WLAN, part 1. Contents
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1 Contents IEEE WLAN architecture Basic routing example IAPP and mobility management Basic frame structure MAC header structure Usage of MAC address fields Management frames Some IEEE standard amendments S Wireless Personal, Local, Metropolitan, and Wide Area Networks 1
2 IEEE WLAN architecture defines two BSS (Basic Service Set) options: Infrastructure BSS AP wired LAN Independent BSS (Ad-Hoc network) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 2
3 Infrastructure BSS This is by far the most common way of implementing WLANs. Infrastructure BSS AP wired LAN The base stations connected to the wired infrastructure are called access points (AP). Wireless stations in an Infrastructure BSS must always communicate via the AP (never directly). Before stations can use the BSS: Association. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 3
4 Independent BSS Mainly of interest for military applications. No access point is required, stations can communicate directly. Efficient routing of packets is not a trivial problem (routing is not a task of ). Independent BSS (Ad-Hoc network) Ad-Hoc WLAN networks are outside the scope of this course. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 4
5 Extended Service Set (ESS) This is a larger WLAN network consisting of a number of BSS networks interconnected via a common backbone AP AP AP supports link-layer mobility within an ESS (but not outside the ESS) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 5
6 Distribution system This is the mechanism by which APs and other nodes in the wired IP subnetwork communicate with each other. Distribution System (DS) AP AP Router External network (LAN or Internet) This communication, using the Inter-Access Point Protocol (IAPP), is essential for link-layer mobility (=> stations can seamlessly move between different BSS networks). S Wireless Personal, Local, Metropolitan, and Wide Area Networks 6
7 Distribution system (cont.) For instance, when a wireless station moves from one BSS to another, all nodes must update their databases, so that the DS can distribute packets via the correct AP. Distribution System (DS) AP 1 AP 2 WS WS moves to another BSS Router AP 1, AP 2 and router: update your databases! Packets for this WS will now be routed via AP 2. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 7
8 Basic routing example When WS associates with AP 2, the router in charge of the IP subnet addressing obtains an IP address from the DHCP (Dynamic Host Configuration Protocol) server. Distribution System (DS) AP 1 AP 2 Router 2 External network (LAN or Internet) 1 2 Association Fetch IP address 1 WS DHCP Server S Wireless Personal, Local, Metropolitan, and Wide Area Networks 8
9 Basic routing example (cont.) The router must maintain binding between this IP address and the MAC address of the wireless station. Distribution System (DS) AP 1 AP 2 Router :90:4B:00:0C:72 External network (LAN or Internet) 00:90:4B:00:0C:72 WS S Wireless Personal, Local, Metropolitan, and Wide Area Networks 9
10 Basic routing example (cont.) The globally unique MAC address of the wireless station is used for routing the packets within the IP subnetwork (DS + attached BSS networks). Distribution System (DS) AP 1 AP 2 Router :90:4B:00:0C:72 External network (LAN or Internet) 00:90:4B:00:0C:72 WS S Wireless Personal, Local, Metropolitan, and Wide Area Networks 10
11 Basic routing example (cont.) The dynamic and local IP address of the wireless station is only valid for the duration of attachment to the WLAN and is used for communicating with the outside world. Distribution System (DS) AP 1 AP 2 Router :90:4B:00:0C:72 External network (LAN or Internet) 00:90:4B:00:0C:72 WS S Wireless Personal, Local, Metropolitan, and Wide Area Networks 11
12 Basic routing example (cont.) The router must also know (and use) the MAC address of the access point via which the packets must be routed. For this purpose, a special protocol (IAPP) is needed! Distribution System (DS) AP 1 AP 2 00:90:4B:00:0C:72 00:03:76:BC:0D:12 WS Router :90:4B:00:0C:72 00:03:76:BC:0D:12 External network (LAN or Internet) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 12
13 IAPP (Inter-Access Point Protocol) IAPP (defined in IEEE f) offers mobility in the Data link layer (within an ESS = Extended Service Set). Distribution System (DS) AP 1 AP 2 AP 3 Router External network (LAN or Internet) 1 2 IAPP: APs must be able to communicate with each other when the station moves around in the WLAN S Wireless Personal, Local, Metropolitan, and Wide Area Networks 13
14 In addition to IAPP IAPP alone is not sufficient to enable seamless handovers in a WLAN. The stations must be able to measure the signal strengths from surrounding APs and decide when and to which AP a handover should be performed (no standardised solutions are available for this operation). In networks, a handover means reassociating with the new AP. There may be two kinds of problems: will handover work when APs are from different vendors? will handover work together with security solutions? S Wireless Personal, Local, Metropolitan, and Wide Area Networks 14
15 Mobility Management (MM) There are basically two objectives of Mobility Management: 1. MM offers seamless handovers when moving from one network/subnetwork/bss to another Active network connection handover 2. MM makes sure that users or terminals can be reached when they move to another network/subnetwork/bss Passive user/terminal reachability S Wireless Personal, Local, Metropolitan, and Wide Area Networks 15
16 MM in cellular wireless networks (1) 1. Handover: In a cellular wireless network (e.g. GSM), the call is not dropped when a user moves to another cell. Handovers are based on measurements performed by the mobile terminal and base stations. BS 1 BS 2 S Wireless Personal, Local, Metropolitan, and Wide Area Networks 16
17 MM in cellular wireless networks (2) 2. Reachability: In a cellular wireless network, the HLR (Home Location Register) knows in which VLR (Visitor Location Register) area the mobile terminal is located. The VLR then uses paging to find the terminal. Paging HLR points to Mobile subscriber number points to VLR HLR S Wireless Personal, Local, Metropolitan, and Wide Area Networks 17
18 MM in cellular wireless networks (3) 3. IP services (e.g. based on GPRS): Reachability in this case is kind of a problem. Conventional IP services use the client server concept where reachability is not an important issue. Typical client - server transaction: Client Request Response Server S Wireless Personal, Local, Metropolitan, and Wide Area Networks 18
19 MM in three different OSI layers Mobility Management (MM) schemes are possible in three different layers of the OSI protocol layer model: Application layer Transport layer Network layer Data link layer Physical layer e.g. SIP (Session Initiation Protocol) Personal mobility Terminal mobility e.g. Mobile IP IAPP (Inter-Access Point Protocol) Handovers S Wireless Personal, Local, Metropolitan, and Wide Area Networks 19
20 MM in the Data link layer Mobility Management (MM) schemes are possible in three different layers of the OSI protocol layer model: Application layer Transport layer Network layer Data link layer Physical layer IAPP (IEEE f): Seamless roaming within an ESS network (= IP subnet). Handover is not possible when moving from one ESS network to another. No reachability solutions. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 20
21 MM in the Network layer Mobility Management (MM) schemes are possible in three different layers of the OSI protocol layer model: Application layer Transport layer Network layer Data link layer Physical layer Mobile IP: Seamless roaming between ESS networks (= IP subnetworks). Handover is possible when moving from one ESS (or WLAN) network to another. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 21
22 MM in the Application layer Mobility Management (MM) schemes are possible in three different layers of the OSI protocol layer model: Application layer Transport layer Network layer Data link layer Physical layer SIP (or other application layer solutions): No seamless handovers as such... However, the terminal can be reached from the outside network, like with Mobile IP. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 22
23 Mobility management summary Within a WLAN, handovers are possible (based on IAPP + proprietary solutions in equipment), but there is no IEEEsupported reachability solution available. Handovers between different WLANs require Mobile IP (which offers also reachability). Unfortunately, Mobile IP includes a non-transparent mechanism (Discovering Careof Address) that must be implemented in all APs. Global reachability of wireless stations can be achieved using SIP or similar Application layer concepts. SIP does not require changes to APs. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 23
24 IEEE frame structure TCP/IP protocol suite (usually) : : IP packet IP IEEE 802 H LLC payload LLC PHY H MAC H MSDU (MAC SDU) MPDU (MAC Protocol Data Unit) PSDU (PLCP Service Data Unit) PPDU (PLCP Protocol Data Unit) MAC PHY S Wireless Personal, Local, Metropolitan, and Wide Area Networks 24
25 PDU vs. SDU Payload of a PDU in layer N = SDU to/from the layer N+1 : IP IP LLC LLC MAC PHY PHY SDU (Service Data Unit) is sent between protocol layers PDU (Protocol Data Unit) is sent between network nodes (in a specific protocol layer) : IP IP LLC LLC MAC PHY PHY S Wireless Personal, Local, Metropolitan, and Wide Area Networks 25
26 Overall frame structure (application = HTML) HTML page HTTP payload HTTP TCP payload TCP TCP/IP H IP payload IP IEEE 802 H LLC payload LLC MAC H MSDU (MAC SDU) MAC PHY H PSDU (PLCP Service Data Unit) PHY S Wireless Personal, Local, Metropolitan, and Wide Area Networks 26
27 MAC header structure MPDU (MAC Protocol Data Unit) Addr 1 Addr 2 Addr 3 Addr 4 (optional) MAC payload FCS Duration field (contains NAV value) Sequence Control field (numbering of frames modulo 4096) Frame Control field (type of frame & various flag bits) One byte (eight bits) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 27
28 Content of Frame Control field One bit Protocol Type Subt. of frame Protocol: Indicates IEEE MAC Type: 00 (Management frames) 01 (Control frames) 10 (Data frames) Subtype of frame: Describes type of management, control, or data frame in more detail (e.g. ACK => 1101) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 28
29 Flags in Frame Control field One bit Protocol Type Subt. of frame : Bit is set if frame is sent to AP 2: Bit is set if frame is sent from AP 3: Used in fragmentation 4: Bit is set if frame is retransmitted 5: Power management bit (power saving operation) 6: More data bit (power-saving operation) 7: Bit is set if WEP is used 8: Strict ordering of frames is required S Wireless Personal, Local, Metropolitan, and Wide Area Networks 29
30 Usage of MAC address fields MPDU (MAC Protocol Data Unit) Addr 1 Addr 2 Addr 3 Addr 4 Address 1: Receiver (wireless station or AP) Address 2: Sender (wireless station or AP) Address 3: Ultimate source/destination (router in DS) Address 4: Only used in Wireless Bridge solutions: LAN AP AP LAN S Wireless Personal, Local, Metropolitan, and Wide Area Networks 30
31 Direction: AP => wireless station Addr 1 Addr 2 Addr 3 Addr 1: Receiver (wireless station) Addr 2: Transmitter = BSSID (AP) Addr 3: Ultimate source (router) 2 Router 3 BSSID: MAC address of AP SSID: Alphanumeric name of AP (or BSS) 1 AP S Wireless Personal, Local, Metropolitan, and Wide Area Networks 31
32 MAC addressing example Frames to the WS must also include the MAC address of the ultimate source to which return frames should be routed (then ultimate destination ). Distribution System (DS) 00:03:76:BC:0D:12 AP 1 AP 2 2 Router 00:20:34:B2:C4:10 3 External network 00:90:4B:00:0C:72 WS 1 S Wireless Personal, Local, Metropolitan, and Wide Area Networks 32
33 Direction: Wireless station => AP Addr 1 Addr 2 Addr 3 Addr 1: Receiver = BSSID (AP) Addr 2: Transmitter (wireless station) Addr 3: Ultimate destination (router) 2 1 Router AP 3 S Wireless Personal, Local, Metropolitan, and Wide Area Networks 33
34 Management frames In addition to the data frames (containing the user data to be transported over the network) and control frames (e.g. acknowledgements), there are a number of management frames. Note that these management frames compete for access to the medium in equal terms (using CSMA/CA) with the data and control frames. Some of these management frames are presented on the following slides. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 34
35 Beacon frames Beacon frames are broadcast (mening that all stations shall receive them and read the information) at regular intervals from the Access Point. These frames contain (among others) the following information: Timestamp (8 bytes) is necessary, so that stations can synchronise to the network Beacon interval (2 bytes) in milliseconds Capability info (2 bytes) advertises network capabilities SSID ( bytes), alphanumeric network name The channel number used by the network (optional). S Wireless Personal, Local, Metropolitan, and Wide Area Networks 35
36 Probe request & response frames A probe request frame is transmitted from a wireless station during active scanning. Access points within reach respond by sending probe response frames. Probe request frames contain the following information: SSID ( bytes), alphanumeric network name Bit rates supported by the station. This is used by APs to see if the station can be permitted to join the network. Probe response frames actually contain the same kind of network information as beacon frames. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 36
37 Association request & response frames Before a station can join an network, it must send an association request frame. The AP responds with an association response frame. Association request frames contain (among others): SSID, capability info, bit rates supported. Association response frames contain (among others): Capability info, bit rates supported Status code (success or failure with failure cause) Association ID (used for various purposes) S Wireless Personal, Local, Metropolitan, and Wide Area Networks 37
38 Passive and active scanning Wireless stations can find out about networks by using passive or active scanning. During passive scanning, the station searches beacon frames, moving from channel to channel through the complete channel set (802.11b => 13 channels). During active scanning, the station selects Channel 1 and sends a probe request frame. If no probe response frame is received within a certain time, the station moves to Channel 2 and sends a probe request frame, and so on. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 38
39 Case study 1: Station connecting to a WLAN When a station moves into the coverage area of a WLAN, the following procedures take place: 1) Scanning: the station searches for a suitable channel over which subsequent communication takes place 2) 3) 4) Association: the station associates with an AP IP address allocation: the station gets an IP address, for instance from a DHCP server Authentication: only if this security option is required. S Wireless Personal, Local, Metropolitan, and Wide Area Networks 39
40 Case study 2: Handover to another AP When a station has noticed that the radio connection to another AP is a better than the existing connection: 1) Reassociation: the station associates with another AP 2) No new IP address is needed; however, the WLAN must be able to route downlink traffic via the new AP 3) Authentication: this security option, if required, will result in a substantially increased handover delay (complete procedure sequence: deauthentication, disassociation, reassociation, authentication). S Wireless Personal, Local, Metropolitan, and Wide Area Networks 40
41 Some IEEE standard amendments f f IAPP IAPP e QoS QoS basic protocol ii Security Security MAC layer h DFS/TCP DFS/TCP a OFDM OFDM 5GHz 5GHz d Scanning Scanning b DSSS DSSS 2.4GHz 2.4GHz g OFDM OFDM 2.4GHz 2.4GHz Physical layer S Wireless Personal, Local, Metropolitan, and Wide Area Networks 41
42 IEEE basic protocol f f IAPP IAPP e QoS QoS basic protocol ii Security Security MAC layer h d Since DFS/TCP DFS/TCP the Scanning Scanning standard is frozen, additions must be a specified in b various amendments. g Many of OFDM these 5GHz are still DSSS in the 2.4GHz draft OFDM phase. 2.4GHz OFDM 5GHz DSSS 2.4GHz OFDM 2.4GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 42
43 IEEE f The objective: to specify the f f e i Inter-Access Point i Protocol QoS QoS Security (IAPP) that enables Securityseamless roaming basic protocol between different Access Points within an ESS. h d Note: Scanning Scanning f is not concerned a with b roaming between g ESS DSSS networks. DSSS 2.4GHz For 2.4GHz OFDM this OFDM 2.4GHz purpose, non- 2.4GHz solutions must be used. IAPP IAPP DFS/TCP DFS/TCP OFDM OFDM 5GHz 5GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 43
44 IEEE e f f IAPP IAPP h DFS/TCP DFS/TCP a OFDM OFDM 5GHz 5GHz Quality of Service e i (QoS) i for better QoS QoS Security Security handling of voice basic protocol traffic, by finding ways of minimizing d jitter and delay Scanning Scanning variations and b maximising g access point throughput. DSSS DSSS 2.4GHz 2.4GHz OFDM OFDM 2.4GHz 2.4GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 44
45 IEEE i Security f issues such f e as TKIP IAPP(Temporary QoS QoS Key Integrity Protocol) e.g. for basic protocol improved key management, h and d 802.1x DFS/TCP DFS/TCP for Scanning Scanning authentication a b (note: OFDM OFDM can 5GHz 5GHz also DSSS DSSS be 2.4GHz 2.4GHz used in wired LAN). ii Security Security g OFDM OFDM 2.4GHz 2.4GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 45
46 IEEE h Transmit Power f f e i Control (TPC) i & QoS QoS Security Dynamic Frequency Security basic Selection protocol (DFS): Required in Europe h d for WLAN systems Scanning Scanning operating in the 5 a bghz band. g IAPP IAPP DFS/TCP DFS/TCP OFDM OFDM 5GHz 5GHz DSSS DSSS 2.4GHz 2.4GHz OFDM OFDM 2.4GHz 2.4GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 46
47 IEEE d f f IAPP IAPP h DFS/TCP DFS/TCP a OFDM OFDM 5GHz 5GHz d supplements e i the i MAC layer to QoS QoS Security Security promote worldwide basic protocol usage of networks (through d further development Scanning Scanning of active & passive b scanning g schemes). DSSS DSSS 2.4GHz 2.4GHz OFDM OFDM 2.4GHz 2.4GHz S Wireless Personal, Local, Metropolitan, and Wide Area Networks 47
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