Protocols for Wireless Communications

Wireless Protocols

Training materials for wireless trainers

Thursday, February 9, 2012 Version 1.0 by Ermanno 2012/2/6 Goals

The goal of this lecture is to introduce: ‣ IEEE wireless protocols coverage ‣802.11 radio protocols terminology ‣ WiFi modes of operation details

Thursday, February 9, 2012 IEEE Wireless Protocols 802.11 – Wireless Local Area Networks (WiFi) 802.11a, 802.11b, 80211g, 802.11n 802.15 – Wireless Personal Access Networks (WPAN) Task Group 1 – Bluetooth (802.15.1) Task Group 2 – Co-existence (802.15.2) Task Group 3 – High Rate WPAN (802.15.3) Task Group 4 – Low Rate WPAN (802.15.4 or 802.15 TG4) Task Group 5 – Mesh Networking (802.15.5) 802.16 – Wireless Metropolitan Area Networks (WiMAX) 802.22 – Wireless Regional Access Network (WRAN) Use free space in the allocated TV spectrum 3

Thursday, February 9, 2012 802.11 and 802.11x refers to a family of specifications developed by the IEEE for wireless LAN technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. Although the terms 802.11 and Wi-Fi are often used interchangeably, the Wi-Fi Alliance uses the term "Wi-Fi" to define a slightly different set of overlapping standards. In some cases, market demand has led the Wi-Fi Alliance to begin certifying products before amendments to the 802.11 standard are complete The 802.15 WPAN™ effort focuses on the development of consensus standards for Personal Area Networks or short distance wireless networks. These WPANs address wireless networking of portable and mobile computing devices such as PCs, Personal Digital Assistants (PDAs), peripherals, cell phones, pagers, and consumer electronics; allowing these devices to communicate and interoperate with one another. The goal is to publish standards, recommended practices, or guides that have broad market applicability and deal effectively with the issues of coexistence and interoperability with other wired and wireless networking solution The IEEE 802.16 Working Group on Broadband Wireless Access Standards develops standards and recommended practices to support the development and deployment of broadband Wireless Metropolitan Area Networks. IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) Working Group, the establishment of which was approved by IEEE Standards Board on December 11, 2002, aims to prepare a formal specification for a packet-based air interface designed for IP-based services. It is hoped that such an interface will allow the creation of low-cost, always-on, and truly mobile broadband wireless networks, nicknamed as Mobile-Fi. EEE 802.22 is a new working group of IEEE 802 LAN/MAN standards committee which aims at constructing Wireless Regional Area Network (WRAN) utilizing white spaces (channels that are not already used) in the allocated TV frequency spectrum. The use of the spectrum will be used in an opportunistic way in order not interfere with any TV channel that is transmitting. IEEE Wireless Protocols Coverage Scope

802.15 802.11 802.16 802.22 W. PAN W. LAN W. MAN W. RAN meters hundred m kilometer hundred km

Thursday, February 9, 2012 Personal, Local, Metropolitan and Regional Area Networks Random access and deterministic access Quality of Service Typical configuration: One BS serves several CPEs

Client Station Client Station

Base Station

Client Station Client Station

Thursday, February 9, 2012 802.11 Terminology

• Station: Device that contains IEEE 802.11 conformant MAC and PHY interface to the wireless medium, but does not provide access to a distribution system. Also called Client.

• Access Point (AP): Device that contains IEEE 802.11 conformant MAC and PHY interface to the wireless medium, and provide access to a distribution system for associated stations. Most often infra-structure products that connect to wired backbones 6

Thursday, February 9, 2012 Keep in mind that this is a functionality diference. Often the same hardware can be conﬁgured either as an AP or as a client, but not at the same time Beacons

• Short frames sent from the access point to stations (infrastructure mode) or station-to- station (ad hoc mode) in order to organize and synchronize wireless communication on the wireless LAN.

Thursday, February 9, 2012 Beacons functions

• Time Synchronization • DSSS Parameter Sets Advertising • SSID Information • Supported Rates • Trafﬁc Indication Map

. •

Thursday, February 9, 2012 Beacons synchronize clients by way of a time-stamp at the exact moment of transmission. When the client receives the beacon, it changes its own clock to reflect the clock of the access point. Once this change is made, the two clocks are synchronized. .In a DSSS (Direct Sequence Spread Spectrum) system, the beacon contains channel information. The SSID contained in the beacon is used by the station which looks at the MAC address of where the beacon originated and sends an authentication request in hopes of associating with that access point. If a station is set to accept any SSID, then the station will attempt to join the network through the first access point that sends a beacon or the one with the strongest signal strength if there are multiple access points. Scanning

Thursday, February 9, 2012 SSID: alphanumeric value between 2 and 32 characters that identifies the network. Can be set to “ANY”. Passive Scanning

• Process of listening for beacons sent by and access point on each channel for a speciﬁc period of time after the station is initialized

Thursday, February 9, 2012 The station searching for a network listens for beacons until it hears a beacon listing the SSID of the network it wishes to join. The station then attempts to join the network through the access point that sent the beacon. Active Scanning

• Active scanning involves the sending of a probe request frame from a wireless station.

Stations send this probe frame when they are actively seeking a network to join. The probe frame will contain either the SSID of the network they wish to join or a broadcast SSID

Thursday, February 9, 2012 If a probe request is sent specifying an SSID, then only access points that are servicing that SSID will respond with a probe response frame. If a probe request frame is sent with a broadcast SSID, then all access points within reach will respond with a probe response frame. The point of probing in this manner is to locate access points through which the station can attach to the network. Once an access point with the proper SSID is found, the station initiates the authentication and association steps of joining the network through that access point MAC Management Frames

• Beacon Timestamp, Beacon Interval, Capabilities, SSID, Supported Rates, parameters Traffic Indication Map Probe SSID, Capabilities, Supported Rates Probe Response Timestamp, Beacon Interval, Capabilities, SSID, Supported Rates, parameters same for Beacon except for TIM

Thursday, February 9, 2012 MAC Management Frames

• Association Request Capability, Listen Interval, SSID, Supported Rates Association Response Capability, Status Code, Station ID, Supported Rates Re-association Request Capability, Listen Interval, SSID, Supported Rates, Current AP Address Re-association Response Capability, Status Code, Station ID, Supported Rates

Thursday, February 9, 2012 Medium Access Control

• IEEE 802.11 uses a random technique to share the medium, called CSMA/CA, similar to CSMA/CD used in wired Ethernet • CSMA/CD cannot be used for wireless networks because it is not practical to detect an over the air collision • So, instead of detecting collisions, the idea is to try to avoid them, and the receiver must send an acknowledgment (ACK) for every successfully received frame 14

Thursday, February 9, 2012 CSMA cannot provide QoS, but if the throughput is high enough this is not a problem, VoIP an Video communications are normally viable. Medium Access Control

Thursday, February 9, 2012 Inter-Frame Spacing

Free access when medium DIFS is free longer than DIFS Contention Window PIFS DIFS SIFS Busy Medium Backoff-Window Next Frame

Slot time

Defer Access Select Slot and Decrement Backoff as long as medium is idle.

SIFS = Short interframe space PIFS = PCF interframe space DIFS = DCF interframe space Back-off timer expressed in terms of number of time slots

Thursday, February 9, 2012 4 types of inter-frame spacings for diferent priorities. SIFS used to separate transmissions belonging to a single dialog and is the minimum inter frame spacing. PIFS, Point Coordination IFS, used by the AP to access the medium before any other station. Is the SIFS + one slot time. NORMALLY not implemented DIFS, Distributed IFS, used for stations willing to start a new transmission, PIFS + one slot time EIFS, Extended IFS, used by a station that received a packet that could not understand, so could not determine the NAV (Network Allocation Vector), used for the virtual carrier sense Data frames and ACK

DIFS

Src Data SIFS

Dest Ack DIFS Contention Window Other Next MPDU Defer Access Backoff after Defer

Acknowledgment must be received within the SIFS The DCF inter-frame space is observed before medium is considered free for use

Thursday, February 9, 2012 Hidden Node

! Station 1 AP Station 2

Thursday, February 9, 2012 Station 1 cannot hear transmissions from Station 2 because it is too far away and vice versa. They are hidden nodes, but both are within the range of the same access point. So Station 1 can initiate a transmission perceiving the channel as free unaware of an ongoing transmission originating in Station 2. RTS/CTS Handshaking

! Receiving Station Sending Station AP Request to Send (RTS) Request to Send (RTS)

Clear to Send (CTS) Clear to Send (CTS)

Data Data

ACK ACK

Thursday, February 9, 2012 This mechanism is used to reserve the channel when there is the need to transmit big ﬁles. It can also be used to deal with the hidden node problem. The drawback is the extra overhead involved in the channel reservation mechanism. Interference

Thursday, February 9, 2012 Interference

Thursday, February 9, 2012 Service Sets

• Basic Service Set (BSS) • Extended Service Set (ESS) • Independent Basic Service set (IBSS)

Thursday, February 9, 2012 There are three corresponding set identiﬁers, also known as network names. Basic Service Set

Wired Network AP

! !

! Cell

Thursday, February 9, 2012 One cell comprises an AP and one or more stations within its range. A set of stations controlled by a single “Coordination Function” (the logical function that determines when a station can transmit or receive).

A BSS can have an Access-Point (both in standalone networks and in building-wide configurations), or can run without and Access-Point (in standalone networks only)

Diameter of the cell is app. twice the coverage-distance between two wireless stations Distribution System A system to interconnect several Basic Service Sets.

Can be:

Integrated; a single Access-Point in a standalone network Wired; using cable to interconnect the Access-Points Wireless; using wireless to interconnect the Access-Points

Thursday, February 9, 2012 Extended Service Set

Wired Network

Thursday, February 9, 2012 Each AP creates its own cell. The cells can overlap to allow roaming, in this case they must have the same ESS Identiﬁer. A set of one or more Basic Service Sets interconnected by a Distribution System (DS) Traffic always flows via Access-Point Distribution System (DS): A system to interconnect a set of Basic Service Sets. Normally an Ethernet cable. network Wired; Using cable to interconnect the Access-Points Wireless; Using wireless to interconnect the Access-Points Independent Basic Service Set

! !

Thursday, February 9, 2012 There is no Access Point. Stations communicate among themselves directly. This is called ad hoc mode. It is used in mesh networks.

A Basic Service Set (BSS) which forms a self-contained network in which no access to a Distribution System is available

A BSS without an Access-Point

One of the stations in the IBSS can be configured to “initiate” the network and assume the Coordination Function

Diameter of the cell determined by coverage distance between two wireless stations Service Set Identifier

• “Network name”

32 bytes long

One network (ESS or IBSS) has one SSID

Thursday, February 9, 2012 Basic Service Set Identifier

• “cell identifier”

6 bytes long (MAC address format)

One BSS has one SSID

Value of BSSID is the same as the MAC address of the radio in the Access-Point

Thursday, February 9, 2012 Roaming

Distribution System !

Thursday, February 9, 2012 The distribution system is normally an Ethernet cable, but can be also wireless when using WDS Inter cell roaming and handof Joining the Wireless Network

• Scanning • Authentication • Association • Data Transfer

Thursday, February 9, 2012

Passive scanning is the process of listening for beacons on each channel for a specific period of time after the station is initialized. These beacons are sent by access points (infrastructure mode) or client stations (ad hoc mode), and the scanning station catalogs characteristics about the access points or stations based on these beacons. The station searching for a network listens for beacons until it hears a beacon listing the SSID of the network it wishes to join. The station then attempts to join the network through the access point that sent the beacon. Active scanning involves the sending of a probe request frame from a wireless station. Stations send this probe frame when they are actively seeking a network to join. The probe frame will contain either the SSID of the network they wish to join or a broadcast SSID. If a probe request is sent specifying an SSID, then only access points that are servicing that SSID will respond with a probe response frame. If a probe request frame is sent with a broadcast SSID, then all access points within reach will respond with a probe response frame. The point of probing in this manner is to locate access points through which the station can attach to the network. Once an access point with the proper SSID is found, the station initiates the authentication and association steps of joining the network through that access point. . Fragmentation

Header Greater collison probability, less overhead

Data-1 Data-2 Data-3 FCS

Data-1 FCS

Less collision probabliity, Data-2 FCS more overhead

Data-3 FCS

Thursday, February 9, 2012 802.1X Authentication

Client Access Point

Request AP blocks all requests until ID request authentication is completed

Identification Identification

The RADIUS server authenticates the client

Thursday, February 9, 2012 Thank you for your attention

For more details about the topics presented in this lecture, please see the book Wireless Networking in the Developing World, available as free download in many languages at: http://wndw.net/

Thursday, February 9, 2012 See Chapter 4 of the book for more detailed information about the material covered in this talk.