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EA ZC451

EA ZC451. Vishal Gupta. Agenda: wireless Communication Basics. Basics. In mobile radio systems, unlike wired networks, electromagnetic signals are transmitted in free space.

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EA ZC451

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  1. EA ZC451 Vishal Gupta

  2. Agenda: wireless Communication Basics

  3. Basics • In mobile radio systems, unlike wired networks, electromagnetic signals are transmitted in free space. • Depending on the frequency and the corresponding wavelength, electromagnetic waves propagate as ground waves, surface waves, space waves or direct waves. • The type of propagation is correlated with the range, or distance, at which a signal can be received. • The general rule is that at a given transmit power the higher the frequency of the wave to be transmitted, the shorter the range reached. • The electromagnetic spectrum extends from low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. • Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz (or, equivalently, wavelengths longer than about 1 mm).

  4. Duplexing Schemes • In general, duplexing schemes provide a separation of the send and receive signals of a terminal or Subscriber Station (SS). • Partitioning of the wireless medium by duplex separation prohibits the self-interference of a station so that it does not receive its own transmitted signal. Time Division Duplex • The alternate transmitting and receiving of data on a single frequency channel is referred to as Time Division Duplex(TDD). • The uplink (UL) and downlink (DL) directions are separated in the time domain. • The point of time when switching from reception to transmission is referred to as switching point. • The position of the switching point allows an asymmetric separation of a frequency channel into DL and UL. • Thus, asymmetric services can be effectively supported that transmit more data into one direction (DL) than into the other.

  5. Duplexing Schemes • Prominent examples for wireless communication systems using TDD are IEEE 802.11 and 802.16. Frequency Division Duplex • Here the UL and DL of a wireless network are separated in the frequency domain. • The frequency channel used by one station for transmitting is used by another station for receiving and vice versa. • In centrally controlled systems, the upper frequency band is usually assigned to the base station to transmit in DL direction, as the pathloss of radio signals increases with frequency. • To communicate over the same distance, the base station therefore requires a higher transmission power (on the upper frequency band) than the subscriber station (transmitting on UL in the lower frequency band). • Since battery power is limited at mobile devices, the lower (less power intensive) frequency band is used for UL transmission. • Today’s cellular radio networks like GSM, UMTS and cdma2000 apply FDD.

  6. Duplexing Schemes

  7. Multiplexing • Multiplexing serves to share the radio channel capacity between competing stations. • A multitude of simultaneous transmission requests is multiplexed to a common channel. • A wireless medium seen as a transmission resource can be divided into multiple dimensions, namely frequency, time, code and space. Frequency Division Multiplex • In a Frequency Division Multiplex(FDM) based system the spectrum is divided into frequency channels that each may be simultaneously used by multiple users.

  8. Multiplexing Time Division Multiplex • The capacity of a frequency channel may be higher than required for a single communication link and therefore may be divided in the time domain among several stations. Code Division Multiplex • With Code Division Multiplex(CDM) a radio signal of small bandwidth is transmitted in a wide frequency channel. • The small bandwidth signal is spread through an adequate code to become a wide bandwidth signal. • CDM allows the simultaneous transmission of multiple stations in the same frequency band.

  9. Multiplexing

  10. Switching in Communication Networks Circuit Switching • Circuit-switched communication implies a dedicated communication link that might be a frequency, time or code based channel between data source and sink. • Circuit switching implies a permanent reservation of channel capacity for the duration of a connection independent of the actual load of the channel. Thus, circuit switching may be rather inefficient. • The establishment of a connection that might run across a number of intermediate router nodes results in an initial delay. However, the delay for user data transmitted on the channel is negligibly low. • Most IEEE 802 systems use reservation-based transmission, which is in a sense circuit switching but with short live duration of the channel reserved.

  11. Switching in Communication Networks Packet Switching • In packet switching, communication is based on data segmented into packets for transmission. • A packet is individually transmitted over the wireless medium and packet transmissions are independent of each other, i.e., each packet has to compete on its own for medium access applying some multiple access rule. • Packets might require a virtual connection to be established in advance, as is usual with ATM transmission systems. • Or it might just be self-contained in that all information to route a packet from source station to sink is contained in the packet header, as is usual for IP datagrams in the Internet or for packets transmitted via a wireless link or LAN.

  12. Medium Access Control (MAC) Protocols • Medium access control protocols are used in communication networks when multiple users compete for using the same channel. Our main Concentration Figure: Overview of different medium access control protocols.

  13. Medium Access Control (MAC) Protocols • In general, MAC protocols can be divided into two classes: deterministic and random, contention-based access. • The main characteristic of random-based medium access is that no predictable or scheduled point in time for medium access of stations exists. QoS is therefore difficult to support under strong competition for medium access. • Deterministic medium access is realized through reservation. Then, coordination of reservations is required that may either be under central or decentral control.

  14.  Carrier Sense Multiple Access • Maximum reachable channel utilization of slotted ALOHA is 37%. • The efficiency of channel utilization can essentially be improved when taking a prominent characteristic of wireless communication into account: “Signal propagation delay on the radio channel is relatively small compared to the packet transmission time” • Listening to (sensing) the radio channel before deciding to transmit is useful to avoid collisions. Such a medium access scheme is referred to as Carrier Sense Multiple Access (CSMA) or Listen-Before-Talk (LBT). • The station’s behavior after detecting a busy channel, and in the case of a collision, leads to different variants of CSMA.

  15. CSMA Variants • In CSMA, rules are required to specify the reaction to a busy channel if a station has a packet ready waiting for transmission. Different variants of reaction are:

  16. CSMA Variants Non-persistent CSMA • If a station would like to transmit and the medium is sensed busy, the station may wait for a random time duration before attempting a retransmission. 1-persistent CSMA • In order to reduce channel idle time, a station may initiate medium access as soon as the medium is idle. • In the case of a collision a backoff period is applied by any station involved (a random waiting time is drawn by each from a specific time interval) before repeating medium access. p-persistent CSMA • The radio channel may be divided into logical time slots. These time slots do not necessarily have the same duration as the packet transmission duration. • If an idle time slot is detected, the station may transmit its packet with probability p and defers from medium access with probability (1 – p). • In the case of a collision this procedure is repeated after a random backoff duration.

  17. CSMA Variants CSMA/CD • The performance of CSMA can be improved when introducing means for Collision Detection (CD) resulting in CSMA/CD. • On a wired medium, stations may observe the shared channel to detect if two transmissions have been started simultaneously, or at least overlap. If such a collision is detected, a jamming signal is transmitted by the respective station and its transmission is terminated immediately. • The capacity of the medium wasted due to a collision can be reduced substantially, since the time spent to detect a collision and issue the jam signal, typically, is much shorter than the time needed to transmit a complete data packet. • CSMA/CD is used in the LAN Ethernet (IEEE 802.3).

  18. CSMA Variants CSMA/CA • Collision Avoidance (CA) is an enhancement of CSMA-based radio channel access leading to CSMA/CA. • Collision avoidance aims at reserving the channel in advance by broadcasting reservation-related information. • Stations that sense the carrier may receive a channel reservation message containing a specific time duration that other stations are being asked to defer from channel access. After expiration of the time reserved all stations may compete again for medium access. • This concept is used for instance in the DCF of 802.11 when using the RTS/CTS frame sequence for channel reservation in the coverage range of both, transmitter and receiver. • The hidden station problem can be essentially mitigated in this way.

  19. Radio Spectrum Regulation • Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz (or, equivalently, wavelengths longer than about 1 mm). • From a technical point of view, the radio spectrum is a public resource that can be used without many limitations. • Use of the spectrum implies constant interference with neighboring radio receivers sharing the same spectrum. Therefore, radio spectrum regulation is required to allow reliable and efficient spectrum usage. • Regulators determine how particular bands of spectrum can be used, make rights available to licensees or unlicensed users and define rules constraining access to this spectrum. • The regulators’ decision making thereby targets at increasing public welfare and reflects public interest.

  20. Radio Spectrum Regulation Regulation of the radio spectrum has different characteristics: • Licensed spectrum for exclusive usage • Licensed spectrum for shared usage • Unlicensed spectrum : is available to all users operating in conformance to regulated technical etiquette or standards, like the Unlicensed National Information Infrastructure (U-NII) bands in the US at 5 GHz, or the Industrial Scientific and Medical (ISM) bands, worldwide, e.g., the band at 2.4 GHz where IEEE 802.11 and Bluetooth are operated. • Open spectrum allows anyone to access any range of spectra without permission under consideration of a minimum set of rules from technical standards or etiquette that are required for sharing spectra.

  21. IEEE 802.11 Wireless Local Area Networks “Just as Ethernet rules the roost when it comes to LAN technologies, modern internetworking is dominated by the TCP/IP Protocol Suite, IEEE 802.11 is the boss of the wireless LAN world.”

  22. How WLAN systems are different Destination address does not equal destination location • In wired LANs, an address is equivalent to a physical location. This is implicitly assumed in the design of wired LANs. • In IEEE Std 802.11, the addressable unit is a station (STA). The STA is a message destination, but not (in general) a fixed location. Media impact on design and performance • IEEE 802.11 PHYs a) Use a medium that has neither absolute nor readily observable boundaries. b) Are unprotected from other signals that may be sharing the medium c) Communicate over a medium significantly less reliable than wired PHYs d) Have dynamic topologies e) Lack full connectivity, and therefore the assumption normally made that every STA can hear every other STA is invalid.

  23. How WLAN systems are different The impact of handling mobile STAs • One of the requirements of IEEE Std 802.11 is to handle mobile as well as portable STAs. • A portable STA is one that is moved from location to location, but that is only used while at a fixed location. • Mobile STAs actually access the LAN while in motion. • Another aspect of mobile STAs is that they may often be battery powered. Hence power management is an important consideration. • For example, it cannot be presumed that a STA’s receiver will always be powered on.

  24. How WLAN systems are different Interaction with other IEEE 802® layers • IEEE Std 802.11 is required to appear to higher layers [logical link control (LLC)] as a wired IEEE 802 LAN. • This requires that the IEEE 802.11 network handle STA mobility within the MAC sublayer. Interaction with non-IEEE-802 protocols • An RSNA (robust security network association) utilizes non-IEEE-802 protocols for its authentication and key management (AKM) services. • Some of these protocols are defined by other standards organizations, such as the Internet Engineering Task Force (IETF).

  25. 802.11 family of specifications • On the surface of it, the 802.11 family of specifications will be very familiar to anyone who has used TCP/IP. • This familiarity is quite intentional, and is in fact one of the major reasons for the raging popularity of these standards over the last few years. • It is quite possible to lift up the TCP/IP stack of practically any device and slide the 802.11 protocol stack underneath it quite successfully. Applications utilizing the stack will quite literally be unaware that they have just been transformed into wireless applications. 802.11 Layered Architecture

  26. IEEE 802.11 Reference Model • Like IEEE 802.3 (Ethernet) and IEEE 802.5 (Token Ring), the 802.11 standard focuses on the two lower layers (1 and 2) of the Open System Interconnection (OSI) reference model.

  27. IEEE 802.11 Reference Model • The Physical Medium Dependent (PMD) sublayer is responsible for sending and receiving data via the wireless channel and defines the transmission scheme, which is different for the different PHYs. • The Physical Layer Convergence Protocol (PLCP) sublayer adapts the requests of the common MAC to the different PHYs into a format specific to the applied PMD. • The MAC user plane is fed with data frames via the MAC Service Access Point (MAC-SAP) at the MAC/LLC boundary.

  28. Components of the IEEE 802.11 architecture The IEEE 802.11 architecture consists of the following components that interact to provide a WLAN that supports STA mobility transparently to upper layers. • BSS (Basic Service Set) • is the basic building block of an IEEE 802.11 LAN. • Following figure shows two BSS’s each of which has two STAs that are members of the BSS.

  29. Components of the IEEE 802.11 architecture • BSS (Basic Service Set) [Contd…..] • It is useful to think of the ovals used to depict a BSS as the coverage area within which the member STAs of the BSS may remain in communication. • This area is called the Basic Service Area (BSA). If a STA moves out of its BSA, it can no longer directly communicate with other STAs present in the BSA. • The independent BSS (IBSS) as an ad hoc network • A minimum IEEE 802.11 LAN may consist of only two STAs. • This mode of operation is possible when IEEE 802.11 STAs are able to communicate directly. • Because this type of IEEE 802.11 LAN is often formed without pre-planning, for only as long as the LAN is needed, this type of operation is often referred to as an ad hoc network.

  30. Components of the IEEE 802.11 architecture • Distribution system (DS) concepts • Instead of existing independently, a BSS may also form a component of an extended form of network that is built with multiple BSSs. • The architectural component used to interconnect BSSs is the DS. • IEEE Std 802.11 logically separates the WM (wireless medium) from the distribution system medium (DSM). • The DS enables mobile device support by providing the logical services necessary to handle address to destination mapping and seamless integration of multiple BSSs. • An access point (AP) is any entity that has STA functionality and enables access to the DS, via the WM for associated STAs. • An AP provides the Distribution System Services (DSS). The DSSs enable the MAC to transport MSDUs between stations that cannot communicate over a single instance of radio channel. • Data move between a BSS and the DS via an AP. • The DS provides the service to transport MAC Service Data Units (MSDUs) between stations that are not in direct communication.

  31. Components of the IEEE 802.11 architecture DS, DSM and AP components of the IEEE 802.11 architecture

  32. Components of the IEEE 802.11 architecture Extended service set (ESS): The large coverage network • A service set identifier (SSID) is a name that identifies a particular 802.11 wireless LAN. • A client device receives broadcast messages from all access points within range advertising their SSIDs. The client device can then either manually or automatically—based on configuration—select the network with which to associate. • An ESS is a set of one or more interconnected BSSs and integrated local area networks that appear as a single BSS to the logical link control layer at any station associated with one of those BSSs. • The set of interconnected BSSs must have a common service set identifier (SSID). • In other words, An ESS is the union of the BSSs connected by a DS. • STAs within an ESS may communicate and mobile STAs may move from one BSS to another (within the same ESS) transparently to LLC.

  33. Components of the IEEE 802.11 architecture Portal • To integrate the IEEE 802.11 architecture with a traditional wired LAN, a final logical architectural component is introduced—a portal. • A portal is the logical point at which MSDUs from an integrated non-IEEE-802.11 LAN enter the IEEE 802.11 DS. • All data from non-IEEE-802.11 LANs enter the IEEE 802.11 architecture via a portal. It is the logical point at which the integration service is provided. • The integration service is responsible for any addressing or frame format changes that might be required when frames pass between the DS and the integrated LAN. • It is possible for one device to offer both the functions of an AP and a portal. • For example, a portal is shown in following Figure connecting to a wired IEEE 802 LAN.

  34. Components of the IEEE 802.11 architecture

  35. 802.11 Architecture Figure: The IEEE 802.11 architecture with typical scenarios of the different service sets (BSS, IBSS, DS). The DCF/PCF coordination functions are concepts for spectrum management and medium access. The PCF uses the DCF coordination function to support QoS.

  36. Services that support the distribution service Association • To deliver a message within a DS, the distribution service needs to know which AP to access for the given IEEE 802.11 STA. This information is provided to the DS by the concept of association. • Before a STA is allowed to send a data message via an AP, it shall first become associated with the AP. The act of becoming associated invokes the association service, which provides the STA to AP mapping to the DS. Reassociation • Additional functionality is needed to support BSS-transition mobility and is provided by the reassociation service. • The reassociation service is invoked to “move” a current association from one AP to another. Disassociation • The disassociation service is invoked when an existing association is to be terminated.

  37. Access control and data confidentiality services Authentication • IEEE 802.11 authentication operates at the link level between IEEE 802.11 STAs. • 802.11 attempts to control LAN access via the authentication service. • This service may be used by all STAs to establish their identity to STAs with which they communicate, Deauthentication • The deauthentication service is invoked when an authentication is to be terminated. • In an ESS, because authentication is a prerequisite for association, the act of deauthentication shall cause the STA to be disassociated. • The deauthentication service may be invoked by either authenticated party (non-AP STA or AP). • Deauthentication is not a request; it is a notification. Deauthentication shall not be refused by either party.

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