Note that they may not be able to hear the data packet! Virtual Carrier Sensing: stations maintain Network Allocation Vector (NAV)» Time that must elapse before a station can sample channel for idle status» Consider the medium to be busy even if it cannot sense a signal Peter A. Steenkiste, CMU 39 Use of RTS/CTS Some More MAC Features Use of RTS/CTS is controlled by an RTS threshold» RTS/CTS is only used for data packets longer than the RTS threshold» Pointless to use RTS/CTS for short data packets high overhead! Number of retries is limited by a Retry Counter» Short retry counter: for packets shorter than RTS threshold» Long retry counter: for packets longer than RTS threshold Packets can be fragmented.» Each fragment is acknowledged» But all fragments are sent in one sequence» Sending shorter frames can reduce impact of bit errors» Lifetime timer: maximum time for all fragments of frame Peter A. Steenkiste, CMU 40 Peter A. Steenkiste, CMU 41

11 Features of MAC protocol Supports MAC functionality» Addressing» CSMA/CA Error detection (FCS) Error correction (ACK frame) Flow control: stop-and-wait Fragmentation (More Frag) Collision Avoidance (RTS-CTS) Peter A. Steenkiste, CMU 42 Page 11

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