1 / 51

شبکه‌های بی‌سیم (873-40) شبکه‌های محلی بی‌سیم با دسترسی تصادفی

شبکه‌های بی‌سیم (873-40) شبکه‌های محلی بی‌سیم با دسترسی تصادفی. نیمسال دوّم 93-92 افشین همّت یا ر. دانشکده مهندسی کامپیوتر. Introduction. FDM-TDMA Networks: Stream traffic over circuit multiplexed cellular networks.

timothy-barber
Download Presentation

شبکه‌های بی‌سیم (873-40) شبکه‌های محلی بی‌سیم با دسترسی تصادفی

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. شبکه‌های بی‌سیم (873-40)شبکه‌های محلی بی‌سیم با دسترسی تصادفی نیمسال دوّم 93-92 افشین همّتیار دانشکده مهندسی کامپیوتر

  2. Introduction • FDM-TDMA Networks: • Stream traffic over circuit multiplexed cellular networks. • A centrally coordinated mechanism for sharing the channels provides capacity on demand to a call. • The call is blocked if the requisite resource is not available. • CDMA Networks: • Allocating resources to stream traffic to satisfy in-call QoS requirements like BER. • An arriving call is blocked if the in-call QoS cannot be met. • Resource allocation for packet multiplexed elastic traffic. • OFMA-TDMA Networks: • Packet multiplexing with a centralized resource allocation of the OFDMA system. • The carrier and timeslots are the resources and, if a packet cannot be transmitted on arrival, it is queued and not dropped or blocked. • WLAN Networks: • Packet multiplexed wireless networks with queueing of packets, but with distributed resource sharing mechanisms. • Several nodes share a wireless medium with possibly no central coordination. • Distributed multiplexing using random access techniques.

  3. Overview (1) • All the nodes of network use the same part of the spectrum. • Random access based Medium Access Control (MAC) • Protocols for distributed access control. • Aloha MAC protocol analysis based on elementary probability theory and Markov chains. • Some current applications of the Aloha protocol in cellular networks and in VSAT networks. • MAC protocols for Wireless Local Area Networks (WLANs). • Hidden and exposed nodes in multihop WLANs. • Handshake mechanism for collision avoidance. • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol of the IEEE 802.11 WLAN MAC standard.

  4. Overview (2) • ETSI HIPERLAN . • A simplified version of a popular saturation throughput analysis of the IEEE 802.11 MAC protocol. • Renewal reward theorem and a fixed point theorem. • Service differentiation mechanisms in IEEE 802.11 networks. • The performance of TCP-based data and voice traffic over WLANs. • Optimal association of an 802.11 node to an access point.

  5. Preliminaries (1) • A minimum SINR (θ) is required by a receiver to decode a packet that is being received. • Like in CDMA networks, in WLANs also all nodes use the entire allocated spectrum. • We use a simplified model of channel usage. • Collision: Whenever two or more transmissions in the same frequency band and of sufficient strength arrive simultaneously at a receiver, neither can be detected. • A collision can occur even after a receiver has successfully decoded a part of a transmission. • Capture: It can happen that if a receiver is simultaneously receiving signals from one or more transmitters, one of them is strong enough for the resulting SINR to be above the threshold, and received packets can be decoded.

  6. Preliminaries (2) • Lower bound of the throughput and capacity obtained by not accounting for capture. • Interference region: carrier sense region • Decode region: a subset of carrier sense region. • If the transmitter is in the interference region of the receiver, the received power at the receiver is significant and causes a collision at the receiver if another transmission is being received at the same time. • If the transmitter is also in the decode region, then the SNR at the receiver is greater than the prescribed threshold (θ), and the receiver can decode the transmission. • The received power from transmitters outside of the interference region is assumed insignificant.

  7. Preliminaries (3) • The solid line is the boundary of the decode region and the dashed line is that of the interference region. • When no other node is transmitting, A can decode transmissions from B (or C). • Transmissions from D or E (or both) when receiving from B (or C) can cause a collision at A. However, the transmissions from D and E cannot be decoded at A. • Transmissions from G or H do not cause a collision at A.

  8. Preliminaries (4) • The decode and interference regions depend on the locations of the other nodes that are transmitting. • We can also define the interference and decode regions for a transmitter. • The interference regions (decode regions) for transmission and reception may not be equal. • In single hop networks (also called broadcast networks or co-located networks), every node is within the decode region of every other nodes. • We will consider only single hop networks. • The channel is also called the medium and a Medium Access Control (MAC) protocol regulates use of the medium by prescribing the rules to initiate a transmission and continue with it.

  9. Preliminaries (5) • In random access networks, collisions may occur and the MAC protocol has to resolve collisions; it has to arbitrate among the nodes contending to use the medium. • The arbitration is a distributed algorithm that typically prescribes forced silences on the nodes. • Some amount of transmission time is lost to collisions and arbitration. • The fraction of time so lost is a measure of the efficiency of the protocol. • Simple protocols, even if of low efficiency, are useful if the per node throughput that the protocol obtains is significant compared to the throughput required by the nodes in the network. • Two key wireless MAC protocols: Aloha and CSMA/CA.

  10. Random Access (1) • An example: • A 100-node multiple access network using a 10 Mbps channel. • Each node requires an average throughput of 1 Kbps, but in bursts. A node may have to transmit 1000-bit packets, on an average once every second. • In a TDM scheme, each node would be statically allocated every 100th slot. • If each slot corresponds to a packet transmission time, the waiting time before the packet transmission is completed could be as high as 10.1 ms and the expected delay would be 5.1 ms. This is assuming that the queue is empty when this packet arrives at the node.

  11. Random Access (2) • Polling scheme: polling overhead and a corresponding delay. • Random access: • Transmission rate is 10 Mbps • Transmission time is just 100 μs • Offered load to the network is low (1% of the maximum possible throughput) • Access delay, the delay between the packet being ready and the beginning of transmission, will also be low. • In TDM and polling schemes, the exact number of nodes needs to be known. • In wireless networks, at any time, number of nodes is typically a random number. Thus random access is possibly the only option, which is not a bad option.

  12. Aloha (1) • Earliest random access MAC protocol: Aloha (pure Aloha) • Simple idea: If a node has a packet to transmit, it just transmits! • Example of a satellite network: • Every node transmits to a satellite, which then reflects the transmission back for every node to receive it. • Data rate is 1 Mbps and the packets are 1000 bits long so the packet transmission time is 1 ms. • Propagation delay is approximately 250 ms. • What a node that wants to transmit is hearing on the channel at time t is actually a transmission from (t−250) ms. • There is no use deferring to a carrier and it is best just to transmit the packet and hope that no other node is transmitting at the same time.

  13. Aloha (2) • Of course, if there indeed was another transmission at the same time, there would be a collision and neither packet can be decoded correctly by the corresponding receivers. • The nodes will have to use additional mechanisms to determine if the packet was successfully received. • If the packet is not correctly received, the packets will have to be retransmitted using a suitable retransmission algorithm. Space-time diagram of a transmission and reception

  14. Aloha (3) • Simple model: • Fixed length packets . • Unit of time: packet transmission time. • Nodes are located along a straight line of length η. • Distances are measured in terms of the propagation delay. • Maximum propagation delay in the network is also η. • Poisson process of rate G attempts per second. • Uniform distribution of transmitting nodes in [0, η]. • Independent transmissions. • Each packet transmission attempt is characterized by an ordered pair (t, y), where t ≥ 0 is the time at which the transmission started, and 0 ≤ y ≤ η is the location of the transmitting node.

  15. Aloha (4) A sample realization of this space-time attempt process • Number of attempts in two non-overlapping areas, A and B are independent. Thus, the number of attempts in A has a Poisson distribution with mean ((G/η) × (Area of A)). • Thus the space-time attempt process in the region ([0,∞) × [0, η]) is a two-dimensional Poisson point process of rate (G/η)attempts per meter-second.

  16. Aloha (5) • Consider a node at location T transmitting a packet to a node at location R. • For this transmission, we can define a collision window in time at each location in [0, η]. If a transmission is begun at that location in the collision window, then it will arrive at R when it is receiving the packet from T, thus causing a collision. Space-time diagram for Aloha (collision cone ” bcdefgb”)

  17. Aloha (6) • The space-time arrival rate, G/η, multiplying by the area of collision cone,2η, gives the mean of the Poisson distribution of the number of arrivals in the collision cone. • Thus, the probability that a reception is successful, Ps, is given by: Ps= Pr(No transmission attempt in collision cone) = e−(G/η×2η) = e−2G • Defining the throughput, S, as the mean number of successful attempts per unit time, we get: S = GPs = Ge−2G • The maximum value of S is achieved for G = 0.5 and Smax = 1/(2e) ≈ 0.18

  18. Slotted Aloha (1) • To make pure Aloha more efficient, let time be slotted and the nodes be allowed to begin transmission only at the beginning of a slot. • The slot length is made equal to the sum of the packet transmission time (unity) and the maximum propagation delay in the network (η). • Nodes begin transmission only at slot boundaries and the transmission and reception of a packet is completed in one slot. • Packets that arrive in a slot are transmitted at the beginning of the next slot. Time slotting in slotted Aloha

  19. Slotted Aloha (2) • Thus, for a collision at a receiver, a second packet should begin transmission in the same slot; that is, another packet should have arrived in the previous slot. • This means that the collision cone is now a rectangle of sides (1 + η)and η. • The Poisson rate of packet arrivals that can cause a collision is the expected number of Poisson arrivals in a slot, G(1 + η). • The probability that a transmission is successfully received is: Ps =e−G(1+η) • We can obtain the throughput as before and it is: S = GPs = Ge−G(1+η) • The maximum achievable throughput is: Smax = 1/(e(1 + η)) = (0.487760) Time slotting in slotted Aloha

  20. Slotted Aloha (3) • The slot length was made equal to (1 + η), rather than 1, to absorb the variations in the propagation delays between the nodes. • In many networks, such as a satellite network the propagation delays between any pair of nodes are very nearly the same, approximately 250 ms, and we can use a slot length of one unit. • In terrestrial networks, like in the cellular and the cable networks, the nodes usually transmit to a central node. • The nodes use ranging to determine the propagation delay to the central node and advance or delay their transmission times to approximate a time slotted link and absorb the differences in the propagation delays.

  21. Slotted Aloha (4) • An Example: GSM Cellular Network • Random Access Channel (RACH), on the reverse link from the mobile node to the base station, is used by the GSM mobile stations to send messages to the network. • The messages are small and are generated at a very low rate compared to the capacity of the RACH channel. • The number of mobile nodes in a cell is not fixed and also quite large and signaling bandwidth cannot be allocated statically to these nodes. • Hence, slotted Aloha is used on this channel. • After transmitting on the RACH, the mobile station waits for a fixed duration to know if the transmission was successful. • If an acknowledgment is not received before this duration, a retransmission is attempted.

  22. Slotted Aloha (5) • Another Example: VSAT Network • Very Small Aperture Terminal (VSAT) network is a satellite network in which there are several geographically widespread, small terminals. • These terminals are attached to individual computers or to the local area networks of small organizations through the Digital Interface Unit (DIU). • The nodes can communicate only with the hub, through a shared satellite channel, and all inter-node communications are over two hops via the hub. • The terminals request for reservations of time on the inbound channel, using the slotted Aloha protocol. • This reservation scheme can be very efficient if the bandwidth allocated for the reservation requests is small and the amount of reserved bandwidth is large.

  23. CSMA (1) • If in a network, the propagation delay is small compared to the packet transmission time, it is possible to infer channel state (busy or idle) through carrier sensing. • In such networks, if a node senses the channel to be busy and yet transmits, it can cause a collision at the receiver of the ongoing transmission, and both transmissions are lost. • Hence, a node should listen to the channel before beginning to transmit and defer to an ongoing transmission. • This is the principle of the Carrier Sense Multiple Access (CSMA) protocol. • In this protocol, once a node begins transmitting, it transmits the complete packet.

  24. CSMA (2) • The collision window for the CSMA is the time since the beginning of a transmission during which another node (not having heard the ongoing transmission) can begin its own transmission, and hence collide with the first transmission. • The maximum collision window is equal to the maximum propagation delay in the network. • Because after this interval, the carrier would have reached every node in the network and all nodes will defer a transmission attempt until the end of the packet transmission that is in progress. • Two or more nodes can begin transmission within a short time of each other (less than the collision window) and collide. • In this case, all the colliding transmissions will be lost.

  25. CSMA (3) • The duration of a collision in the network is the time from the beginning of the first transmission in the collision until the earliest time at which a fresh transmission can begin. Then: Maximum duration of a collision = ttrans+ 2tpropgn where ttrans is the packet transmission time and tpropgn is the maximum propagation delay. • Collision can be detected if the node continuing to monitor the channel after beginning transmission. • If it senses a collision on the channel, then the node can immediately stop transmission and minimize the loss of channel capacity. • This is called CSMA with Collision Detect or CSMA/CD. Then: Maximum duration of a collision = 3tpropgn

  26. CSMA (4) Collisions in CSMA and CSMA/CD networks

  27. CSMA (5) • In wireless networks, spatial reuse allows the spectrum to be simultaneously used in different parts of the network and significantly increases the traffic carrying capacity. • Spatial reuse requires that the interference region of the transmitters be much smaller than the geographical spread of the network. • This allows different transmitter-receiver pairs to be active in geographically different parts of the network. • Challenges of carrier detection: • Hidden and exposed nodes. • Difficulty to design reliable collision detection hardware. • Hence in wireless LANs, the emphasis is on avoiding collisions, rather than detecting them.

  28. Collision Avoidance (1) Node b is hiddenfrom node a with reference to a transmission to node c. Node d is exposed to a transmission from node a. Hidden and exposed nodes in a wireless network (Propagation delays are assumed to be zero.)

  29. Collision Avoidance (2) • Hidden nodes reduce the capacity by causing collisions at receivers without the transmitter knowing about it. • Exposed nodes force a node to unnecessarily defer in its transmission attempts, thus reducing spatial reuse. • Collision Avoidance (CA) mechanisms prevent collisions due to transmissions by hidden node. • These mechanisms assume that the interference regions, and also the decode regions, for transmission and reception are identical. • A simple CA mechanism is to have a narrowband auxiliary signaling channel in addition to the data channel. • A node actively receiving data on the data channel transmits a busy tone on the signaling channel to enable the hidden nodes to defer to receiving nodes in their interference regions.

  30. Collision Avoidance (3) • Dividing the available spectrum into two parts is both cumbersome and inefficient. • The effect of the busy tone is achieved by preceding the actual data transfer by a handshake between the transmitter and the receiver. • This handshake is used to convey an imminent reception to the hidden nodes. • Before transmitting a data packet, a source node transmits a (short) Request To Send (RTS) packet to the destination. • If the destination receives the RTS correctly, it means that it is not receiving any other packet and it acknowledges the RTS with a Clear To Send (CTS) packet. • CTS informs the neighborhood of a receiver about an impending packet reception. • The source then begins the packet transmission.

  31. Collision Avoidance (4) • If CTS is not received within a specified timeout period, the source assumes that the RTS had a collision at the receiver (most likely with another RTS packet) and a retransmission is attempted after a random back-off period. • The RTS serves to inform nodes in the decode region of the transmitter about the imminent transmission of a packet. • The CTS serves the same purpose for nodes in the decode region of the receiver. • Thus nodes that are not in the interference region of the transmitter, but are in the decode region of the receiver, are informed of the imminent packet transmission. • If the transmission duration information is included in the RTS and CTS packets, then the nodes in the decode region of both the transmitter and the receiver can maintain a Network Allocation Vector (NAV).

  32. Collision Avoidance (5) • NAV indicates the remaining time in the current transmission and schedule their own transmissions to avoid collision. • Thus this handshake is a collision avoidance scheme and the protocol is the Carrier Sense, Multiple Access with Collision Avoidance (CSMA/CA). • After completion of the RTS/CTS exchange, the medium is reserved in the region that is the union of the decode regions of the transmitter and the receiver. • Hence this basic channel access mechanism was called Multiple Access with Channel Acquisition (MACA) when it was first proposed. • It is an adaptation of the handshake protocols used in RS-232-C and Apple talk.

  33. Collision Avoidance (6) • Collision avoidance helps to reduce the inefficiency that is introduced by not being able to do collision detection in wireless networks. • Only short RTS packets collide, and hence the time lost due to collisions is small. • The RTS/CTS scheme helps ameliorate the hidden terminal problem but does not eliminate it. • Only the nodes in the decode region of the receiver have been alerted by the CTS. Those in the interference region, but not in the decode region of the receiver, have just sensed a carrier but they do not know of an impending packet transmission. • During the packet transmission, they do not sense the carrier and can still cause a collision.

  34. Collision Avoidance (7) • In the CSMA/CA scheme, it seems difficult to be able to allow an exposed node to transmit. • Any node in the interference region of the transmitter of the ongoing packet is exposed. Even if such a node were allowed to transmit a RTS to a node (outside the interference region of the transmitter of the ongoing packet), it will itself not be able to receive the subsequent CTS and hence it will not know if it can transmit. • Improvements: MACA for Wireless (MACAW) • An acknowledgment from the receiver after the successful reception of a packet. • Transmission of a short Data Sending (DS) packet preceding the actual data transfer. This is also useful in networks where the nodes do not have carrier sensing capability.

  35. Collision Avoidance (8) Handshake and data exchange sequence in MACAW

  36. IEEE 802.11 WLAN Standard (1) • The basic ideas of the CSMA/CA protocol of MACA and MACAW have been formalized in IEEE 802.11 (Wi-Fi) wireless LAN standards. • Two modes of network configuration: • Independent or ad hoc network mode • Nodes form an independent multi-hop wireless network and they communicate directly with one another. • A routing protocol and a corresponding routing algorithm will need to be used so that the packets find paths to the destinations. An IEEE 802.11 ad hoc network

  37. IEEE 802.11 WLAN Standard (2) • Infrastructure mode • Data communication is always between a mobile station (MS) and an Access Point (AP). • The AP is connected to the wired network and provides a service similar to the base station of a cellular network. • MSs need to associate with an AP using an association protocol. • The AP and the MSs associated with it form a Basic Service Set (BSS), and a set of BSSs is called an Extended Service Set (ESS). • The association, and dissociation, allows the MSs to be mobile within the ESS.

  38. IEEE 802.11 WLAN Standard (3) • A typical IEEE 802.11 ESS architecture

  39. IEEE 802.11 WLAN Standard (4) PHYsical (PHY) layer • Different frequency bands and data rates • The initial 802.11 standard had three PHY standards: 1) Infrared 2) 1 and 2 Mbps over Frequency Hopping Spread Spectrum (FHSS) in the 2.4 GHz band 3) 1 and 2 Mbps over Direct Sequence Spread Spectrum (DSSS) in the 2.4 GHz band. • The transmitter and the receiver can choose the data rate to suit the channel conditions.

  40. IEEE 802.11 WLAN Standard (5) • A second version of the IEEE 802.11 standard defined the following physical layer standards: • 802.11a • 5.8 GHz band • OFDM (52 carriers, 48 for data) • 20 MHz bandwidth • Data rates: 6, 9, 12, 18, 24, 36, 48, or 54 Mbps • 802.11b • 2.4 GHz band • DSSS • 11MHz bandwidth • Data rates: 1, 2, 5.5, 11 Mbps • 802.11g (an extension of 802.11b) • DSSS and OFDM • Data rates: 5.5 to 54 Mbps

  41. IEEE 802.11 WLAN Standard (6) 2.4 GHz Wi-Fi channels (802.11b,g WLAN)

  42. IEEE 802.11 WLAN Standard (7) Medium Access Control (MAC) layer • Two basic protocols: 1) Polling-based: Point Coordination Function (PCF) 2) Random-access: Distributed Coordination Function (DCF) • PCF and DCF can coexist in the same BSS. • Time is divided into super-frames and each super-frame has two parts: (1) Contention Free Period (CFP) (2) Contention Period (CP) • PCF is used in the CFP and DCF in the CP.

  43. IEEE 802.11 WLAN Standard (8) • PCF in CFP: • AP polls all the nodes in the BSS by transmitting a beacon frame. • Data that need to be transmitted to them are transmitted along with the polling message. • If any polled node has packets to transmit, it will also transmit them in response to the polling packet. • When all the nodes are polled by the AP, the end of CFP is signaled using the “End frame”. • DCF in CP: • Using the DCF-based MAC, until the end of the super-frame period. • In a super-frame , the CFP and the CPalternate.

  44. IEEE 802.11 WLAN Standard (9) • Short Inter-Frame Space (SIFS) timer starts at the end of the data transmission, and allows the receiving node to turn around its radio and send back a MAC level ACK packet. • DCF Inter-Frame Space (DIFS) timer starts by all the nodes, after completion of ACK transmission and the channel is sensed to be idle. • DIFS duration is more than SIFS. Beacon frame, PCF, and DCF periods in an IEEE 802.11 network

  45. IEEE 802.11 WLAN Standard (10) • Target Beacon Transmission Time (TBTT) specifies the period of the super-frame. • AP will begin trying to initiate a new CFP, after TBTT has elapsed from the time that the previous one was initiated. • In addition to the RTS and CTS based handshake mechanism, the standard specifies the following: • Minimum silence periods between transmissions is different for different kinds of packets according to their priorities. • Back-off mechanism is used to resolve collisions.

  46. IEEE 802.11 WLAN Standard (11) • All nodes enter a backoff phase, when the DIFS timers expire. • Random backoff is used to avoid collision. • If a node was already in a backoff when another node started its data transmission, then during the transmission backoff timer is frozen. • Upon completion of a node transmission, each other node that had deferred to the node continues the remainder of itsbackoff. • The backoff durations are multiples of the basic slot time. • When a new backoff is sampled, this multiple is sampled uniformly from the integers {0, 1, . . . ,CWmin − 1}. • In the standard, CWmin =32.

  47. IEEE 802.11 WLAN Standard (12) • When a backoff period of some node expires, then this node transmits an RTS packet to its destination node. • Upon hearing activity on the medium, all other nodes freeze their backoff timers. • The node to which the RTS was directed then sends back a CTS packet (a SIFS elapses in between the two). • Transmitter node then waits for a SIFS and sends its data packet, after which an ACK is sent by the receiver node. • A collision occurs if two nodes finish their backoffs within one slot of each other. • It is assumed that the maximum propagation delay in the network is such that all nodes are able to sense a transmission within one slot time. • In this case, both RTS packets collide.

  48. IEEE 802.11 WLAN Standard (13) • A CTS time-out then follows, after which the colliding nodes sample a backoff from a doubled collision window; that is, from the window {0, 1, 2, . . . , 2 · CWmin − 1}. • After the collision event, the nodes that were not involved in the collision continue their backoffs with their residual backoff timers. • Repeated collisions lead to a doubling of the collision window until it reaches CWmax, after which the collision window remains fixed. • In the standard, CWmax= 1024. • Extended Inter-Frame Space (EIFS) is equal to sum of the transmission times of SIFS, CTS (and ACK), and DIFS packets and the PHY headers.

  49. IEEE 802.11 WLAN Standard (14) • EIFS is longer than DIFS and is used by nodes that cannot decode a transmission, because of a collision or because the received power is such that the SNR is below the decoding threshold. • This prevents collision during reception of CTS and ACK. • Also, when the point coordinator wants to initiate the CFP, it waits for PCF Interframe Space (PIFS) after the end of an ongoing transmission before transmitting a beacon or the polling packet. • SIFS < PIFS < DIFS implies that the initiation of the CFP has priority over new transmission initiations. • Multi-valued DIFS can be used to prioritize among different traffic classes.

  50. IEEE 802.11 WLAN Standard (15) Specification of 802.11a, b, and g • RTS packet: 20 octets • CTS packet: 14 octets • ACK packet: 14 octets • All transmitted at the lowest transmission rate. • Payload packet: Up to 2312 bytes • MAC header and trailers: 34 octets • PHY headers: 192 bits.

More Related