MAC Architecture Module-7 Jerry Bernardini Community College of Rhode Island 6/18/2015Wireless Networking J. Bernardini1

Presentation Reference Material CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-9 The purpose of is to transfer data from computer to another --- not to be forgotten 6/18/2015Wireless Networking J. Bernardini2

Bits, Bytes, Octets, Frames, Packets Bits =1 or 0 Bytes = 8 bits Octets = 8 bits = Byte – Octet is used by telecommunication people – Byte is used by IT people Frames = grouping of bits at layer-2 Packets = grouping of bits at layer-3 Datagrams = another term for packets 6/18/2015Wireless Networking J. Bernardini3

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini 4 OSI data flow

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini 5 IEEE Physical Layer Standards IEEE wireless standards follow OSI model, with some modifications Data Link layer divided into two sublayers: – Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control – Media Access Control (MAC) sublayer: Appends physical addresses to frames Physical layer divided into two sublayers: – Physical Medium Dependent (PMD) sublayer: Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data – Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions Reformats data received from MAC layer into frame that PMD sublayer can transmit “Listens” to determine when data can be sent

6 Where the IEEE Standard Fits

Data Link Layer - Physical Layer- Data Units Data Link Layer (Layer-2) Physical Layer (Layer-1) Logical Link Control Media Access Control PHY Layer Convergence Protocol Physical Medium Dependent LLC MAC PLCP PMD PHY = Physical Layer MSDU (MAC Service Data Unit) MPDU (MAC Protocol Data Unit) PSDU (PLCP Service Data Unit) PPDU (PLCP Protocol Data Unit) Modulated Radio Signal ( MPDU = PSDU name change to indicated service needed) (From upper layers 2304 bytes max)

Ethernet and Frames Ethernet Frame Wireless Frame Preamble SourceDestinationDataFCS Start Of Frame Type or Length Field Frame Cntrl SourceDestinationData FCS 2 Rec. Adr Sequence Cntrl 66 Trans. Adr Duration ID Sync 10 or 18 Start Of Frame 2 4 or 6 PLCP Header MAC Packet DATA Unit, (MPDU) 1518

Frame Categories / Types Management Frames oBeacon Frame oProbe Frames oAssociation Frames… more Control Frames oRTS and CTS Frames oACK – Acknowledgement Frames… more Data Frames oData Payload Frames

Twelve Management Frame Types 6/18/2015Wireless Networking J. Bernardini10

Eight Control Frames Used to assist with the delivery of data frames 6/18/2015Wireless Networking J. Bernardini11

Fifteen Data Frames The frames that actually carry application data 6/18/2015Wireless Networking J. Bernardini12

IEEE Frame Formats CWNA Guide to Wireless LANs, Second Edition13 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)

Frame Types and Sizes Protocol Data Unit (PDU) Frame Type and Vendor SupportMTU (Bytes) MTU + Overhead (Bytes) TCPTransport maximum segment size1460 IPLayer-3 default size1500 MACIEEE Ethernet default MPDUIEEE default1534 MPDUIEEE maximum2304 MACJumbo Frame>1500 MACCisco Baby giant MACCisco Catalyst MACCisco Catalyst /18/2015Wireless Networking J. Bernardini14

Beacons Beacon – information frame sent by an AP. 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 PayloadTrailer Beacon Frame

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 6/18/2015Wireless Networking J. Bernardini 16 S1 S2 AP Control Point Beacon

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 6/18/2015Wireless Networking J. Bernardini 17 S1 S2 AP Control Point Beacons

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 6/18/2015Wireless Networking J. Bernardini 18 S1 S2 AP Control Point Probe Request Probe Response

Authentication and Association Using the IEEE State Machine Stations are in one of three states 1.Unauthenticated / Unassociated 2.Authenticated / Unassociated 3.Authenticated / Associated You cannot transmit data frames for processing until you are associated You cannot transmit associated frames for processing until you are Authenticated 6/18/2015Wireless Networking J. Bernardini19

Wireless Connection Process 2-Step Connection Process – Authentication Phase – Association Phase Authentication Phase Association Phase

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

Authentication Authentication - A means to establish or prove identity; verifying eligibility of users, devices, or applications. – Only authorized clients are allowed to gain access to the network. For this level of authentication it applies to the stations and not the user.

Association Association – The binding of a wireless network client to an Access Point for the purpose of data transfer. Again the stations not the user.

Connection State Machine Unauthenticated and Unassociated Authenticated and Unassociated Authenticated and Associated Successful Authentication De-authentication Notification Successful Association Disassociation Notification De-authentication Notification

Unauthenticated & Unassociated In the first state: – Wireless Client/Node is not connected to the network. Wireless Client Access Points Wired LAN

Authenticated & Unassociated In the Second State: – Wireless Client/Node has passed the authentication process but is not associated with the AP. Wireless Client Access Points Wired LAN Association Request Association Response

Authenticated & Associated In the Third State: – Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wireless Client Access Points Wired LAN Data Link

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

Open System Authentication Wireless Client Access Points Wired LAN Authentication Management Frame - SSID Authentication Management Frame ACK

Closed System Authentication Closed System – IEEE – Authentications based only on SSID – Client sends SSID – Receiving station, (AP) sends acknowledgment

Shared Key Authentication Shared Key – IEEE Wired Equivalent Privacy, (WEP). – Authentications based on Text and WEP Keys. – Challenge – Response Scheme

Shared Key Authentication Wireless Client Access Points Wired LAN Authentication Management Frame - SSID Challenge Phrase Encrypted Phrase Authentication WEP Key WEP Key Secure Channel

Roaming Roaming - The ability for a user to function when the serving network is different from their home/associated network. The process of a client moving seamlessly from one area or cell to another while maintaining a data link.

Roaming 1 Wireless Clients Access Points Wired LAN 1 12 Usually a 20%-30% cell overlap

Unauthenticated & Unassociated In the first state: – Wireless Client/Node is not connected to the network. Wireless Client Access Points Wired LAN

Authenticated & Unassociated In the Second State: – Wireless Client/Node has passed the authentication process but is not associated with the AP. Wireless Client Access Points Wired LAN Association Request Association Response

Authenticated & Associated In the Third State: – Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wireless Client Access Points Wired LAN Data Link

IEEE MAC Functions 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 6/18/2015Wireless Networking J. Bernardini38

Open Association Process 6/18/2015Wireless Networking J. Bernardini39

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