1、Chapter 6: Medium Access Control Layer,Chapter 6: Roadmap,Overview Wireless MAC protocols Carrier Sense Multiple Access， Multiple Access with Collision Avoidance (MACA) and MACAW， MACA By Invitation， IEEE 802.11， IEEE 802.15.4 and ZigBee Characteristics of MAC Protocols in Sensor Networks Energy Eff

2、iciency， Scalability， Adaptability， Low Latency and Predictability， Reliability Contention-Free MAC Protocols Contention-Based MAC Protocols Hybrid MAC Protocols,Medium Access Control,In most networks, multiple nodes share a communication medium for transmitting their data packets The medium access

3、control (MAC) protocol is primarily responsible for regulating access to the shared medium The choice of MAC protocol has a direct bearing on the reliability and efficiency of network transmissions due to errors and interferences in wireless communications and to other challenges Energy efficiency a

4、lso affects the design of the MAC protocol trade energy efficiency for increased latency or a reduction in throughput or fairness,Overview,Responsibilities of MAC layer include: decide when a node accesses a shared medium resolve any potential conflicts between competing nodes correct communication

5、errors occurring at the physical layer perform other activities such as framing, addressing, and flow control Second layer of the OSI reference model (data link layer) or the IEEE 802 reference model (which divides data link layer into logical link control and medium access control layer),X,MAC Prot

6、ocol Categorization,Contention-Free Medium Access,Collisions can be avoided by ensuring that each node can use its allocated resources exclusively Examples of fixed assignment strategies: FDMA: Frequency Division Multiple Access,Contention-Free Medium Access,Collisions can be avoided by ensuring tha

7、t each node can use its allocated resources exclusively Examples of fixed assignment strategies: TDMA: Time Division Multiple Access,This GSM digital cellular method shows how up to eight users can share a 200-kHz channel in different time slots within a frame of 1248 bits,Contention-Free Medium Acc

8、ess,Examples of fixed assignment strategies: CDMA: Code Division Multiple Access simultaneous accesses of the wireless medium are supported using different codes if these codes are orthogonal, it is possible for multiple communications to share the same frequency band forward error correction (FEC)

9、at the receiver is used to recover from interferences among these simultaneous communications,Contention-Free Medium Access,Fixed assignment strategies are inefficient it is impossible to reallocate slots belonging to one device to other devices if not needed in every frame generating schedules for

10、an entire network can be a taunting task these schedules may require modifications every time the network topology or traffic characteristics in the network change,Contention-Free Medium Access,Dynamic assignment strategies: allow nodes to access the medium on demand polling-based protocols a contro

11、ller device issues small polling frames in a round-robin fashion, asking each station if it has data to send if no data to be sent, the controller polls the next station token passing stations pass a polling request to each other (round-robin fashion) using a special frame called a token a station i

12、s allowed to transmit data only when it holds the token,Contention-Free Medium Access,Dynamic assignment strategies: allow nodes to access the medium on demand reservation-based protocols static time slots used to reserve future access to the medium e.g., a node can indicate its desire to transmit d

13、ata by toggling a reservation bit in a fixed location these often very complex protocols then ensure that other potentially conflicting nodes take note of such a reservation to avoid collisions,Contention-Based Medium Access,Nodes may initiate transmissions at the same time requires mechanisms to re

14、duce the number of collisions and to recover from collisions Example 1: ALOHA protocol uses acknowledgments to confirm the success of a broadcast data transmission allows nodes to access the medium immediately addresses collisions with approaches such as exponential back-off to increase the likeliho

15、od of successful transmissions,Contention-Based Medium Access,Nodes may initiate transmissions at the same time requires mechanisms to reduce the number of collisions and to recover from collisions Example 2: slotted-ALOHA protocol requires that a station may commence transmission only at predefined

16、 points in time (the beginning of a time slot) increases the efficiency of ALOHA introduces the need for synchronization among nodes,Contention-Based Medium Access,Carrier Sense Multiple Access (CSMA) CSMA with Collision Detection (CSMA/CD) sender first senses the medium to determine whether it is i

17、dle or busy if it is found busy, the sender refrains from transmitting packets if the medium is idle, the sender can initiate data transmission CSMA with Collision Avoidance (CSMA/CA) CSMA/CD requires that sender aware of collisions instead, CSMA/CA attempts to avoid collisions in the first place,Hi

18、dden and Exposed Terminal Problems,Hidden-terminal problem senders A and C are able to reach B, but cannot overhear each others signals it is possible for A and C to transmit data to B at the same time, causing a collision at B, without being able to directly detect this collision,Hidden and Exposed

19、 Terminal Problems,Exposed-terminal problem C wants to transmit data D, but decides to wait because it overhears an ongoing transmission from B to A Bs transmission could not interfere with data reception at C,Wireless MAC protocols Carrier Sense Multiple Access Multiple Access with Collision Avoida

20、nce (MACA) and MACAW MACA By Invitation IEEE 802.11 IEEE 802.15.4 and ZigBee,Carrier Sense Multiple Access,Nodes first sense the medium before they begin a transmission (reduces number of collisions) Non-persistent CSMA,Carrier Sense Multiple Access,1-persistent CSMA node wishing to transmit data co

21、ntinuously senses the medium for activity once the medium is found idle, the node transmits data immediately if a collision occurs, the node waits for a random period of time before attempting to transmit again,Carrier Sense Multiple Access,p-persistent CSMA node continuously senses the medium node

22、transmits data with a probability p once the medium becomes idle delays transmission with a probability 1 p random back-off values are either continuous values in the case of un-slotted CSMA or multiples of a fixed slot size in slotted CSMA,Carrier Sense Multiple Access,CSMA/CA (CSMA with Collision

23、Avoidance) nodes sense the medium, but do not immediately access the channel when it is found idle instead, a node waits for a time period called DCF interframe space (DIFS) plus a multiple of a slot size in case there are multiple nodes attempting to access the medium, the one with the shorter back

24、-off period will win,Carrier Sense Multiple Access,Example: node A waits for DIFS + 4 s (where s represents the slot size), while node Bs back-off is DIFS + 7 s once node A begins with its transmission, node B freezes its own back-off timer and resumes the timer after node A completes its transmissi

25、on plus another period of DIFS once node Bs back-off timer expires, it can also begin its transmission,MACA and MACAW,Multiple Access with Collision Avoidance (MACA) dynamic reservation mechanism sender indicates desire to send with ready-to-send (RTS) packet intended receiver responds with clear-to

26、-send (CTS) packet if sender does not receive CTS, it will retry at later point in time nodes overhearing RTS or CTS know that reservation has taken place and must wait (e.g., based on the size of data transmission) address hidden terminal problem and reduces number of collisions,MACA and MACAW,MACA

27、 for Wireless LANs (MACAW) receiver responds with acknowledgment (ACK) after data reception other nodes in receivers range learn that channel is available nodes hearing RTS, but not CTS do not know if transmission will occur MACAW uses data sending (DS) packet, sent by sender after receiving CTS to

28、inform such nodes of successful handshake,MACA and MACAW,MACA for Wireless LANs (MACAW),MACA By Invitation,In MACA-BI, destination device initiates data transfers by sending a Ready To Receive (RTR) packet to the source source then responds with the data message Compared to MACA, MACA-BI reduces ove

29、rhead increases the theoretical maximum throughput depends on the destination knowing when to receive data Source nodes can use an optional field within the data message to indicate the number of queued messages providing the destination with an indication that more RTS packets will be required,IEEE

30、 802.11,Published in 1999 by the Institute of Electrical and Electronics Engineers (IEEE) specifies the physical and data link layers of the OSI model for wireless connections Often referred to as Wireless Fidelity (Wi-Fi) certification given by Wi-Fi Alliance, a group that ensures compatibility bet

31、ween hardware devices that use the 802.11 standard Wi-Fi combines concepts found in CSMA/CA and MACAW, but also offers features to preserve energy,IEEE 802.11,Two modes of operation Point Coordination Function (PCF) mode communication among devices goes through a central entity called an access poin

32、t (AP) or base station (BS): managed mode Distributed Coordination Function (DCF) mode devices communicate directly with each other: ad-hoc mode,IEEE 802.11,IEEE 802.11 is based on CSMA/CA before a node transmits, it first senses the medium for activity the node is allowed to transmit, if the medium

33、 is idle for at least a time period called the DCF interframe space (DIFS) otherwise the device executes a back-off algorithm to defer transmission to a later time this algorithm randomly selects a number of time slots to wait and stores this value in a back-off counter for every time slot that pass

34、es without activity on the network, the counter is decremented and the device can attempt transmission when this counter reaches zero if activity is detected before the counter reaches zero, the device waits until the channel has been idle for a period of DIFS before it continues to decrement the co

35、unter value,IEEE 802.11,After a successful transmission receiver device responds with an acknowledgment after waiting for a time period called the short interframe space (SIFS) the value of SIFS is smaller than the value of DIFS to ensure that no other device accesses the channel before the receiver

36、 can transmit its acknowledgment Once a node A makes a reservation using RTS and CTS control messages another neighboring node B, overhearing the RTS message, must refrain from accessing the medium until node As transmission has been completed and acknowledged however, this would mean that node B ha

37、s to continuously sense the medium to detect when it becomes idle again,Node ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type

38、) If the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness),IEEE 802.11

39、,After a successful transmission receiver device responds with an acknowledgment after waiting for a time period called the short interframe space (SIFS) the value of SIFS is smaller than the value of DIFS to ensure that no other device accesses the channel before the receiver can transmit its ackno

40、wledgment Once a node A makes a reservation using RTS and CTS control messages another neighboring node B, overhearing the RTS message, must refrain from accessing the medium until node As transmission has been completed and acknowledged however, this would mean that node B has to continuously sense

41、 the medium to detect when it becomes idle again,Sending unicast packets node has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors,Sender,Rece

42、iver,Others,IEEE 802.11,Instead, senders RTS message carries the size of the data it will transmit allowing receiver to estimate how long the transmission will take and to decide whether to enter a low-power sleep mode some neighboring nodes may only overhear CTS (but not RTS), therefore, the data s

43、ize is also carried in the CTS message using the data size information, neighboring nodes set a network allocation vector (NAV) that indicates how long the medium will be unavailable reduces the need for continuously sensing the medium, allowing a node to save power,IEEE 802.11,Sender,Receiver,Other

44、s,IEEE 802.11,PCF mode access point (AP) coordinates channel access to ensure collision-free communication periodically broadcasts a beacon to its client devices (includes list of devices with data pending at AP) during contention-free period, AP transmits these packets to its client devices AP can

45、also poll client devices to allow them to initiate data transfers AP uses a wait period called the PCF interframe space (PIFS) PIFS is shorter than DIFS, but longer than SIFS ensures that PCF traffic has priority over traffic generated by devices operating in the DCF mode, without interfering with c

46、ontrol messages in the DCF mode such as CTS and ACK,IEEE 802.11,Focus of IEEE 802.11 is on providing fair access to the medium with support for high throughput and mobility since devices spend a large amount of time listening to the medium and collisions occur frequently, this standard incurs large

47、overheads, including significant energy costs Energy consumption problem IEEE 802.11 offers a power saving mode (PSM) for devices operating in the PCF mode devices can inform the AP that they wish to enter a low-power sleep mode using special control messages these devices wake up periodically to re

48、ceive beacon messages from the AP to determine if they should stay awake to receive incoming messages saves energy, but only works in the infrastructure mode and it is not specified when or how long devices should sleep,IEEE 802.15.4,IEEE 802.15.4 created for low-power devices in the 868 MHz, 915 MH

49、z, and 2.45 GHz frequency bands supports two transmission modes:,UWB： Ultra wideband. CSS: Chirp Spread Spectrum.,ZigBee,Before 802.15.4, ZigBee Alliance worked on low-cost communication technology for low data rates and low power consumption IEEE and ZigBee Alliance joined forces and ZigBee has bec

50、ome the commercial name for the IEEE 802.15.4 technology Star mode: communication via the Personal Area Network (PAN) coordinator,ZigBee,Peer-to-peer mode: devices are free to communicate directly with each other but they still must associate with the PAN coordinator before they can participate in p

51、eer-to-peer communication Data transfer between the device and its PAN coordinator is always initiated by the device,IEEE 802.15.4 and ZigBee,Challenges: standard does not clearly define the operation of the peer-to-peer approach in large WSNs, it is unlikely that all devices will be able to use the

52、 same PAN coordinator standard does allow communication among PAN coordinators, but this again is not well defined,Chapter 6: Roadmap,Overview Wireless MAC protocols Carrier Sense Multiple Access， Multiple Access with Collision Avoidance (MACA) and MACAW， MACA By Invitation， IEEE 802.11， IEEE 802.15

53、.4 and ZigBee Characteristics of MAC Protocols in Sensor Networks Energy Efficiency， Scalability， Adaptability， Low Latency and Predictability， Reliability Contention-Free MAC Protocols Contention-Based MAC Protocols Hybrid MAC Protocols,Characteristics of MAC Protocols in WSNs,Most MAC protocols ar

54、e built for fairness everybody should get an equal amount of resources no one should receive special treatment In a WSN, all nodes cooperate to achieve a common purpose, therefore fairness is less of a concern Instead, wireless nodes are mostly concerned with energy consumption Sensing applications

55、may value low latency or high reliability over fairness,Energy Efficiency,Sensor nodes must operate using finite energy sources, therefore MAC protocols must consider energy efficiency Common technique: dynamic power management (DPM) a resource can be moved between different operational modes such a

56、s active, idle, and asleep for resources such as the network, the active mode can group together multiple different modes of activity, e.g., transmitting and receiving Periodic traffic models are very common in WSNs significant energy savings can be obtained by putting a device into a low-power slee

57、p mode fraction of time a sensor nodes spends in active mode is called the duty cycle often very small due to the infrequent and brief data transmissions occurring in most sensor networks,Energy Efficiency,Characteristics of typical radios used by state-of-the-art sensor nodes,Energy Efficiency,Reas

58、ons for energy inefficiency idle listening inefficient protocol designs (e.g., large packet headers) reliability features (collisions requiring retransmissions or other error control mechanisms) control messages to address the hidden-terminal problem choice of modulation scheme choice of transmissio

59、n rate overemitting,Scalability,Many wireless MAC protocols have been designed for use in infrastructure-based networks access points or controller nodes arbitrate access to the channel and perform some centralized coordination and management functions Most wireless sensor networks rely on multi-hop and peer-to-peer communications without centralized coordinators MAC protocols must be able to allow for efficient use of resources without incurring unacceptable overheads, particularly in very large networks MAC protocols based on CDMA have to cache a large number of code (may be i