Wireless Networking. Media Access Control Layer Standards Module-06

Transcription

1 Wireless Networking Media Access Control Layer Standards Module-06 Jerry Bernardini Community College of Rhode Island CCRI J. Bernardini 1 REFERENCES LANs, 3rd Edition Chapter-6 CWTS Certified Wireless Technology Specialist Official Study Guide, Chapter-7 CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-7 The California Regional Consortium for Engineering Advances in Technological Education (CREATE) project CCRI J. Bernardini 2 1

2 Objectives Describe the three WLAN service sets Explain the features of MAC frames and MAC frame types Describe the MAC functions of discovering, joining, and transmitting on a WLAN 3 IEEE Topologies or Operating Modes The purpose of is to interconnect radio cards Every wireless device has a radio card All wireless devices are referred to as Stations (STA) Three topologies defined by Service Sets Basic Service Set (BSS) Extended Service Set (ESS) Independent Basic Service Set (IBSS) Nonstandard Topologies Bridging, Repeating, Workgroup bridging Mesh networking (growing in importance) 10/8/2014 Wireless Networking J. Bernardini 4 2

3 Service Set Identifiers - SSID and BSSID SSID -Service Set Identifier is a 1-32 byte alphanumeric sequence that uniquely names an ESS (the network name). Any SSID or Null SSID is a blank SSID used to associate with anyone. BSSID- Basic Service Set Identifier is a 48-bits that uniquely identifies a BSS BSA Basic service Area Physical Coverage Area ESS Id = SSID Wired LAN AP BSS Id = BSSID Basic Independent Basic Service Set (IBSS) Independent Basic Service Set (IBSS): Wireless network that does not use an AP Wireless devices communicate between themselves Peer-to-peer or ad hoc mode BSS more flexible than IBSS in being able to connect to other wired or wireless networks IBSS useful for quickly and easily setting up wireless network When no connection to Internet or external network needed 6 3

4 Basic Service Set (BSS) BSS - The Basic Service Set is a term used to describe the collection of Stations which may communicate together within an WLAN. Basic Service Set (BSS): Group of wireless devices served by single AP infrastructure mode BSS must be assigned unique identifier Service Set Identifier (SSID) Serves as network name for BSS Basic Service Area (BSA): Geographical area of a BSS Max BSA for a WLAN depends on many factors Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases 7 Extended Service Set (ESS) ESS - is comprised of a number BSS s ESS stations must have the same SSID The BSSID is the name of the BSS (not same as SSID) APs can be positioned so that cells overlap to facilitate roaming Wireless devices choose AP based on signal strength Stations going from one BSS to another will deal with Handoff Wired LAN ESS SSID BSS2 (BSSID2) BSS1 (BSSID1) BSS3 (BSSID3) 4

5 Extended Service Set and Mobile IP If STs move from one AP to another AP resides in a separate subnet a new IP address must be assigned Connectivity can be temporarily lost Running applications may have to be restarted Called Layer 3 roaming TCP/IP protocol support of mobile computing Computers given home address Static IP number on home network Home agent: Forwarding mechanism that keeps track of where mobile computer located When computer moves to foreign network, a foreign agent provides routing services Assigns computer a care-of address Computer registers care-of address with home agent 9 Mobile IP Components MN Mobile Node (VMD) FA Foreign Agent (MAP) HA Home Agent (MAP) AAP Authoritative Access Point MAP Mobile IP Access Point (MN) VMD Visiting Mobile Device (MN) COA Care-of-Address HAN Home Agent Network FAN Foreign Agent Network CN Correspondent Node CCRI J. Bernardini 10 5

6 Mobile IP Process - Discovery Mobile IP 3-Step Process 2 MN Mobile Node 1-Discovery 2-Registration FA Foreign Agent HA Home Agent 3-Tunneling 1- Mobile Node (MN) gets address from Home Agent (HA) 2- MN moves to Foreign Agent (FA ) network 3- MN discovers FA or solicits for an agent. 4- MN gets Care-of-Address (COA) from FA CCRI J. Bernardini 11 Mobile IP Process - Registration Mobile IP 3-Step Process 1-Discovery 2-Registration 3-Tunneling MN Mobile Node FA Foreign Agent HA Home Agent CN Correspondent Node 5- MN send registration request to FA 6- FA checks request, adds MN to pending list 7- FA relays request to HA 8- HA checks request and creates a mobile binding 9- HA sends registration reply to FA 10- FA checks reply and adds MN to visitor list and relays reply to MN 11-FA creates a routing entry and tunnel to HA 12- MN check reply and creates a tunnel to the HA Through the FA CCRI J. Bernardini 12 6

7 Mobile IP- Tunneling and Packet Forwarding MN Mobile Node FA Foreign Agent HA Home Agent CN Correspondent Node COA- Care-of-Address 13- Correspondent Node (CN) sends packet to MN and is routed to HA 14- HA intercepts packets and tunnels to the FA using the COA 15- FA relays the packets to the MN CCRI J. Bernardini 13 Subnets and Mobile IP Subnets provide many advantages: make network management easier Subnets reduce broadcasts Subnets provide security Subnets use subnet masks ( ) WLAN subnets creates problems for roaming devices Example: /24 can seamlessly roam within x /24 Note /24 is a subnet mask /24 can not seamlessly roam within x /24 without changing the IP address Mobile IP offers a solution can roam as a guest on the x network CCRI J. Bernardini 14 7

8 Mobile IP -Explained With mobile IP, computers are given a home address (static IP number on home network) Home agent: forwarding mechanism that keeps track of where the mobile computer is located When the computer roams to another network (foreign network) a foreign agent provides routing services to the computer Foreign agent assigns a temporary IP number (known as care-of-address) Computer then registers the care-of-address with its home agent 15 Mobile IP (continued): Mobile IP (continued) When a frame is sent to computer s home address, the home agent intercepts the frame Encapsulates that frame into a new frame with the care-ofaddress as the destination address Redirects it to the foreign agent, which send it to the computer located on the foreign network Mobile IP enables a host to be identified by a single IP number even as it moves from one network to another 16 8

9 Distribution system (DS) Distribution system (DS): used by an AP to determine what communication needs to take place with other APs in the ESS or with the wired network Decides if it is necessary to exchange frames in their own BSSs, with a wired network, or to forward frames to another BSS Distribution system media: media that interconnects APs Wireless distribution system (WDS) - A wireless configuration that is used to connect APs 17 Medium Access Control Layer Frame Formats and Types IEEE has divided the Data Link Layer into two sublayers: Logical Link Control (LLC) sublayer: provides a common interface, reliability, and flow control Medium Access Control (MAC) sublayer: appends physical addresses to the frame Functions performed at the MAC sublayer involve different frame formats and types There are three main types of MAC frames: Management frames Control frames Data frames Ethernet one frame type 18 9

10 MAC Frame Formats OSI model uses the term data unit to describe sets of data that move through the OSI layers Service Data Unit (SDU): specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsulated by that lower layer Protocol Data Unit (PDU): specifies that data will be sent to the peer protocol layer at the receiving device Changing an SDU to a PDU involves an encapsulation process in which the lower layer adds headers and footers 19 MAC Frame Formats Process in a network using SDUs and PDUs: Layer 3 send data to LLC sublayer of Layer 2. Unit of data is called the MAC Service Data Unit (MSDU) LLC sends data unit to MAC sublayer where MAC header information is added. Data unit is now called MAC Protocol Data Unit (MPDU) also known as frame MPDU is sent to PLCP sublayer in the Physical Layer and is then called the PLCP Service Data Unit (PSDU) PSDU is passed to the PMD sublayer that creates the PLCP Protocol Data Unit (PPDU) by adding header/footer PPDU is then transmitted as a series of bits 20 10

11 SDUs and PDUs LANs, Third Edition 21 MAC Frame Formats Aggregate MAC Service Data Unit (A-MSDU): allows multiple MSDUs to be combined Aggregate MAC Protocol Data Unit (A-MPDU): allows multiple MPDUs to be combined Figure 6-9 A-MSDU and A-MPDU 22 11

12 Ethernet and Frames Ethernet Frame Preamble Source Destination Data FCS Start Of Frame Wireless Frame 10 or 18 Sync Type or Length Field 2 4 or 6 PLCP Header Start Of Frame Source Destination Rec. Adr Trans. Adr Data FCS Duration ID Frame Cntrl MAC Packet DATA Unit, (MPDU) Sequence Cntrl Management Frames Beacon Frame Probe Frames Association Frames more Control Frames Frame Categories / Types RTS and CTS Frames ACK Acknowledgement Frames more Data Frames Data Payload Frames 12

13 IEEE Frame Formats Management Frame Control (2) Duration (2) Des. Address (6) Source Address (6) BSSID (6) Seq. Control (2) Frame Body ( 1 to 2311) Frame Check Seq. (6) Control Frame Control (2) Duration (2) Receiver Address (6) Transmit Address ( 6) Frame Check Seq. (6) Data Frame Control (2) Duration (2) Address 1 (6) Address 2 (6) Address 3 (6) Seq. Control (2) Address 4 (6) Data ( 1 to 2311) Frame Check Seq. (6) (Bytes per field) LANs, Second Edition 25 Twelve Management Frame Types 10/8/2014 Wireless Networking J. Bernardini 26 13

14 Beacons Beacon information frame sent by an AP every 100 ms Beacon frames are approximately 50-bytes with the following information: Timestamp Beacon Interval Capability Info Service Set Identifier Support Rates Parameter Sets Traffic Indication Map Header Payload Trailer Beacon Frame Eight Control Frames Used to assist with the delivery of data frames 10/8/2014 Wireless Networking J. Bernardini 28 14

15 Fifteen Data Frames The frames that actually carry application data 10/8/2014 Wireless Networking J. Bernardini 29 Protocol Data Unit (PDU) MTU and Fragmentation Frames have different MTUs and may require fragmentation Frame Type and Vendor Support MTU (Bytes) TCP Transport maximum segment size 1460 IP Layer-3 default size 1500 MTU + Overhead (Bytes) MAC IEEE Ethernet default MPDU IEEE default 1534 MPDU IEEE maximum 2304 MAC Jumbo Frame >1500 MAC Cisco Baby giant A-MPDU IEEE Aggregate Service Unit 7935 MAC Cisco Catalyst MAC Cisco Catalyst /8/2014 Wireless Networking J. Bernardini 30 15

16 IEEE MAC Functions Main Functions Discovering a WLAN - Joining the WLAN - Transmitting on a WLAN Scanning- discover AP or BSS Synchronization- all stations have the same clock Frame Transmission- rules for frame transfer Authentication-allow device in network Association-after authentication associate with AP Reassociation-roaming and association with new AP Data Protection-data encryption protects data Power Management-save power by sleeping transceiver Fragmentation-breakup frame for efficiency and interfer. RTS/CTS- solution to hidden node problem 10/8/2014 Wireless Networking J. Bernardini 31 Beacon Management Frame A special management frame that is used by a client stations seeking a wireless network to join. Instead of beacon frames a station could use probe request and probe response frames In an ad hoc (IBSS ) wireless network all stations take turns broadcasting the beacon frame Beacon Beacon S1 S2 AP Control Point 10/8/2014 Wireless Networking J. Bernardini 32 16

17 Passive Scanning (Beacons) 1. Client stations listens for a beacon from an access point (AP) 2. If multiple beacons are received the strongest one is selected 3. The listening station then requests authentication and association S1 Beacons Beacons AP Control Point S2 10/8/2014 Wireless Networking J. Bernardini 33 Active Scanning (Probes) A station could use probe request and probe response frames Instead of beacon frames 1. Station is configured with SSID and switched to a channel 2. Probe request sent by requesting station 3. All stations that have the same SSID and have normal configurations respond with a Probe Response frame The process also involves waiting for ProbeDelay and MinChannel Timers S1 Probe Request AP Control Point Probe Response S2 10/8/2014 Wireless Networking J. Bernardini 34 17

18 Wireless Connection Process Authentication is the process used by a station to: verify that another station is approved for communications. this is a station authentication and not a user authentication. Authentication Step Two IEEE standards: Open System Authentication Shared Key Authentication 2-Step Connection Process Authentication Phase Association Phase Authentication Phase Association Phase IEEE Connection State Machine Unauthenticated and Unassociated Successful Authentication De-authentication Notification Authenticated and Unassociated Successful Association Disassociation Notification Authenticated and Associated De-authentication Notification 18

19 Unauthenticated & Unassociated In the first state: Wireless Client/Node is not connected to the network. Wired LAN Access Points Wireless Client Authenticated & Unassociated In the Second State: Wireless Client/Node has passed the authentication process but is not associated with the AP. Wired LAN Association Response Access Points Association Request Wireless Client 19

20 In the Third State: Authenticated & Associated Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wired LAN Access Points Data Link Wireless Client Open System Authentication Open System / Null Authentication IEEE Default Authentications based on empty string SSID Client sends empty string SSID Receiving station, (AP) sends acknowledgment Wired LAN Authentication Management Frame - SSID Wireless Client ACK Authentication Management Frame ACK Access Points 20

21 Shared Key Authentication Wired LAN Authentication Management Frame - SSID Challenge Phrase Encrypted Phrase Access Points Wireless Client Authentication WEP Key Secure Channel WEP Key Roaming Roaming - The process of a client moving seamlessly from one area or cell to another while maintaining a data link. Wired LAN Access Points Wireless Clients Usually a 20%-30% cell overlap 21

22 Transmitting on the WLAN IEEE specifies three procedures for transmitting on the WLAN: Distributed coordination function Point coordination function Hybrid coordination function 43 MAC Layer Access Methods Communications Options DCF RTS/CTS (optional) Distributed function Wireless MAC Avoids hidden node problem DCF PCF (optional) AP polls stations Superframes to allow station to eventually get access Superframe = Beacon + CFP + CP CFP = Contention-Free Period CP = Contention Period 22

23 RTS/CTS Sending unicast packets Station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) Acknowledgement via CTS after SIFS by receiver (if ready to receive) Sender can now send data at once, acknowledgement via ACK Other stations store medium reservations distributed via RTS and CTS MAC Frame Formats Due to significant differences between highthroughput (HT) n and non-ht a/b/g, an AP can tell n devices to change to one of four HT Operation Modes: HT Greenfield Mode (Mode 0) HT Nonmember Protection Mode (Mode 1) HT 20 MHz Protection Mode (Mode 2) HT Mixed Mode (Mode 3) 46 23

24 Distributed Coordination Function The Distributed Coordination Function (DCF) is the fundamental access mechanism in IEEE Medium Access Control (MAC). DCF can be used in all wireless topologies: IBSS, BSS, and ESS. Distributed Coordination Function (DCF) Distributed coordination function (DCF) defines two procedures: Carrier Sense Multiple Access with Collision Detection and Request to Send/Clear to Send Channel access methods: Rules for cooperation among wireless devices Contention: Computers compete to use medium If two devices send frames simultaneously, collision results and frames become unintelligible Must take steps to avoid collisions 48 24

25 Distributed Coordination Function (DCF) Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Before networked device sends a frame, listens to see if another device currently transmitting If traffic exists, wait; otherwise send Devices continue listening while sending frame If collision occurs, stops and broadcasts a jam signal CSMA/CD cannot be used on wireless networks: Difficult to detect collisions Hidden node problem (when stations are out of range of each other) 49 Distributed Coordination Function (DCF) Distributed Coordination Function (DCF): Specifies modified version of CSMA/CD Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Attempts to avoid collisions altogether Time when most collisions occur is immediately after a station completes transmission All stations must wait random amount of time after medium clear Slot time 50 25

26 Distributed Coordination Function (DCF) CSMA/CA also reduces collisions via explicit frame acknowledgment Acknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intact If ACK not returned, transmission error assumed IEEE n adds a feature known as block acknowledgment Supports multiple MPDUs in an A-MPDU CSMA/CA does not eliminate collisions Does not solve hidden node problem 51 Distributed Coordination Function (DCF) Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem Also known as virtual carrier sensing Significant overhead upon the WLAN with transmission of RTS and CTS frames Especially with short data packets RTS threshold: Only packets that are longer than RTS threshold are transmitted using RTS/CTS 52 26

27 Distributed Coordination Function (DCF) Fragmentation: Divide data to be transmitted from one large frame into several smaller ones Reduces probability of collisions Reduces amount of time medium is in use If data frame length exceeds specific value, MAC layer fragments it Receiving station reassembles fragments Alternative to RTS/CTS High overhead ACKs and additional SIFS time gaps 53 Distributed Coordination Function (DCF) Variations of RTS/CTS are used as protection mechanisms: CTS-to-self: process used when g devices are mixed with b devices HT Dual-CTS Protection: used with n devices in a mixed environment with a/b/g devices n devices sends a RTS to the AP, which responds with two CTS frames: one in n format and one in non n format HT L-SIG Protection: used with n devices in a mixed environment 54 27

28 Distributed Coordination Function (DCF) Interframe spaces (IFS): Intervals between transmissions of data frames Short IFS (SIFS): For immediate response actions such as ACK Point Coordination Function IFS (PIFS): Time used by a device to access medium after it has been asked and then given approval to transmit Distributed Coordination Function IFS (DIFS): Standard interval between transmission of data frames 55 Distributed Coordination Function (DCF) Interframe spaces (IFS) continued: Extended IFS: used when frames must be retransmitted Arbitration IFS: used when setting priorities to different types of transmissions Reduced IFS: reduces amount of dead space required between OFDM transmissions 56 28

29 Point Coordination Function (PCF) Polling: Channel access method in which each device asked in sequence if it wants to transmit Effectively prevents collisions Point Coordination Function (PCF): AP serves as polling device or point coordinator Point coordinator has to wait only through point coordination function IFS (PIFS) time gap Shorter than DFIS time gap 57 Point Coordination Function (PCF) If point coordinator hears no traffic after PIFS time gap, sends out beacon frame Field to indicate length of time that PCF (polling) will be used instead of DCF (contention) Receiving stations must stop transmission for that amount of time Point coordinator then sends frame to specific station, granting permission to transmit one frame standard allows WLAN to alternate between PCF (polling) and DCF (contention) 58 29

30 Hybrid Coordination Function (HCF) Hybrid Coordination Function (HCF): allows for different types of wireless traffic to be given different levels of priority Enhanced Distributed Channel Access (EDCA): Contentionbased but supports different types of traffic Four access categories (AC) Provides relative QoS but cannot guarantee service Hybrid Coordination Function Controlled Channel Access (HCCA): based upon polling Serves as a centralized scheduling mechanism 59 Timing Diagrams Event Timing or Sequence Diagrams - A graph showing events/levels as a function of time. Event 2ms 4ms 6ms 8ms 10ms Time tp1 sync mxc rst ack data A Time flag en tp2 data pd Time 30

31 SIFS SIFS - Shortest and highest priority time space sent before and/or after RTS, CTS, and ACK frames. For DSSS, SIFS is 10 microseconds or 10 S. sender DIFS data receiver SIFS ACK others DIFS DIFS data deferring mode contention waiting time PIFS PIFS Are used only in Point Coordination Mode by the APs. This mode is enabled by the administrator. It has medium priority and therefore always wins over DIFS, so that the AP can take control in polling. For DSSS, PIFS is 30 S. 31

32 Distributed Coordination Function The Distributed Coordination Function (DCF) is the fundamental access mechanism in IEEE Medium Access Control (MAC). DCF can be used in all wireless topologies: IBSS, BSS, and ESS. DIFS DIFS Is used by default on all stations. DIFS is the lowest priority and is used for data and management frames. For DSSS, DIFS is 50 s. DIFS PIFS Station Backoff Timers sender DIFS medium busy SIFS contention frame A look at all three: 10 s, 30 s, 50 s, for DSSS. 32

33 Hidden Node Wireless Client 1 2 Wireless Client Access Points 65 Hidden Node Detection and Solution Symptoms Increase corruption Increased retransmission Use protocol analyzer near to the AP and count corrupted frames Use protocol analyzer near to the STA and count retransmission frames Solutions Use RTS/RTS Increase STA power Remove obstacles and move STA if possible APs and STA at same IEEE h power level 66 33

34 Protection Mechanisms Protection Mechanisms allow for backward compatibility Older technology and newer technology compatibility Two Broad Categories of protection mechanism Extended rate physical (ERP) High throughput (HT) 10/8/2014 Wireless Networking J. Bernardini 67 Review g Protection Before an g client can transmit to an g AP it must reserve the medium. Must transmit so b will understand. Two Protection Methods CTS-to self at b modulation (slow Clear to Send) RTS-CTS at b modulation CTS-to-self is more efficient but may not be seen by hiddennode RTS-CTS is more reliable but has more overhead Both Methods dramatically reduce the g throughput CCRI J. Bernardini 68 34

35 802.11b/g Mixed Mode Operation Station g Station g AP g Station b 1-Slow CTS 2-Slow CTS 3-Fast Data 3-Fast Data 2-Slow CTS 3-Fast Data CCRI J. Bernardini Summary Characteristics Protocol Release Date Op. Frequency Throughput (Typical) Data Rate (Max) Modulation Technique Range (Radius Indoor) Depends, # and type of walls Range (Radius Outdoor) Loss includes one wall a GHz 23 Mbps 54 Mbps OFDM ~35 Meters ~120 Meters b GHz 4.3 Mbps 11 Mbps DSSS -CCK ~38 Meters ~140 Meters g GHz 19 Mbps 54 Mbps OFDM & DSSS ~38 Meters ~140 Meters n June GHz 5 GHz 74 Mbps 248 Mbps OFDM MIMO ~70 Meters ~250 Meters CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCRI J. Bernardini 70 35

36 HT Mode Non-HT Legacy Mandatory for n Only 20 MHz channels Same format as ag HT Mixed Two part preamble First part can be decoded by ag Second part can not be decoded by ag HT Greenfield Preamble can not be decoded by ag Can use both 20 MHz and 40 MHz channels 10/8/2014 Wireless Networking J. Bernardini 71 Dynamic Rate Switching Dynamic Rate Switching =Dynamic Rate Selection =Automatic Rate Shifting----All mean the same thing Process of reducing or increasing the data rate based upon RF signal levels RF signals attenuate over distance or by absorption AP will reduce data rate for weaker signals AP will increase data rate for higher signals Data Rate (Mb/sec) Received Signal (dbm) /8/2014 Wireless Networking J. Bernardini 72 36

37 Data Rate Coverage Areas -85 dbm -76 dbm -72 dbm 24 Mbps 18 Mbps 9 Mbps Summary A Basic Service Set (BSS) is defined as a group of wireless devices that is served by a single access point (AP) An Extended Service Set (ESS) is comprised of two or more BSS networks that are connected through a common distribution system An Independent Basic Service Set (IBSS) is a wireless network that does not use an access point A Service Data Unit (SDU) is a specific unit of data passed down from a higher OSI layer A Protocol Data Unit (PDU) specifies data that will be sent to the peer layer at the receiving device 74 37

38 Summary Because of the differences between n HT and non-ht a/b/g devices an AP can tell n devices to change to one of four HT Operation Modes in order to interoperate Three main types of MAC frames: management frames, control frames, and data frames WLAN discovery can be done by passive scanning or active scanning Passive scanning depends on the AP advertising itself Active scanning station send out a management probe request on an available channel 75 Summary Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association The IEEE standard specifies three procedures for transmitting on the WLAN, distributed coordination function (DCF), point coordination function (PCF), and hybrid coordination function (HCF) 76 38