Wireless ABC

This is Work in Progress!

Contents

3GPP - 3rd Generation Partneship Project

collaboration between groups of telecom associations to develop third-generation (3G) mobile devices.

3GPP specifications are based on GSM specs and include Radio, Core Network and Service interfaces.

3GPP2 (Project 2) specifies standards for 3G technology based on IS-95 also known as CDMA2000.

Original scope was later amended to include the development of 4G/LTE standards.

ARQ - Automatic Repeat Request

is error control protocol for data transmission that involves acknoledgements

(messages sent by receiver to indicate that it has received a frame/packet)

and timeouts (periods of time allowed before an acknoledgement is received)

to achieve reliable data transmission over unreliable medium. If the sender

does not receive an acknoledgement from receiver before timeout expires it

retransmits the frame/packet until it receives an acknoledgement.

There are several types of ARQs: Stop and wait (wait for ack of each packet),

Go-Back-N (use sequence number of frames without ack for each packet),

Selective Repeat ARQ (accept and ack frames after an initial error).

ARQ protocols belong to Data Link or Transport Layers.

Beacon

in Wireless Local Area Networks (IEEE802.11/15) it is data transmission from wireless Access Point

which contains SSID, the channel number and security protocols such as WEP or WPA.

This data does not contain the link layer address of destination device and can be received by any client.

Distress or emergency beacons used in Cospas-Sarsat Search and Rescue satalite system (406 MHz)

may contain encoded GPS position which can be used by Search and Rescure (SAR) aircraft and ground

search parties who can come to the aid of the boat/aircraft/person who activated the beacon.

Chip

is a puls of pseudo-noise code sequence used in DSSS.

Typically a rectangular pulse of 1/-1 amplitude produced by code generator, multiplied first by

a transmitted data sequence (1/-1 message bits) and then by carrier waveform to make transmitted signal.

Chip rate is a number of pulses (chips) per second. The chip rate is larger than symbol rate, which means

each symbol is represented by multiple chips. The ratio of chip rate and symbol rate is known as

Spreading Factor (SF).

In CDMA users codes (chips) are chosen to be mutually orthogonal, and each user is given a unique code.

Codes are produced from Orthogonal Variable Spreading Factor (OVSF) binary code tree, constructed

from Hadamard matrices (1/-1 entries with mutually orthogonal rows/columns).

CDMA/CA - Collision Detection Multiple Access with Collision Avoidance

is MAC sub-layer network access method in wich a station willing to transmit has to first listen

to the channel for a predefined time so as to detect any activity on the channel.

If the channel is sensed "idle" then the station is permitted to transmit. If the channel is sensed

as "busy" the station has to defer its transmission. This is the essence of both CSMA/CA and CSMA/CD.

In CSMA/CA, once the channel is clear, a station sends a signal telling *all* other stations not

to transmit, and then sends its packet. CSMA/CA is used in wireless LANs (802.11 Wi-Fi) and

PANs (802.15.4 ZigBee).

One of the problems of wireless LANs/PANs is that it is not possible to listen while sending

and therefore collision detection is not possible. Another one is *hidden terminal* problem

where a node A is in the range of reciever R but not in the range of sender S, and therefore

cannot know that S is sending to R.

CSMA/CA may be optionally supplemented (for wireless LANs) by exchange of Request to Send (RTS)

packet sent by the sender, and Clear to Send (CTS) packet sent by the intended reciever,

alerting *all* nodes within range of sender/reciever or both to stay quiet for the duration of packet.

This is known as the IEEE 802.11 RTS/CTS exchange.

CDMA/CD - Collision Detection Multiple Access with Collision Detection

is MAC sub-layer network access method in wich a carrier sensing scheme is used.

Transmitting station that detects another signal while transmitting a frame, transmits a jam signal

and then waits for a random time interval (known as *backoff delay* and determined using truncated binary

exponential backoff algorithm) before trying to re-send the frame again. Jam signal will cause *all*

transmitters to back off by random intervals.

CSMA/CD monitors the physical medium for traffic all the time by watching the carrierSense signal

provided by PLS (Physical layer signals to MAC layer). Collisions are detected by monitoring

collisionDetect signal provided by Physical Layer.

CSMA/CD improves CSMA performance by terminating transmission as soon as collision is detected

and reducing the probability of new collision on re-send. CSMA/CD is no longer used in 10 Gigabit

Ethernet because of switches replacing all hubs and repeaters.

DSSS - Direct Sequence Spread Spectrum

is a modulation technique where transmitted signal takes more bandwidth than information signal.

DSSS modulates sine wave with a string of pseudo-noise (PN) code symbols known as "chips".

Whis means that each information bit is modulated by a sequence of chips (1/-1 values) at a frequency

much higher than that of original signal, thus spreading the information into much wider band.

DSSS signal uses a sequence of chips (produced by transmitter) which is known by the receiver.

Receiver then uses the same pseudo-random sequence to reconstruct the original signal.

This requires synchronization of sequences between transmitters and recievers via timing search process.

If several transmitters are synchronized, the relative synchronizations the reciever makes between them

can be used to determine relative timing and therefore to calculate the reciever's position.

This is the foundation of satellite navigation process.

The longer PN sequence used the better signal to noise ratio (SNR) on the channel ("prosess gain").

If some other transmitter transmitts on the same channel using differnt PN sequence, de-spreading results

in no processing gain for that signal. This is the basis of CDMA property of DSSS, which allows multiple

transmitters to share the same channel.

DSSS is used in GPS and Galileo satellite navigation systems, in DS-CDMA,

as well as in WiFi (802.11b) and ZigBee (802.15.4).

E-UTRAN - Evolved UTRAN

is radio interface standard of LTE which is a replacement of UMTS and HSPA.

On the network side E-UTRAN consists only of enodeBs.

EnodeB plays the role of nodeB and RNC (Radio Network Controller) in one.

This simplifies radio interface and reduces its latency.

On physical (L1) layer EUTRA uses OFDM, MIMO antenna on the downlink,

and both OFDM and SC-FDMA (precoded version of OFDM) on the uplink.

E-UTRAN protocol stack consists of the following: L1, MAC, RLC, PDCP, RRC, NAS and IP.

FDD - Frequency Division Duplexing

multiplexing technique when transmitter and receiver operate at different carrier frequencies.

Uplink and downlink are separated by frequency offset. Most efficient in case of symmetric traffic.

Makes deployment in dense areas easier since neighboring base stations transmit in different sub-bands

and do not "hear" each other. Used in UMTS/WCDMA as well as in wired ADSL/VDSL systems.

LTE - Long Term Evolution

LTE is a set of enhancements to the UMTS introduced in 3GPP Release 8.

Specification provides peak rates of at least 100 Mbps in downlink and 50 Mbps in uplink.

LTE supports flexible carrier bandwidths, from 1,4 MHz up to 20 MHz and both FDD and TDD duplexing.

Big advantage of LTE is flat IP based network architecture known as System Architecture Evolution (SAE),

designed to replace complex GPRS Core Network and guarantee mobility between legacy and non-3GPP systems

like WiMAX. Which was one of the major reason for major North American operators like Verizon and AT&T

to announce plans to convert their networks to LTE.

LTE provides improved spectrum efficiency - 2 to 4 times compared with HSPA Release 6,

as well as cost-effective migration from UTRA radio inteface and architecture.

In the downlink, LTE uses OFDM , with one resource element carrying QPSK, 16QAM or 64QAM.

In the uplink, LTE uses pre-coded version of OFDM called Single Carrier FDMA (SC-FDMA), which compensates

for standard OFDM drawback - high Peak to Average Power Ratio requring costly power amplifiers with high

linearity which increases cost of device and drains the battery faster.

Originally there were 10 FDD and 4 TDD bands defined by 3GPP:


FDD frequency bands (MHz)
BandUplinkDownlink
11920-19802110-2170
21850-19101930-1990
31710-17851805-1880
41710-17552110-2155
5824-849869-894
6830-840875-885
72500-25702620-2690
8880-915925-960
91749.9-1784.91844.9-1879.9
101710-17702110-2170

TDD frequency bands (MHz)
BandFrequencies UL and DL
a1900-1920, 2010-2025
b1850-1910, 1930-1990
c1910-1930
d2570-2620

In the US, FCC (Federal Communications Commission) allocated several blocks (A to D) of 700 MHz band

for different network and handset vendors. Verizon Wireless aquired most of the C block spectrum,

while many AT&T's 700 MHz lincences sit in lower C and B block of regional licences.

Group of smaller US wireless operators known as "700 MHz Block A Good Faith Purchasers Alliance"

purchased A block spectrum and D block is expected to be reallocated to public safety systems.

Verizon and Motorola plan to provide roaming between Verizon's network and D block spectrum.

MAC - Media Access Control

is data communication protocol sub-layer of Data Link Layer (DLL) specified in seven-layer OSI model (layer 2).

It provides addressing and channel access control mechanism so that several network nodes can communicate

within a multi-point network (MAN/LAN/PAN). The hardware that is used to implement MAC is referred to as

Medium Access Controller (MAC).

MAC sub-layer is an interface between Logical Link Controll (LLC) and network Physical Layer (PHY).

It provides full-duplex logical communication channel via unicast, multicast and broadcast service.

MSDU - MAC Service Data Unit

is a SDU recieved from upper LLC sub-layer.

LLC and MAC are sub-layers collectively referred to as Data Link Layer (DLL).

OFDM - Orthogonal Frequency Division Multiplexing

is a FDM scheme where large number of orthogonal sub-carriers are used to carry data.

Orthogonality of sub-carriers means that cross-talk between subcarriers is eliminated and guard bands

are not required. This also eliminates the need for separate filter for each sub-channel.

But it requires accurate frequency synchronization between receiver and transmitter.

Orthogonality is implemented using FFT on the receiver, and inverse FFT on the transmitter using

low cost DSP chips. Each sub-carrier is modulated with a conventional modulation scheme like QAM

or PSK at low symbol rate. Data is divided into several data streams or channels one for each sub-carrier.

Key advantages of OFDM - resistance to severe channel conditions, co-channel interference and fading

caused by multipath propagation, high spectral efficiency. It also makes possible design of single

frequency networks where several transmitters send same signal at the same frequency.

Main disadvantages - sensitivity to Doppler effect (shift of frequency at high speed), frequency

synchronization problems, high peak-to-average power ratio, requiring linear transmitter circuitry,

loss of efficiency due to cyclic prefix/guard interval between symbols.

OFDM is used in WiFi (802.11a/g/n), WPAN (UWB 802.15.3a), WiMAX (802.16e), LTE air interface

as well as in many cable systems (ADSL, VDSL, DVB-C2).

OFDMA - Orthogonal Frequency Division Multiple Access

is FDMA where different OFDM channels are assigned to different users.

OFDMA supports differentiated QoS by assigning different number of sub-carriers to different users

like in CDMA, thus avoiding complex packet scheduling or MAC schemes.

PDCP - Packet Data Convergence Control Protocol

E-UTRA protocol that provides ciphering and data integrity protection for RRC layer.

For IP layer it provides Robust Header Compression (ROHC), ciphering, in-sequence

delivery, duplicate detection and retransmission of SDUs during handover.

PDU - Protocol Data Unit

is a set of data sent to the *peer* protocol layer.

PDU at layer (n) is the SDU of the protocol layer (n-1).

RAN - Radio Access Network

is part of wireless system that is located between user equipment (UE) and core network (CN).

It implements a radio access technology specific for a wireless network.

In case of GSM it is known as GRAN (GSM Radio Access Network) and consists of Base Transiver Station (BTS) and

Base Station Controllers (BSC) and provides access to both Circuit Switched (CS) and Packet Switched (PS) core networks.

In case of GSM EDGE it is known as GERAN and is essentially the GRAN plus packet radio services.

In case of UMTS it is known as UTRAN (UMTS Terrestial RAN).

Dual-mode devices could be simulteneously connected to two RANs - GSM and UMTS.

ROHC - Robust Header Compression

is method used to compress IP/UDP/RTP/TCP packet headers.

The overhead of packet header is 40 bytes for IPv4 and 60 bytes for IPv6 which

is not acceptable for wireless systems where bandwidth is prime resource.

ROHC compresses those 40 or 60 bytes into 1 or 3 bytes by placing a compressor

before the link and a decompressor after that link. ROHC scheme has three modes

of operation: Unidirectional (packets only sent in one direction),

Bidirectional Optimistic (feedback channel is used to send error recovery requests)

and Bidirectional Reliable (Optimistic with context sync). ROHC algorithm is similar

to video compression: base frame and several diff frames are used as an IP packet flow.

RLC - Radio Link Control

transports PDCP's PDUs to MAC. Depending on reliability mode can provide ARQ error correction,

segmentation/concatenation of PDUs, re-ordering for in-sequence delivery, duplication detection.

RRC - Radio Resource Control

transports non-access stratum (NAS) messages, paging, establishment and release of RRC connection,

security key management, handover, UE measurements related to inter-RAT mobility and QoS.

RRM - Radio Resource Management

IEEE 802.11k and 802.11r are the key industry standards that enable seamless Basic Service Set (BSS)

transitions in the WLAN environment. The 802.11k standard provides information to discover the best

available access point (AP). 802.11k is intended to improve the way traffic is distributed within a network.

In a wireless LAN, each device normally connects to the AP that provides the strongest signal.

Depending on the number and geographic locations of the subscribers, this arrangement can sometimes lead

to excessive demand on one AP and underutilization of others, resulting in degradation of overall network

performance.

In a network conforming to 802.11k, if the AP having the strongest signal is loaded to its full capacity,

a wireless device is connected to one of the underutilized APs. Even though the signal may be weaker,

the overall throughput is greater because more efficient use is made of the network resources.

The following steps are performed before switching to a new access point:

  1. AP determines that client is moving away from it
  2. Informs client to prepare to switch to a new access point
  3. Client requests list of nearby access points
  4. AP gives site report
  5. Client moves to best AP based on report

RSSI - Received Signal Strength Indication

is a measurement of power present in a received radio signal.

RSSI could be done in the Intermediate Frequency (IF) stage before amplifier (heterodyne).

In a direct-conversion receiver (homodyne or zero-IF system), it is done in the baseband signal chain,

before the baseband amplifier. RSSI output is often a DC analog level. It can also be sampled by an

internal ADC and the resulting codes available directly or via peripheral or internal processor bus.

In IEEE 802.11 system RSSI is the relative received signal strength in a wireless environment, in arbitrary units.

RSSI can be used internally in a card to determine when the amount of radio energy in the channel is below

a certain threshold at which point the network card is Clear To Send (CTS).

Once the card is clear to send, a packet can be sent. Device user can observe an RSSI value when measuring

the signal strength of a wireless network through the use of a wireless network monitoring tool

like Wireshark, Wildpacket, Kismet or Inssider.

RSSI measurements are unitless and in the range 0 to 255, expressible as a one-byte unsigned integer.

The maximum value, RSSI_Max, is vendor dependent:

Cisco Systems cards have a RSSI_Max value of 100 and will report 101 different power levels (from 0 to 100).

Atheros based Wi-Fi card will return an RSSI value of 0 to 127 (0x7f) with 128 (0x80) indicating an invalid value.

There is no specified relationship of any particular physical parameter to the RSSI reading.

The 802.11 standard does not define any relationship between RSSI value and power level in dBm.

Vendors provide their own granularity and range for the actual power (measured as mW or dBm)

and their range of RSSI values (from 0 to RSSI_Max).

RSSI is acquired during the preamble stage of receiving an 802.11 frame.

To this extent 802.11 RSSI has (for the most part) been replaced with Received Channel Power Indicator (RCPI).

RCPI is a functional measurement covering the entire received frame with defined absolute levels of accuracy

and resolution. RSSI is stored on the TX/RX descriptor and is measured by baseband and PHY for each individual packet.

Service Data Unit

is a set of data sent by a *user* of a given service/layer

which is transmitted as it is (unchanged) to a peer service below.

SDU is essentially a *payload* of a PDU.

UMTS - Universal Mobile Telecommunication System

is 3G technology specified by 3GPP.

UMTS is complete wireless network system and includes radio access network

(UMTS Terrestial Radio Access Network or UTRAN), core network (Mobile Application Part or MAP)

as well as user authentication via USIM card. The most popular form of UMTS uses WCDMA as air interface,

but standard also covers TD-CDMA and TD-SCDMA. Deployment of UMTS requires new base stations and new frequencies.

Maximum theoretical data transfer rate is 43Mbit/s (with HSPA+), but in most deployed networks it is up to 384 kbit/s

or 7.2Mbit/s for HSDPA devices.

Frequency bands originally defined in the standard are 1885-2025 MHz (uplink) and 2110-2200 MHz (downlink).

But in USA 1710-1755 MHz and 2120-2155 MHz are used since 1900 MHz band was already occupied.

UTRAN - UMTS Terrestial Radio Access Network

is composed of UMTS base stations known as NodeB and Radio Network Controllers (RNC).

NodeB and RNC could be implemented as one device but typically RNC located at some central office is serving

multiple NodeBs via interface known as "lub". Interface connecting RNC to Core Network (CN) is known as "lu",

interface between UE and NodeB is known as "Uu", interface between two RNCs is known as "lur".

RNC and its NodeBs are known as Radio Network Subsystem (RNS).

W-CDMA - Wideband Code Division Multiple Access

air interface standard used in 3G networks, most popular version of UMTS family.

Developed in 1990s initially by NTT DoCoMo for 3G FOMA network, it was accepted by ITU as 3G standard

known as IMT-2000 and later selected as an air interface for UMTS.

Key features:


WiFi

is a trademark of Wi-Fi Alliance as well as WLAN devices based on IEEE 802.11 standards.

Certification process requires conformance to 802.11 radio standards, WPA and WPA2 security standards

and EAP authentication protocol.

WiFi pruducts use both single-carrier (DSSS in 802.11b) and multi-carrier (OFDM in 802.11g) technologies

in unlicensed spectrum - 2450 MHz and 5 GHz (ISM band). WiFi uses CSMA/CA, frames are always aknowledged.

Number of channels used by WiFi: in USA - 11 channels, in Europe - 13, and in Japan - 14 for 2450 MHz band.

Equivalent isotropical radiated power (EIRP) is limited to 20dBm (100mW) in Europe.

WiFi access points called "hotspots" frequently used as part of a router (e.g. part of DSL modem),

devices also can be connected in "ad-hoc" mode - client to client connection without a router.

WiFi supports device roaming between hotspots and mesh networks.

References: