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Routing in a (W)LAN Routing in a (W)LAN is based on MAC addresses, never IP addresses. A router (e.g. integrated with an access point) performs mapping between these two address types: IP network (W)LAN (W)LAN device 00:90:4B:00:0C:72 Δίκτυα Υπολογιστών II Router 124.2.10.57 00:90:4B:00:0C:72 Server 124.2.10.57 Δρ. Γεώργιος Δημητρακόπουλος Address allocation MAC addresses are associated with the hardware devices. IP addresses can be allocated to (W)LAN devices either on a permanent basis or dynamically from an address pool using the Dynamic Host Configuration Protocol (DHCP). The DHCP server may be a separate network element (or for example integrated into a RADIUS server that offers a set of additional features), or may be integrated with the address-mapping router and/or access point. RADIUS = Remote Authentication Dial-In User Service Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Network Address Translation (NAT) On the (W)LAN side of the network address translator (NAT device), different (W)LAN users are identified using private (reusable, globally not unique) IP addresses. On the Internet side of the NAT device, only one (globally unique) IP address is used. Users are identified by means of different TCP/UDP port numbers. In client - server type of communication, the application on the server is usually behind a certain TCP/UDP port number (e.g. 80 for HTTP) whereas clients can be allocated port numbers from a large address range. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος NAT example IP network (W)LAN User 1 NAT device IP address for all users in (W)LAN: Server 124.0.6.12 User 2 User 1 IP address 10.2.1.57 User 1 TCP port number 14781 User 2 IP address 10.2.1.58 User 2 TCP port number 14782 Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Case study: ADSL WLAN router 1) The ADSL connection to the wide area network (WAN) is allocated a globally unique IP address using DHCP. 2) We assume that the router has NAT functionality. Behind the router, in the private LAN network, wireless and cabled LAN devices are allocated private IP addresses, again using DHCP (this is a kind of "double DHCP" scenario). Although routing in the LAN is based on MAC addresses, the IP applications running on the LAN devices still need their own "dummy" IP addresses. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Contents IEEE 802.11 MAC layer operation • Basic CSMA/CA operation • Network Allocation Vector (NAV) • Backoff operation • Wireless medium access example Usage of RTS / CTS Fragmentation Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Medium Access Control (MAC) Medium access control: Different nodes must gain access to the shared medium (for instance a wireless channel) in a controlled fashion (otherwise there will be collisions). Access methods: FDMA TDMA CDMA CSMA Δίκτυα Υπολογιστών II : Assigning channels in frequency domain LLC Assigning time slots in time domain MAC Assigning code sequences in code domain PHY Assigning transmission opportunities in time domain on a statistical basis Δρ. Γεώργιος Δημητρακόπουλος CSMA/CD vs. CSMA/CA (1) CSMA/CD (Collision Detection) is the MAC method used in a wired LAN (Ethernet). Wired LAN stations can (whereas wireless stations cannot) detect collisions. Basic CSMA/CD operation: 1) 2) 3) 4) Δίκτυα Υπολογιστών II CSMA/CD rule: Backoff after collision Wait for free medium Transmit frame If collision, stop transmission immediately Retransmit after random time (backoff) Δρ. Γεώργιος Δημητρακόπουλος CSMA/CD vs. CSMA/CA (2) CSMA/CA (Collision Avoidance) is the MAC method used in a wireless LAN. Wireless stations cannot detect collisions (i.e. the whole packets will be transmitted anyway). Basic CSMA/CA operation: 1) 2) 3) 4) 5) Δίκτυα Υπολογιστών II CSMA/CA rule: Backoff before collision Wait for free medium Wait a random time (backoff) Transmit frame If collision, the stations do not notice it Collision => erroneous frame => no ACK returned Δρ. Γεώργιος Δημητρακόπουλος AP CSMA: One packet at a time Δίκτυα Υπολογιστών II wired LAN Basic wireless medium access We shall next investigate Infrastructure BSS only. As far as medium access is concerned, all stations and AP have equal priority transmission in downlink (from the AP) and uplink (from a station) is similar. Δρ. Γεώργιος Δημητρακόπουλος DCF (CSMA/CA) vs. PCF Designed for contention-free services (delay-sensitive real-time services such as voice transmission), but has not been implemented (yet) Point Coordination Function (PCF) Used for contention services (and basis for PCF) MAC extent Distributed Coordination Function (DCF) based on CSMA/CA Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (1) Cyclic Redundancy Check (CRC) is used for error detection Transmitted frame (A=>B) ACK (B=>A) DIFS SIFS If the received frame is erroneous, no ACK will be sent When a frame is received without bit errors, the receiving station (B) sends an Acknowledgement (ACK) frame back to the transmitting station (A). Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (2) Earliest allowed transmission time of next frame Transmitted frame (A=>B) ACK (B=>A) Next frame (from any station) DIFS SIFS DIFS During the transmission sequence (Frame + SIFS + ACK) the medium (radio channel) is reserved. The next frame can be transmitted at earliest after the next DIFS period. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (3) Transmitted frame (A=>B) ACK (B=>A) Next frame DIFS SIFS DIFS There are two mechanisms for reserving the channel: Physical carrier sensing and Virtual carrier sensing using the socalled Network Allocation Vector (NAV). Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (4) Information about the length of the frame is in the PHY header Transmitted frame (A=>B) ACK (B=>A) Next frame DIFS SIFS DIFS Physical carrier sensing means that the physical layer (PHY) informs the MAC layer when a frame has been detected. Access priorities are achieved through interframe spacing. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (5) The two most important interframe spacing times are SIFS and DIFS: SIFS (Short Interframe Space) = 10 ms (16 ms) DIFS (DCF Interframe Space) = 50 ms (34 ms) 802.11b 802.11g When two stations try to access the medium at the same time, the one that has to wait for the time SIFS wins over the one that has to wait for the time DIFS. In other words, SIFS has higher priority over DIFS. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (6) Transmission is not allowed as long as NAV is non-zero Transmitted frame ACK NAV value is given here DIFS NAV SIFS Next frame DIFS Virtual carrier sensing means that a NAV value is set in all stations that were able to receive a transmitted frame and were able to read the NAV value in this frame. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (7) Transmitted frame Long transaction ACK ACK NAV DIFS SIFS DIFS Virtual carrier sensing using NAV is important in situations where the channel should be reserved for a ”longer time” (RTS/CTS usage, fragmentation, etc.). Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος NAV value is carried in MAC header MPDU (MAC Protocol Data Unit) Addr 1 Addr 2 Addr 3 Addr 4 (optional) MAC payload FCS Duration field: 15 bits contain the NAV value in number of microseconds. The last (sixteenth) bit is zero. All stations must monitor the headers of all frames they receive and store the NAV value in a counter. The counter decrements in steps of one microsecond. When the counter reaches zero, the channel is available again. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (8) Channel was idle at least DIFS seconds Transmitted frame (A=>B) ACK (B=>A) Next frame (from any station) DIFS SIFS t > DIFS When a station wants to send a frame and the channel has been idle for a time > DIFS (counted from the moment the station first probed the channel) => can send immediately. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (9) Channel was busy when station wanted to send frame Transmitted frame (A=>B) ACK (B=>A) Backoff DIFS SIFS Next frame DIFS When a station wants to send a frame and the channel is busy => the station must wait a backoff time before it is allowed to transmit the frame. Reason? Next two slides… Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος No backoff => collision is certain Suppose that several stations (B and C in the figure) are waiting to access the wireless medium. When the channel becomes idle, these stations start sending their packets at the same time => collision! Station A ACK Station B Collision! Station C DIFS Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Backoff => collision probability is reduced Contending stations generate random backoff values bn. Backoff counters count downwards, starting from bn. When a counter reaches zero, the station is allowed to send its frame. All other counters stop counting until the channel becomes idle again. Station A ACK Backoff Remaining backoff time Station B bn is large Station C DIFS Δίκτυα Υπολογιστών II bn is small Δρ. Γεώργιος Δημητρακόπουλος Contention window (CW) for 802.11b If transmission of a frame was unsuccessful and the frame is allowed to be retransmitted, before each retransmission the Contention Window (CW) from which bn is chosen is increased. 802.11b CW Initial attempt DIFS 1st retransm. DIFS … CW = 25-1 = 31 slots (slot = 20 ms) … CW = 26-1 = 63 slots : 5th (and further) retransmissions Δίκτυα Υπολογιστών II DIFS … CW = 210-1 = 1023 slots Δρ. Γεώργιος Δημητρακόπουλος Contention window (CW) for 802.11g In the case of 802.11g operation, the initial CW length is 15 slots. The slot duration is 9 ms. The backoff operation of 802.11g is substantially faster than that of 802.11b. 802.11g CW Initial attempt DIFS 1st retransm. DIFS … CW = 24-1 = 15 slots (slot = 9 ms) … CW = 25-1 = 31 slots : 6th (and further) retransmissions Δίκτυα Υπολογιστών II DIFS … CW = 210-1 = 1023 slots Δρ. Γεώργιος Δημητρακόπουλος Selection of random backoff From the number CW (= 15 / 31 … 1023 slots) the random backoff bn (in terms of slots) is chosen in such a way that bn is uniformly distributed between 0 … CW. Since it is unlikely that several stations will choose the same value of bn, collisions are rare. The next slides show wireless medium access in action. The example involves four stations: A, B, C and D. ”Sending a packet” means ”Data+SIFS+ACK” sequence. Note how the backoff time may be split into several parts. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (1) Station A ACK Defer Station B Backoff 1 Station C Contention Window Defer 2 1) While station A is sending a packet, stations B and C also wish to send packets, but have to wait (defer + backoff) 2) Station C is ”winner” (backoff time expires first) and starts sending packet Station D DIFS Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (2) 3) Station D also wishes to send a packet Station A Station B 4 Station C ACK 3 Defer Station D DIFS Δίκτυα Υπολογιστών II 4) However, station B is ”winner” and starts sending packet DIFS Δρ. Γεώργιος Δημητρακόπουλος Wireless medium access (3) 5) Station D starts sending packet. Now there is no competition. Station A Station B ACK Station C 5 Station D DIFS Δίκτυα Υπολογιστών II DIFS Δρ. Γεώργιος Δημητρακόπουλος No shortcuts for any station… Transmitted frame (A=>B) Backoff ACK (B=>A) DIFS SIFS Next frame (A=>B) DIFS When a station wants to send more than one frame, it has to use the backoff mechanism like any other station (of course it can ”capture” the channel by sending a long frame, for instance using fragmentation). Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος ACK frame structure Frame type = control MPDU Frame subtype = ACK NAV 0 0 1 0 1 0 1 1 FCS No MAC payload Address of station from which frame was sent that is now acknowledged Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Usage of RTS & CTS The RTS/CTS (Request/Clear To Send) scheme is used as a countermeasure against the “hidden node” problem: Hidden node problem: WS 1 WS 1 and WS 2 can ”hear” the AP but not each other => If WS 1 sends a packet, WS 2 does not notice this (and vice versa) => collision! Δίκτυα Υπολογιστών II AP WS 2 Δρ. Γεώργιος Δημητρακόπουλος Reservation of medium using NAV The RTS/CTS scheme makes use of “SIFS-only” and the NAV (Network Allocation Vector) to reserve the medium: SIFS WS 1 RTS Data frame CTS AP SIFS NAV in RTS NAV in CTS Δίκτυα Υπολογιστών II DIFS ACK SIFS NAV = CTS + Data + ACK + 3xSIFS NAV = Data + ACK + 3xSIFS Δρ. Γεώργιος Δημητρακόπουλος Danger of collision only during RTS WS 2 does not hear the RTS frame (and associated NAV), but can hear the CTS frame (and associated NAV). WS 1 RTS AP Data frame CTS ACK Danger of collision NAV in RTS NAV in CTS Δίκτυα Υπολογιστών II NAV = CTS + Data + ACK + 3xSIFS NAV = Data + ACK + 3xSIFS Δρ. Γεώργιος Δημητρακόπουλος Advantage of RTS & CTS (1) Usage of RTS/CTS offers an advantage if the data frame is very long compared to the RTS frame: WS 1 AP RTS Data frame CTS (RTS/CTS used) ACK Short interval: collision not likely WS 1 AP Data frame (RTS/CTS not used) ACK Long interval: collision likely Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Advantage of RTS & CTS (2) A long “collision danger” interval (previous slide) should be avoided for the following reasons: Larger probability of collision Greater waste of capacity if a collision occurs and the frame has to be retransmitted. A threshold parameter (dot11RTSThreshold) can be set in the wireless station. Frames shorter than this value will be transmitted without using RTS/CTS. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος Fragmentation Fragmentation makes use of the RTS/CTS scheme and the NAV mechanism: SIFS WS 1 RTS AP SIFS Frag 0 CTS DIFS Frag 1 ACK 0 SIFS ACK 1 SIFS SIFS RTS NAV in WS NAV in AP Δίκτυα Υπολογιστών II Frag 0 CTS ACK 0 Δρ. Γεώργιος Δημητρακόπουλος Sequence control field MPDU (MAC Protocol Data Unit) Addr 1 Addr 2 Fragment number (for identifying fragments) Δίκτυα Υπολογιστών II Addr 3 Addr 4 (optional) MAC payload FCS Frame sequence number (for identifying frames) Δρ. Γεώργιος Δημητρακόπουλος Advantage of fragmentation Transmitting long data frames should be avoided for the following reasons: Larger probability that the frame is erroneous Greater waste of capacity if a frame error occurs and the whole frame has to be retransmitted. A threshold parameter (dot11FragmentationThreshold) can be set in the wireless station. Frames longer than this value will be transmitted using fragmentation. Δίκτυα Υπολογιστών II Δρ. Γεώργιος Δημητρακόπουλος