15 Medium Access (simple view 1 week)

  Medium Access Control

Slide ke 15 MK Wireless Network with Carrier Sense Multiple Access

Collision Avoidance (CSMA/CA) (1)

   Metode ini digunakan pada jaringan wireless LAN, yang

dimaksudkan untuk menghindari terjadinya tabrakan

antara data yang dikirimkan oleh beberapa node / user _ yang hendak melakukan transmisi.

  Pada komunikasi menggunakan kabel, suatu node dapat melakukan Sense terhadap media transmisi yang _ digunakan.

  Namun pada jaringan wireless, suatu node tidak

mungkin dapat untuk mendeteksi seluruh sinyal yang berlalu lalang pada media (udara). with

Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) (2)

   _{disamping yaitu jaringan digambarkan pada gambar Cara kerja CSMA/CA dapat  Dimana Y merupakan media yaitu node X, Y, dan Z.} wireless dengan tiga titik _{Access Point. dapat diamsumsikan sebagai} perantara yang secara mudah  _{Karena keterbatasan hanya dapat merakan kekuatan sinyal, maka X media menuju Y, dan Y hanya hanya bisa merasakan kondisi} keadaan media menuju Y, Z _{ collision jika X dan Z mengirimkan data Kondisi tersebut akan menyebabkan dengan dirinya. ke node yang terkoneksi} bisa merasakan kondisi media _{menjangkau Z dan sebaliknya. secara bersamaan, karena X tidak dapat} with Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) (3) _  _{Untuk menghindari collision, gini-gini. TRUZZ GIMANA DONKKK ?? …} baik X dan Z musti _{yaitu Y, sedang dalam kondisi mendeteksi perantaranya,  akan mengirimkan sinyal Salah satu node, misal X,} sibuk apa tidak. _{RTS berisi permintaan untuk melakukan transmisi beserta} (Request to Send) _{kepada Y yang  Setelah Y menerima , untuk melakukan transmisi.} waktu yang dibutuhkan oleh X _{RTS  berkomunikasi dengan Y, dan bagi Z, Informasi CTS menandakan bahwa X dapat Z. CTS (Clear to Send)} maka Y akan mengirimkan _{baik ke X dan}

_sibuk.

_{informasi ini menandakan bahwa Y sedang}

with Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) (4) _  _{Untuk menghindari collision, gini-gini. TRUZZ GIMANA DONKKK ?? …} baik X dan Z musti _{yaitu Y, sedang dalam kondisi mendeteksi perantaranya,  akan mengirimkan sinyal Salah satu node, misal X,} sibuk apa tidak. _{RTS berisi permintaan untuk melakukan transmisi beserta} (Request to Send) _{kepada Y yang  Setelah Y menerima , untuk melakukan transmisi.} waktu yang dibutuhkan oleh X _{RTS  berkomunikasi dengan Y, dan bagi Z, Informasi CTS menandakan bahwa X dapat Z CTS (Clear to Send)} maka Y akan mengirimkan _{baik ke X dan}

_sibuk.

_{informasi ini menandakan bahwa Y sedang}

with Carrier Sense Multiple Access

Collision Avoidance (CSMA/CA) (5)

  

IEEE Wireless LAN Configurations:

Basic Service Set

• 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
• 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

  IEEE Wireless LAN Configurations:

Basic Service Set (continued)

Basic Service Set (BSS)

  

IEEE Wireless LAN Configurations:

Extended Service Set

• Extended Service Set (ESS): Comprised of two or more BSS networks connected via a common distribution system
• APs can be positioned so that cells overlap to facilitate roaming
• – Wireless devices choose AP based on signal strength
• – Handoff
IEEE Wireless LAN Configurations:

Extended Service Set (continued)

Extended Service Set (ESS)

  

IEEE Wireless LAN Configurations:

Independent Basic Service Set

• 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
IEEE Wireless LAN Configurations: Independent Basic Service Set

(continued)

  Independent Basic Service Set (IBSS)

  

IEEE 802.11 Media Access Control

(MAC) Layer Standards

• Media Access Control (MAC) layer performs several vital functions in a WLAN
• – Discovering WLAN signal
• – Joining WLAN
• – Transmitting on WLAN
• – Remaining connected to WLAN
• Mechanics of how functions performed center around frames sent and received in WLANs

  

MAC Frame Formats

• Packet: Smaller segments of a digital data transmission
• – Strictly speaking, other terms used to describe these smaller segments
• Frames: Packet at MAC layer
• – Or Data Link layer in OSI model
• – IEEE MAC frames different from 802.3 Ethernet frames in format and function
• – Used by wireless NICs and APs for communications and managing/controlling wireless network

  

MAC Frame Formats (continued)

• Frame control field identifies:
• – Specific 802.11 protocol version
• – Frame type
• – Indicators that show WLAN configuration
• All frames contain

• – MAC address of the source and destination device

• – Frame sequence number
• – Frame check sequence for error detection

  

MAC Frame Formats (continued)

• Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode)
• – Maintain connection

  Structure of a management frame

  

MAC Frame Formats (continued)

• Types of management frames:
• – Authentication frame
• – Association request frame
• – Association response frame
• – Beacon frame
• – Deauthentication frame
• – Disassociation frame
• – Probe request frame
• – Probe response frame
• – Reassociation request frame
• – Reassociation response frame
Control frame

  

MAC Frame Formats (continued)

• Control frames: Provide assistance in delivering frames that contain data

Data frame

  

MAC Frame Formats (continued)

• Data frame: Carries information to be transmitted to destination device

  X 802.11 MAC _{xxx Distribution System (DS)} Y

  Addressing

_{Access Point 1}

_{111 Access Point 2} Host A to Host B

  C A B _{General 802.11 Frame}

_aaa

_bbb D

• Address 1 – Receiver address
• Address 2 – Transmitter address
• Address 3 – Ethernet SA, Ethernet DA, or BSSID
• Transmitter: Sends a frame on to the wireless medium, but _{doesn’t necessarily create the frame.}
• Receiver: Receives a frame on the wireless medium, but may not _{be the destination, i.e. may be the access point.}

  

Discovering the WLAN: Beaconing

• At regular intervals, AP (infrastructure network) or wireless device (ad hoc network) sends beacon frame
• – Announce presence
• – Provide info for other devices to join network
• Beacon frame format follows standard structure of a management frame
• – Destination address always set to all ones
Discovering the WLAN: Beaconing

(continued)

Beaconing

  

Discovering the WLAN: Beaconing

(continued)

• Beacon frame body contains following fields:
• – Beacon interval
• – Timestamp – Service Set Identifier (SSID)
• – Supported rates
• – Parameter sets
• – Capability information
>In ad hoc networks, each wireless device assumes responsibility for beaconing
• In infrastructure networks beacon interval normally 100 ms, but can be modified

  

Discovering the WLAN: Scanning

• Receiving wireless device must be looking for beacon frames
• Passive scanning: Wireless device simply listens for beacon frame

• – Typically, on each available channel for set period

• Active scanning: Wireless device first sends out a management probe request frame on each available channel
• – Then waits for probe response frame from all available APs

  Discovering the WLAN: Scanning

(continued)

Active scanning

  

Joining the WLAN: Authentication

• Unlike standard wired LANS, authentication performed before user connected to network

• – Authentication of the wireless device, not the user

• IEEE 802.11 authentication: Process in which AP accepts or rejects a wireless device
• Open system authentication: Most basic, and default, authentication method
• Shared key authentication: Optional authentication method
• – Utilizes challenge text

  Joining the WLAN: Authentication

(continued)

Open system authentication

  Joining the WLAN: Authentication

(continued)

Shared key authentication

  

Joining the WLAN: Authentication

(continued)

• Open system and Shared key authentication techniques are weak
• – Open System: Only need SSID to connect
• – Shared Key: Key installed manually on devices
>Can be discovered by examining the dev
• Digital certificates: Digital documents that associate an individual with key value
• – Digitally “signed” by trusted third party
• – Cannot change any part of digital certificate without being detected

  ID and supported data rates

  

Joining the WLAN: Association

• Association: Accepting a wireless device into a wireless network
• – Final step to join WLAN
• After authentication, AP responds with association response frame
• – Contains acceptance or rejection notice
• If AP accepts wireless device, reserves memory space in AP and establishes association ID
• Association response frame includes association

  

Transmitting on the WLAN: Distributed

Coordination Function (DCF)

• MAC layer responsible for controlling access to wireless medium
• 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

  _{All stations detect the collision}

Medium Access – CSMA/CA

ACK _CSMA/CD

• Both CSMA/CD and CSMA/CA are half-duplex architectures
• Ethernet uses CSMA/CD – Collision Detection _{– Ethernet devices detect a collision as when the data is transmitted}
• 802.11 uses CSMA/CA – Collision Avoidance _{– 802.11 devices only detect a collision when the transmitter has not received an Acknowledgement (coming).}
• – Stations also use CS/CCA – coming

  

Medium Access – CSMA/CA

• The 802.11 standard makes it mandatory that all stations implement the DCF _{(Distributed Coordination Function), a form of carrier sense multiple access with collision avoidance (CSMA/CA). Coming!}
• CSMA is a contention-based protocol making sure that all stations first sense _{the medium before transmitting (physically and virtually). Coming! • The main goal of CSMA/CA is to avoid having stations transmit at the same}

  time, which will then result in collisions and eventual retransmissions. Coming!

  _{• However, collisions may still occur and when they do stations may or may not be able to detect them (hidden node problem). Coming!} ^CSMA/CD
^{CSMA/CA ACK All stations detect the collision}

  

DCF and PCF

• • IEEE mandated access mechanism for 802.11 is DCF (Distributed

  _{Coordination Function) – Discussed in detail next} – Basis for CSMA/CA
• There is also the PCF (Point Coordination Function) _{– Point Coordinators (PC), ie.Access Points, provide point coordination – Restricted to Infrastructure BSSs} for contention-free services.
• – Stations can only transmit when allowed to do so by PC (AP).

  DCF Operation

• In DCF operation, a station wanting to transmit :

  _{– Checks to see if radio link is clear, CS/CCA – Carrier Sense,}

  _{Clear Channel Assessment (Later in PHY presentation)}
• – Checks its Network Allocation Vector (NAV) timer to see if _{someone else is using the medium. – If medium is available DCF uses a random backoff timer to avoid} collisions and sends the frame.
>Transmitting station only knows the 802.11 frame got there if it receives an ACK.
• May also use RTS/CTS to reduce collisions (coming)

  Duration Field General 802.11 Frame (more on this later)

• • Duration/ID field – The number of microseconds (millionth of a second)

  _{progress. that the medium is expected to remain busy for transmission currently in – Includes time to:} – Transmitting device sets the Duration time in microseconds. _{• The returning ACK} • Transmit this frame to the AP (or to the client if from an AP) _{• All stations monitor this field!}
• The time in-between frames, IFS (Interframe Spacing) • All stations update their NAV (Network Allocation Vector) timer.

  NAV Timer General 802.11 Frame (more on this later)

• _{• Virtual carrier-sensing function} • All stations have a NAV (Network Allocation Vector) timer. _{• Martha sends a frame to George.} • Protects the sequence of frames from interruption. _{• Vivian updates her NAV timer with the duration value.} • Since wireless medium is a “broadcast-based” (not broadcast frame) _{shared medium, all stations including Vivian receive the frame.}

  _{• Stations will only update their NAV when the duration field value received}

  • Vivian will not attempt to transmit until her NAV is decremented to 0.

  

Interframe Spacing (IFS)

• 802.11 uses four different interframe spaces used to determine _{medium access (note: microsecond = millionth of a second): – DIFS – DCF Interface Space (50 microseconds in DSSS)}
• Minimum amount of medium idle time until contention-based services _{begin. – PIFS – PCF Interframe Space (30 microseconds in DSSS)}
• Used by PCF _{– SIFS – Short Interframe Space (10 microseconds in DSSS) • Used for highest priority transmission, ACKs, RTS, CTS}

  

Wanting to transmit (1/3)

_{Random backoff slots • Carrier Sensing:} • Station wanting to transmit. _{– Physical: Physically senses medium is idle (CS/CCA – coming). • Waits DIFS (DCF Interface Space) period of 50 microseconds}

_{– Minimum amount of medium idle time until contention-based services begin.}

– Virtual: NAV timer is 0 _{• Contention window begins. – Uses random backoff algorithm to determine when it can attempt to access} – Once DCF is over, stations can contend for access. the medium. (next)

  Wanting to transmit (2/3) _{Contention Window Begins • The random backoff algorithm randomly selects a value from 0 to 255}

• (Detail of random backoff algorthim has been left out, but this will be sufficient.) _{(maximum value varies by vendor and stored in the NIC).}

_{• Stations pick a random slot and wait for that slot before attempting to access} • The random value is the number of 802.11 slot times the station must wait _{after the DIFS, during the contention window before it may transmit. • With several stations attempting to transmit, the station that picks the lowest slot, lowest random number, wins.} the medium. Example I’m _{waiting • Both Vivian and George want to transmit frames. Scenario:} I’m _waiting

• Both stations have same NAV values and physically sense when the _{medium is idle.}
• Both are waiting for Martha’s transmission to end and the medium to _{become available.}
• The medium now becomes available.

  Example _{Random backoff slots • Both perform the following:} • George and Vivian are both wanting to transmit.

• Both sense that medium is available using Physical and Virtual _{Carriers Sensing: – Physical: Physically senses medium is idle (CS/CCA – coming). • Both waits DIFS (DCF Interface Space) period of 10 microseconds}
• – Virtual: NAV timer is 0 • Contention window begins. _{– Uses random backoff algorithm to determine when it can attempt to access the medium. (next)}

  Example _{Vivian (7), George (31)}

• Both Vivian and George calculate their random backoff algorithm to randomly selects a value from 0 to 255.
• Vivian has a slot time of 7, George a slot time of 31.
• Vivian wins.
• The destination of her frame is George

  _{based” 802.11 frame. receive “broadcast-} Martha and George

  Example _{update NAV Others} ( ( ( ) ) ) General 802.11 Frame (more on this later)

• Vivian transmits, setting the Duration ID to the time needed to transmit, ACK and IFSs.
• George with a higher slot will see the 802.11 frame from Vivian and wait to transmit.
• Assuming their was not a collision from another station, Martha and George update their NAVs.

  

Transmitting on the WLAN: Distributed

Coordination Function (continued)

Hidden node problem

  RTS/CTS Solution • The hidden node stations cannot see the RTS. _{• The AP replies to Vivian with a CTS, which all nodes, including the hidden node can see.}

• ^{Vivian attempts to reserve the medium using} _{an RTS control frame to the AP.}
• ^{The RTS frame indicates to the AP and all} _{stations within range, that Vivian wants to reserve the medium for a certain duration of} time, message, ACK, and SIFS.
• Vivian transmits the frame. _{• The AP returns an ACK to Vivian. • The AP sends the message to George who returns an ACK to the AP.}

  RTS/CTS Solution _{additional latency. capacity and overhead, resulting in} ^{RTS/CTS consumes a fair amount of}

• _{environments.} ^{Normally used in high capacity}
>The RTS/CTS procedure can be enabled/controlled by setting the RTS threshold on the 802.11 client NIC.

• • RTS/CTS is also used during frame fragmentation (coming).

  Setting the RTS Threshold on a Cisco Client _{Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS • Threshold} RTS _{occur in environments with obstructions or metallic surfaces that create complex multipath However, small values help the system recover from interference or collisions, which can}

_{more of the available bandwidth and reduces the throughput of other network packets.}

flow control. A small value causes RTS packets to be sent more often, which consumes

signals.

  

Improving WLAN Performance with RTS/

CTS by Jim Geier (wi-fiplanet.com)

• If you enable RTS/CTS on a particular station (just the _{frame until the station completes a RTS/CTS handshake with hidden node station), it will refrain from sending a data}

  _{• Keep in mind, though, that an increase in performance using}

  another station, such as an access point. RTS/CTS is the net result of introducing overhead (i.e., RTS/

  _{retransmissions). If you don't have any hidden nodes, then}

  _{CTS frames) and reducing overhead (i.e., fewer}

  _{problem may also result in performance degradation if you}

  _{overhead, which reduces throughput. A slight hidden node}

  the use of RTS/CTS will only increase the amount of _{by reducing retransmissions. Thus, be careful when}

  _{frames cost more in terms of overhead than what you gain}

  implement RTS/CTS. In this case, the additional RTS/CTS

  implementing RTS/CTS.

  

Improving WLAN Performance with RTS/

• _{• One of the best ways to determine if you should activate RTS/}

  CTS by Jim Geier (wi-fiplanet.com)

  _{and likely out of range, then try enabling RTS/CTS on the}

  _{large number of collisions and the users are relatively far apart}

  CTS is to monitor the wireless LAN for collisions. If you find a

  _{clicking "enable RTS/CTS" somewhere in the user setup screens.}

  applicable user wireless NICs. You can activate the function by

  _{You don't need to enable RTS/CTS at the access point in this • Of course, keep in mind that user mobility can change the access point will always respond with a CTS frame.}

  case. After receiving a RTS frame from a user's radio NIC, the

  _{other stations most of the time. If collisions are occurring}

  _{time, perhaps when you perform the testing, then be closer to}

  results. A highly mobile user may be hidden for a short period of _{the result of high network utilization or possibly RF interference.}

  between users within range of each other, the problem may be

  

Frame Fragmentation

• Since we have already discussed RTS/CTS, let’s also discuss frame _{fragmentation. • Later, we will see that RTS/CTS and fragmentation are typically} combined. _{• Frame fragmentation is a MAC layer function that is designed to increase the reliability of transmitting frames across a wireless medium.}

  

Frame Fragmentation

• In a “hostile wireless medium” (interference, noise) larger frames may _{have more of a problem reaching the receiver without any errors.}

  _{• By decreasing the size of the frame, the probability of interference during}

  transmission can be reduced. _{• Breaking up a large frame into smaller frames, allows a larger percentage of frames to arrive undamaged (without errors).}

  

Frame Fragmentation

• Frame fragmentation can increase the reliability of frame _{transmissions but there is additional overhead:} – Each frame fragment includes the 802.11 MAC protocol header. _{– Each frame fragment requires a corresponding acknowledgement.}
• If a frame fragment encounters errors or a collision, only that _{fragment needs to be retransmitted, not the entire frame.}

• • The frame control field includes information that this is a fragmented

  _frame.

  

Transmitting on the WLAN: Quality of

Service (QoS) and 802.11e

• DCF does not work well for real-time, time- dependent traffic
• Quality of Service (QoS): Capability to prioritize different types of frames
• Wi-Fi Multimedia (WMM): Modeled after wired network QoS prioritization scheme
• 802.11e draft: defines superset of features intended to provide QoS over WLANs
• – Proposes two new mode of operation for 802.11 MAC Layer
Transmitting on the WLAN: Quality of

Service and 802.11e (continued)

Wi-Fi Multimedia (WMM)

  

Transmitting on the WLAN: Quality of

Service and 802.11e (continued)

• 802.11e draft (continued):
• – Enhanced Distributed Channel Access (EDCA): Contention-based but supports different types of traffic
• Four access categories (AC)
• Provides “relative” QoS but cannot guarantee service
• – Hybrid Coordination Function Controlled

  Channel Access (HCCA): New form of PCF based upon polling

• • Serves as a centralized scheduling mechanism

  

Remaining Connected to the WLAN:

Reassociation

• Reassociation: Device drops connection with one

  AP and establish connection with another

• – Several reason why reassociation may occur:
• Roaming • Weakened signal
• – When device determines link to current AP is poor, begins scanning to find another AP
• Can use information from previous scans

  

Remaining Connected to the WLAN:

Power Management

• When laptop is part of a WLAN, must remain

  “awake” in order to receive network transmissions

• – Original IEEE 802 standard assumes stations always ready to receive network messages
• Power management: Allows mobile devices to conserve battery life without missing transmissions
>– Transparent to all protocols
• – Differs based on WLAN configuration
• – AP records which stations awake and sleeping
• – Buffering: If sleeping, AP temporarily stores frames
Remaining Connected to the WLAN:

Power Management (continued)

Power management in infrastructure mode

  

Remaining Connected to the WLAN:

Power Management (continued)

• At set times AP send out beacon to all stations
• – Contains traffic indication map (TIM)
• – At same time, all sleeping stations switch into active listening mode
• Power management in ad hoc mode:
• – Ad hoc traffic indication message (ATIM)

  window: Time at which all stations must be awake

• Wireless device sends beacon to all other devices
• – Devices that previously attempted to send a frame to a sleeping device will send ATIM frame indicating that receiving device has data to receive and must remain awake

  

WLAN Network Layer Standards:

WLAN IP Addressing

• In standard networking, IP protocol responsible for moving frames between computers
• – Network layer protocol
• TCP/IP works on principle that each network host has unique IP address
• – Used to locate path to specific host
• – Routers use IP address to forward packets
• – Prohibits mobile users from switching to another network and using same IP number
• Users who want to roam need new IP address on every network

  

WLAN Network Layer Standards:

Mobile IP

• Provides mechanism within TCP/IP protocol to support 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
WLAN Network Layer Standards:

Mobile IP (continued)

Mobile IP components

  WLAN Network Layer Standards:

Mobile IP (continued)

Computer relocated in Mobile IP

  WLAN Network Layer Standards:

Mobile IP (continued)

Encapsulated Mobile IP frame

  

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
• Frames are used by both wireless NICs and access points for communication and for managing and controlling the wireless network

  

Summary (continued)

• The MAC layer provides four major functions in

  WLANs: discovering the WLAN signal, joining the WLAN, transmitting on the WLAN, and remaining connected to the WLAN

• Discovery is a twofold process: the AP or other wireless devices must transmit an appropriate frame (beaconing), and the wireless device must be looking for those frames (scanning)
• Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association

  

Summary (continued)

• The IEEE 802.11 standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF)
• The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF)
• The 802.11e draft defines a superset of features that is intended to provide QoS over WLANs

  

Summary (continued)