Δίκτυα Υπολογιστών II 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 Router Server (W)LAN device 00:90:4B:00:0C:  00:90:4B:00:0C: Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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 Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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. Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II NAT example IP network (W)LAN Server NAT device User 1 User 2 User 1 IP address User 2 IP address IP address for all users in (W)LAN: User 1 TCP port number User 2 TCP port number Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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. Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Contents IEEE MAC layer operation Basic CSMA/CA operation Network Allocation Vector (NAV) Backoff operation Wireless medium access example Usage of RTS / CTS Fragmentation Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Medium Access Control (MAC) LLC MAC PHY : 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). FDMA TDMA CDMA CSMA Assigning channels in frequency domain Assigning time slots in time domain Assigning code sequences in code domain Assigning transmission opportunities in time domain on a statistical basis Access methods: Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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)Wait for free medium 2)Transmit frame 3)If collision, stop transmission immediately 4)Retransmit after random time (backoff) CSMA/CD rule: Backoff after collision Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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)Wait for free medium 2)Wait a random time (backoff) 3)Transmit frame 4)If collision, the stations do not notice it 5)Collision => erroneous frame => no ACK returned CSMA/CA rule: Backoff before collision Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Basic wireless medium access AP 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. CSMA: One packet at a time wired LAN Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II DCF (CSMA/CA) vs. PCF Distributed Coordination Function (DCF) based on CSMA/CA Point Coordination Function (PCF) MAC extent Used for contention services (and basis for PCF) Designed for contention-free services (delay-sensitive real-time services such as voice transmission), but has not been implemented (yet) Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (1) DIFSSIFS ACK (B=>A) Transmitted frame (A=>B) When a frame is received without bit errors, the receiving station (B) sends an Acknowledgement (ACK) frame back to the transmitting station (A). If the received frame is erroneous, no ACK will be sent Cyclic Redundancy Check (CRC) is used for error detection Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (2) DIFSSIFSDIFS ACK (B=>A) Transmitted frame (A=>B) 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. Next frame (from any station) Earliest allowed transmission time of next frame Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (3) DIFSSIFSDIFS ACK (B=>A) Transmitted frame (A=>B) There are two mechanisms for reserving the channel: Physical carrier sensing and Virtual carrier sensing using the so- called Network Allocation Vector (NAV). Next frame Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (4) DIFSSIFSDIFS ACK (B=>A) Transmitted frame (A=>B) 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. Next frame Information about the length of the frame is in the PHY header Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (5) The two most important interframe spacing times are SIFS and DIFS: SIFS (Short Interframe Space) = 10  s (16  s) DIFS (DCF Interframe Space) = 50  s (34  s) 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 b g Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (6) DIFSSIFSDIFS ACK Transmitted frame NAV 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. NAV value is given here Next frame Transmission is not allowed as long as NAV is non-zero Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (7) DIFSSIFS Transmitted frame NAV Virtual carrier sensing using NAV is important in situations where the channel should be reserved for a ”longer time” (RTS/CTS usage, fragmentation, etc.). ACK Long transaction DIFS Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II NAV value is carried in MAC header MPDU (MAC Protocol Data Unit) MAC payloadAddr 1Addr 2Addr 3Addr 4 (optional) 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. Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (8) DIFSSIFSt > DIFS ACK (B=>A) Transmitted frame (A=>B) 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. Next frame (from any station) Channel was idle at least DIFS seconds Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (9) DIFSSIFSDIFS ACK (B=>A) Transmitted frame (A=>B) 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… Next frame Channel was busy when station wanted to send frame Backoff Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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 Station B Station C DIFS Collision! ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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 Station B Station C DIFS bn is large bn is small Backoff Remaining backoff time ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Contention window (CW) for b 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. DIFS … CW = = 31 slots (slot = 20  s) Initial attempt DIFS … CW = = 63 slots 1st retransm. DIFS CW = = 1023 slots 5th (and further) retransmissions : … CW b Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Contention window (CW) for g In the case of g operation, the initial CW length is 15 slots. The slot duration is 9  s. The backoff operation of g is substantially faster than that of b. DIFS … CW = = 15 slots (slot = 9  s) Initial attempt DIFS … CW = = 31 slots 1st retransm. DIFS CW = = 1023 slots 6th (and further) retransmissions : … CW g Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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. Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (1) Station A Station B Station C Station D DIFS Defer Contention Window Backoff 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 2 1 ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (2) Station A Station B Station C Station D DIFS Defer 3) Station D also wishes to send a packet 4) However, station B is ”winner” and starts sending packet 3 4 ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Wireless medium access (3) Station A Station B Station C Station D DIFS 5) Station D starts sending packet. Now there is no competition. DIFS 5 ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II No shortcuts for any station… DIFSSIFSDIFS ACK (B=>A) Transmitted frame (A=>B) Next frame (A=>B) Backoff 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). Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II ACK frame structure MPDU Address of station from which frame was sent that is now acknowledged FCS No MAC payload NAV Frame type = control Frame subtype = ACK Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Usage of RTS & CTS The RTS/CTS (Request/Clear To Send) scheme is used as a countermeasure against the “hidden node” problem: AP WS 1 WS 2 Hidden node problem: 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! Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Reservation of medium using NAV The RTS/CTS scheme makes use of “SIFS-only” and the NAV (Network Allocation Vector) to reserve the medium: RTS SIFS DIFS NAV = CTS + Data + ACK + 3xSIFS CTS Data frame ACK SIFS WS 1 AP NAV = Data + ACK + 3xSIFS NAV in RTS NAV in CTS Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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). RTS NAV = CTS + Data + ACK + 3xSIFS CTS Data frame ACK WS 1 AP NAV = Data + ACK + 3xSIFS NAV in RTS NAV in CTS Danger of collision Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Advantage of RTS & CTS (1) Usage of RTS/CTS offers an advantage if the data frame is very long compared to the RTS frame: RTS CTS Data frame ACK WS 1 AP Short interval: collision not likely Data frame ACK WS 1 AP Long interval: collision likely (RTS/CTS not used) (RTS/CTS used) Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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. Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Fragmentation Fragmentation makes use of the RTS/CTS scheme and the NAV mechanism: RTS SIFS DIFS RTS CTS Frag 0 ACK 0 SIFS WS 1 AP CTS NAV in WS NAV in AP Frag 1 ACK 1 SIFS Frag 0 ACK 0 Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II Sequence control field MPDU (MAC Protocol Data Unit) MAC payloadAddr 1Addr 2Addr 3Addr 4 (optional) FCS Fragment number (for identifying fragments) Frame sequence number (for identifying frames) Δρ. Γεώργιος Δημητρακόπουλος

Δίκτυα Υπολογιστών II 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. Δρ. Γεώργιος Δημητρακόπουλος