Mobile Communication MDM College

1Prof. Shingare A. A.

Master Deenanath Mangeshkar college Aurad[sha]

Chapter 3

Motivation for Specialized MAC

Can we apply media access methods from fixed networks?

Example CSMA/CD Carrier Sense Multiple Access with Collision Detection. send as soon as the medium is free, listen into the medium if a collision

occurs (original method in IEEE 802.3).

Problems in wireless networks signal strength decreases proportional to the square of the distance. the sender would apply CS and CD, but the collisions happen at the

receiver. it might be the case that a sender cannot “hear” the collision, i.e., CD does

not work. furthermore, CS might not work if, e.g., a terminal is “hidden”.



Most (if not all) radios are half-duplex Listening while transmitting is not possible Collision might not occur at sender Collision at receiver might not be detected by sender!



CS – Carrier Sense Nodes can distinguish between an idle and a busy link.

MA - Multiple Access A set of nodes send and receive frames over a shared link.

CA – Collision Avoidance Nodes use protocol to prevent collisions from Occurring



Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C

Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not

necessary C is “exposed” to B

Motivation - hidden and exposed terminalsMaster Deenanath Mangeshkar college Aurad[sha]


Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A

Motivation - near and far terminalsMaster Deenanath Mangeshkar college Aurad[sha]


MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet

CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

Signaling packets contain sender address receiver address packet size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)



MACA avoids the problem of hidden terminals A and C want to

send to B A sends RTS first C waits after receiving

CTS from B

MACA avoids the problem of exposed terminals B wants to send to A, C

to another terminal now C does not have

to wait for it cannot receive CTS from A

MACA examplesMaster Deenanath Mangeshkar college Aurad[sha]




Controls radiated energy in space. Use spot beam antennas. Different areas may be served by same frequency: TDMA or CDMA; or different frequencies: FDMA. Adaptive antennas dynamically adapt to number of users, etc Reverse link may be a problem: limited battery capability, interference problems.

[Space Division Multiple Access]

[Frequency Division Multiple Access]

•Individual channels (frequency) to individual users.

• On demand channel assignment.



If channel not in use, sits idle.

Channel bandwidth relatively narrow (30kHz), i.e, usually narrowband systems.

Symbol time >> average delay spread little or no equalization ⇒required.

Simplest.

Best suited for analog links.

Continuous transmission implies no framing or synchronization bits needed.

Requires tight filtering to minimize interference.

Usually combined with FDD for duplexing



[Time Division Multiple Access]

• Divide radio spectrum into time slots.

• Only 1 user allowed to either transmit or receive in a slot

• Slots assigned cyclically

• Non-continuous transmission: buffer and burst

• Digital data and modulation must be used

• Guard time allows for different prop delays bet mobile and BS

• 20～ 30 % of data rate is overhead

• Tradeoffs in overhead, size of data payload, and latency





Advantages:

• Shares single carrier frequency with multiple users.• Non-continuous transmission makes handoff simpler (mobile assisted handoff possible)• Slots can be assigned on demand (concatenation and reassignment): bandwidth supplied on demand.• Less stringent power control due to reduced interuser. Interference

Disadvantages:

• Higher synchronization overhead.• Equalization necessary for high data rates.• Frequency/slot allocation complexity.• Pulsating power envelop: interference with other devices.



All users use same frequency and may transmit simultaneously. Narrowband message signal multiplied by wideband spread in signal, or codeword. Each user has its own pseudo-codeword (orthogonal to others). Receivers detect only the desired codeword. All others appear as noise. Receivers must know transmitter’s codeword.

[Code Division Multiple Access]Master Deenanath Mangeshkar college Aurad[sha]


CDMA Encode / DecodeOne Sender Interference



CDMA Encode / Decode

Two Sender Interference



Relationship Code used in CDMA



Soft capacity limit: system performance degrades for all users as number of users increase.

Wide frequency spectrum reduces fading.

Rake receiver: Separate multipath signals of different delays by “chip” unit.

Cell frequency reuse 1 : No frequency planning.

Soft Handover increases capacity.

“make before break” vs. “break before make”.

Utilization of voice activity (talk spurts) Power control necessary for mitigating near-far problem.

Tradeoff between precision of power control and capacity.

Complex network support for implementing soft handoff.

Self-jamming problem due to spreading sequences not being exactly orthogonal.



[Global System For Mobile Communication]

Through the evolution of cellular telecommunications. Various System have Been Developed without the benefit Of the standardized specification. This presented Many problem directly related to Compatibility, especially with the development of digital radio technology. The GSM Standards is intended to address these problems. From 1982 to 1985 discussions were held to decide between buildings an Analogs or digital system. After multiple field tests. A digital system was adopted for GSM.



GSM is a PLMN(Public Land Mobile Network)

several providers setup mobile networks following the GSM standard within each country

GSM system comprises3 subsystems

RSS (radio subsystem): covers all radio aspects MS (mobile station) BSS (base station sub system) or RAN (radio access network)

BTS (base transceiver station) BSC (base station controller)

NSS (network and switching subsystem): call forwarding, handover, switching MSC (mobile services switching center) LR (location register): HLR and VLR

OSS (operation subsystem): management of the network OMC (operation and maintenance centre) AuC (authentication centre) EIR (equipment identity register)

GSM Architecture Master Deenanath Mangeshkar college Aurad[sha]


Handover

The problem Change the cell while communicating Reasons for handover:

Quality of radio link deteriorates Communication in other cell requires less radio resources Supported radius is exceeded(e.g. Timing advance in GSM) Overload in current cell Maintenance



Handover procedure(change of BSC)Master Deenanath Mangeshkar college Aurad[sha]


GSM -authenticationBit Challenge-Response:• Authentication • center provides RAND to Mobile• AuC generates SRES using Ki of subscriber and RAND via A3• Mobile (SIM) generates SRES using Ki and RAND• Mobile transmits SRES to network (MSC)• network (MSC) compares received SRES with one generated by AuC



GSM -key generation and encryptionMaster Deenanath Mangeshkar college Aurad[sha]


GSM Evolution - OverviewMaster Deenanath Mangeshkar college Aurad[sha]


Introducing packet switching in the network Using shared radio channels for packet transmission over the air:

multiplexing multiple MS on onetime slot flexible (also multiple) allocation of times lots to MS

(scheduling by PCU Packet Control Unit in BSC or BTS) using free slots only if data packets are ready to send (e.g., 115 k bit /s using 8 slots temporarily)standardization 1998, introduction 2001advantage: first step towards UMTS, flexible data services

GPRS network elements

GSN (GPRS Support Nodes): GGSN and SGSNGGSN(Gateway GSN)

interworking unit between GPRS and PDN (Packet Data Network)SGSN(Serving GSN)

supports the MS (location, billing, security)HLR(GPRS Register –GR)

Maintains location and security information

GSM - GPRS (General Packet Radio Service)Master Deenanath Mangeshkar college Aurad[sha]




GPRS protocol architectureMaster Deenanath Mangeshkar college Aurad[sha]


This Presentation is Designed by

Patel Anwar Rashid ( BCA Ty)

Mobile Communication MDM College

Education

signal c

c exposed terminals

necessary c

sender collision

b maca

c waits

c senses

b motivation