12-1 multiple access random access controlled access channelization

12-1

Multiple Access

• Random Access• Controlled Access• Channelization

12-2

Data Link Layer: Two sublayers

• Data link layer divided into two functionality-oriented sublayers

• IEEE made this division for LANs

12-3

Medium Access Protocols

12-4

Random Access

• Each station has the right to the medium without being controlled by any other station

• Collision, an access conflict, if more than one station tries to send

12-5

ALOHA

• The earliest random access method developed at the Univ. of Hawaii in the early 1970s

• Designed for a radio (wireless) LAN• Pure ALOHA and Slotted ALOHA• Frames in a pure ALOHA network

12-6

Pure ALOHA Protocol: Procedure• Binary exponential back-off algorithm

12-7

Pure ALOHA Protocol

• Pure ALOHA vulnerable time = 2 x Tfr

• The throughput for pure ALOHA is S = G × e −2G .

• The maximum throughput Smax = 0.184 when G= (1/2).

12-8

Slotted ALOHA• Pure ALOHA vulnerable time = 2 x Tfr because there is no rule that defines

when the station can send• Slotted ALOHA was invented to improve the efficiency of pure ALOHA

12-9

Slotted ALOHA

• throughput for slotted ALOHA is S = G × e−G .

• The maximum throughput Smax = 0.368 when G = 1• Slotted ALOHA vulnerable time = Tfr

12-10

Carrier Sense Multiple Access (CSMA)

• CSMA– “Sense before transmit”– “Listen before talk”

• CSMA can reduce the possibility of collision, but it can not eliminate it

12-11

Collision in CSMA

12-12

CSMA: Vulnerable Time

• Vulnerable time for CSMA is the propagation time Tp

needed for a signal to propagate from one end of the medium to the other

12-13

CSMA: Persistence Methods• Behavior of 1-persistent, Nonpersistent, p-persistent method

12-14

CSMA: Persistence Methods

• Flow diagram for 1-persistent, Nonpersistent, p-persistent method

12-15

Persistence Strategy

• Nonpersistent strategy

– Reduces the chance of collision

– Reduces the efficiency of the network

• 1-persistent

– Increases the chance of collision

• p-persistent

– Reduces the chance of collision and improves the efficiency by combining the other two strategies.

12-16

CSMA/CD (Collision Detection)

12-17

CSMA/CD: Min. Frame Size

• Example: A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal, as we see later) is 25.6 μs, what is the minimum size of the frame?

SolutionThe frame transmission time is Tfr = 2 × Tp = 51.2 μs. This means, in the worst case, a station needs to transmit for a period of 51.2 μs to detect the collision. The minimum size of the frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet.

12-18

CSMA/CD: Flow Diagram

12-19

CSMA/CD: Energy Level & Throughput

• Energy level during transmission, idleness, or collision

• Throughput of CSMA/CD is greater than that of ALOHA• The max. throughput occurs at a different value of G and is based on the persistent method and the value of p in the p-persistent approach• The max throughput is around 50% when G=1 for 1-persistent, up to 90% when G is between 3 and 8 for non-persistent

12-20

CSMA/CA (Collision Avoidance)

• Invented for wireless network where we cannot detect collisions• Collision are avoided through the use of CSMA/CA’s three strategies:

the interframe space, the contention windows, and acknowledgement

• IFS can also be used to define the priority of a station or a frame• If the station finds the channel busy, it does not restart the timer of the contention window; it stops the timer and restarts it when the channel becomes idle

12-21

CSMA/CA: Flow Diagram

12-22

Controlled Access• The stations consult one another to find which station has the right to

send• Reservation/Polling/ Token passing

• Reservation access method

12-23

Polling: Select and Poll Functions

12-24

Token Passing

• Logical Ring and physical topology

12-25

Channelization: FDMA

• FDMA– Available bandwidth of the common channel is divided into bands

that are separated by guard bands– FDMA is an access method in data link layer protocol. But, FDM

is a physical layer technique

12-26

Channelization: TDMA• TDMA

– The bandwidth is just one channel that is timeshared between different stations

– TDMA is an access method. But, TDM is a physical layer technique

12-27

Channelization: CDMA

• One channel carries all transmissions simultaneously• Two properties: If we multiply each code by another, we get 0. If we

multiply each code by itself, we get 4• Data = (d1

.c1 + d2.c2 + d3

.c3 + d4.c4) .c1

= d1.c1

.c1 + d2.c2

.c1 + d3.c3

.c1 + d4.c4

.c1 = 4.d1

12-28

CDMA: Chips

• Sequence of numbers called chips

• Orthogonal sequences have the following properties:– Each sequence is made of N elements, where N is the number of stations– If we multiply a sequence by a number, every element in the sequence is multiplied by that

element (scalar multiplication)– If we multiply two equal sequence, element by element, and add the results, we get N (inner

product)– If we multiply two different sequence, element by element, and add the results, we get 0– Adding two sequence means adding the corresponding elements. The result is another sequence

• Data representation in CDMA

12-29

CDMA: Encoding and Decoding

• Show how four stations share the link during a 1-bit interval

12-30

CDMA: Signal Level

• Digital signal created by four stations in CDMA using NRZ-L for simplicity

12-31

CDMA: Decoding

• Show how station 3 can detect the data by station 2 by using the code for station 2

• Decoding of the composite signal for one in CDMA

12-32

CDMA: Sequence Generation

• To generate chip sequence, we use a Walsh table• The number of sequence in a Walsh table needs to be N = 2m

12-33

Sequence Generation: Example

• Find the chips for a network with a. Two stations b. Four stations

Solution

a. For a two-station network, we have [+1 +1] and [+1 −1].

b. For a four-station network we have [+1 +1 +1 +1], [+1 −1 +1 −1], [+1 +1 −1 −1], and [+1 −1 −1 +1].

13-34

Wired LANs: Ethernet

1. IEEE Standards2. Standard Ethernet3. Changes in the Standard4. Fast Ethernet5. Gigabit Ethernet

13-35

IEEE Standards• In 1985, the Computer Society of the IEEE started a project, called In 1985, the Computer Society of the IEEE started a project, called

Project 802, to set standards to enable intercommunication among Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of specifying functions of the physical layer and the data link layer of major LAN protocols.major LAN protocols.

13-36

IEEE 802 Working Group

Active working groups Inactive or disbanded working groups

802.1 Higher Layer LAN Protocols Working Group 802.3 Ethernet Working Group 802.11 Wireless LAN Working Group 802.15 Wireless Personal Area Network (WPAN) Working Group 802.16 Broadband Wireless Access Working Group 802.17 Resilient Packet Ring Working Group 802.18 Radio Regulatory TAG 802.19 Coexistence TAG 802.20 Mobile Broadband Wireless Access (MBWA) Working Group 802.21 Media Independent Handoff Working Group 802.22 Wireless Regional Area Networks

802.2 Logical Link Control Working Group802.4 Token Bus Working Group802.5 Token Ring Working Group802.7 Broadband Area Network Working Group802.8 Fiber Optic TAG802.9 Integrated Service LAN Working Group802.10 Security Working Group802.12 Demand Priority Working Group802.14 Cable Modem Working Group

13-37

Logical Link Control (LLC)• Framing: LLC defines a protocol data unit (PDU) that is similar to Framing: LLC defines a protocol data unit (PDU) that is similar to

that of HDLCthat of HDLC• To provide flow and error control for the upper-layer protocols that To provide flow and error control for the upper-layer protocols that

actually demand these servicesactually demand these services

13-38

Standard Ethernet• The original Ethernet was created in 1976 at Xerox’s Palo Alto The original Ethernet was created in 1976 at Xerox’s Palo Alto

Research Center (PARC). Since then, it has gone through four Research Center (PARC). Since then, it has gone through four generationsgenerations

13-39

MAC Sublayer

• Preamble: alerting the receiving system to the coming frame and Preamble: alerting the receiving system to the coming frame and enabling it to synchronize its input timingenabling it to synchronize its input timing

• CRC: CRC-32CRC: CRC-32

13-40

Addressing

• Ethernet address in hexadecimal notationEthernet address in hexadecimal notation

• The least significant bit of the first byte defines the type of address.If the bit is 0, the address is unicast; otherwise, it is multicast

• The broadcast destination address is a special case of the multicast address in which all bits are 1s

13-41

Ethernet

• Access method: 1-persistent CSMA/CDAccess method: 1-persistent CSMA/CD• Slot time = rount-trip time + time required to send the jam sequenceSlot time = rount-trip time + time required to send the jam sequence

– 512 bits for Ethernet, 51.2 512 bits for Ethernet, 51.2 μss for 10 Mbps Ethernet for 10 Mbps Ethernet

• Slot time and collisionSlot time and collision

• Slot time and maximum network lengthSlot time and maximum network length• MaxLength = PropagationSpeed x SlotTime/2MaxLength = PropagationSpeed x SlotTime/2• MaxLength = (2 x 10MaxLength = (2 x 1088) x (51.2 x 10) x (51.2 x 10-6-6/2) = 5120 m/2) = 5120 m• MaxLength = 2500 m 48 % of the theoretical calculation by MaxLength = 2500 m 48 % of the theoretical calculation by

considering delay times in repeaters and interfaces, and the time considering delay times in repeaters and interfaces, and the time required to send the jam sequencerequired to send the jam sequence

13-42

Physical Layer: Ethernet

13-43

10Base5: Thick Ethernet

10Base2: Thin Ethernet

13-44

10BaseT: Twisted-Pair Ethernet

10Base-F: Fiber Ethernet

13-45

Summary of Standard Ethernet

13-46

Changes in the Standard

• Bridged Ethernet: Raising bandwidth and separating collision Bridged Ethernet: Raising bandwidth and separating collision domainsdomains

13-47


• Switched Ethernet: N-port bridgeSwitched Ethernet: N-port bridge

13-48


• Full-duplex (switched) Ethernet: no need for CSMA/CDFull-duplex (switched) Ethernet: no need for CSMA/CD

13-49

Fast Ethernet

• Under the name of IEEE 802.3uUnder the name of IEEE 802.3u• Upgrade the data rate to 100 MbpsUpgrade the data rate to 100 Mbps• Make it compatible with Standard EthernetMake it compatible with Standard Ethernet• Keep the same 48-bit address and the same frame format Keep the same 48-bit address and the same frame format • Keep the same min. and max. frame lengthKeep the same min. and max. frame length

• MAC SublayerMAC Sublayer• CSMA/CD for the half-duplex approachCSMA/CD for the half-duplex approach• No need for CSMA/CD for full-duplex Fast EthernetNo need for CSMA/CD for full-duplex Fast Ethernet

• Autonegotiation: allow two devices to negotiate the mode or data rate Autonegotiation: allow two devices to negotiate the mode or data rate of operationof operation

13-50

Fast Ethernet: Physical Layer

• TopologyTopology

• ImplementationImplementation

13-51

Fast Ethernet: Encoding

13-52

Summary of Fast Ethernet

13-53

Gigabit Ethernet

• Under the name of IEEE 802.3zUnder the name of IEEE 802.3z• Upgrade the data rate to 1 GbpsUpgrade the data rate to 1 Gbps• Make it compatible with Standard or Fast EthernetMake it compatible with Standard or Fast Ethernet• Keep the same 48-bit address and the same frame format Keep the same 48-bit address and the same frame format • Keep the same min. and max. frame lengthKeep the same min. and max. frame length• Support autonegotiation as defined in Fast EthernetSupport autonegotiation as defined in Fast Ethernet

• MAC SublayerMAC Sublayer• Most of all implmentations follows full-duplex approachMost of all implmentations follows full-duplex approach• In the full-duplex mode of Gigabit Ethernet, there is no collision;

the maximum length of the cable is determined by the signal attenuation in the cable.

• Half-duplex mode (very rare)Half-duplex mode (very rare)• Traditional: 0.512 Traditional: 0.512 μs s (25m)(25m)• Carrier Extension: 512 bytes (4096 bits) min. lengthCarrier Extension: 512 bytes (4096 bits) min. length• Frame bursting to improve the inefficiency of carrier extensionFrame bursting to improve the inefficiency of carrier extension

13-54

Gigabit Ethernet: Physical Layer• TopologyTopology

13-55

Gigabit Ethernet: Physical Layer• ImplementationImplementation

• EncodingEncoding

13-56

Gigabit Ethernet: Summary

Data Communications, Kwangwoon University 14-57

Wireless LANs

1. IEEE 802.11

2. Bluetooth


Basic Service Set (BSS)

• BSS

– The building block of a wireless LAN

• BSS with an AP

– Access Point (AP): central base station

• BSS without an AP

– Stand-alone network

– Cannot send data to other BSSs

– Ad hoc architecture

• BSS with an AP

– Called an infrastructure network


BSS


Extended Service Set (ESS)

• BSSs are connected through a distribution system: infrastructure network (usually wired LAN)

• Station Types: No-transition, BSS-transition, and ESS-transition mobility


MAC Sublayer

• Two MAC sublayers: DCF and PCF

• DCF uses CSMA/CA as the access method


CSMA/CA Flow Chart


CSMA/CA and NAV

• Network allocation vector (NAV) shows how much time must pass before these stations are allowed to check the channel for idleness

• Collision During handshaking


Point Coordination Function (PCF)• An optional access method that can be implemented in an AP• A centralized, contention-free polling access method• To give priority to PCF over DCF, another set of interframe spaces has

been defined: PIFS and SIFS• PIFS (PCF IFS) is shorter than the DIFS AP using PCF has priority• Repetition interval starts with a special control frame, called a beacon

frame


MAC Layer Frame Format


Frame Types

• Three categories of frames• Management frames for initial communication between stations and APs• Control frames for accessing the channel and acknowledging frames• Data frame for carrying data and control information


Addressing Mechanism


Hidden Station Problems

• The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.


Exposed Station Problems


Physical Layer

• Industrial, scientific, and medical (ISM) band which defines three unlicensed bands in three ranges 902-928 MHz, 2.400-4.835 GHz, and 5.725-5.850 GHz


Physical Layer• IEEE 802.11 FHSS

• IEEE 802.11 DSSS


Physical Layer• IEEE 802.11 Infrared

• IEEE 802.11a OFDM– Common data rates are 18 Mbps (PSK) and 54 Mbps (QAM)

• IEEE 802.11b DSSS– High-rate direct sequence spread spectrum (HR-DSSS)

– Similar to DSSS method except for the encoding method called complementary code keying (CCK), Four data rates; 1, 2, .5, 11 Mbps

• IEEE 802.11g– Forward error correction and OFDM using 2.4 GHz ISM, 22- or 54-Mbps data rate


Bluetooth: Piconet

• Bluetooth is a wireless LAN technology designed to connect devices Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneouslyhoc network, which means that the network is formed spontaneously

• Bluetooth network is called piconet, or a small net


Bluetooth: Scatternet

• Piconet can be combined to form what is called a scatternet.Piconet can be combined to form what is called a scatternet.

• Bluetooth device has a built-in short range radio transmitter with 2.4 Bluetooth device has a built-in short range radio transmitter with 2.4 GHz bandwidth. A possibility of interference with IEEE 802.11bGHz bandwidth. A possibility of interference with IEEE 802.11b


Bluetooth Layers

• Radio layer

– 2.4 GHz ISM band divided into 79 channels of 1 MHz each

– FHSS: Bluetooth hops 1600 times per second, dwell time is 625 μsec (= 1/1600 sec)

– Modulation: GFSK (FSK with Gaussian bandwidth filtering)


Bluetooth Layers

• Baseband layer

– TDD-TDMA (time division duplex TDMA): half-duplex communication

– Single-secondary communication


Bluetooth Layers

• Baseband layer

– TDD-TDMA: multiple-secondary communication


Bluetooth Layers• Physical links

– SCO (synchronous connection-oriented)

– ACL (asynchronous connectionless link)

• Frame format

• L2CAP (Logical Link Control and Adaptation Protocol)

– Multiplexing, segmentation and reassembly, QoS, group management

14-79

• Cellular Telephony• Satellite Networks

Cellular Telephony

• to provide communications between two moving units, called mobile stations

(MSs), or between one mobile unit and one land line unit.

• Cellular service area is divided into small regions called cells.

• Cell size is not fixed and can be increased or decreased depending on the population of the area.

• Typical radius of a cell is 1 to 12 mi.

• Each cell contains an antenna and is controlled by a base station (BS).

• Each base station, in turn, is controlled by a switching office, called a mobile

switching center (MSC).

14-80

Cellular Telephony

• MSC coordinates communication between all BSs and the telephone central office.

• MSC is a computerized center that is responsible for connecting calls, recording

call information, and billing.

• A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the channel from one BS to another BS as the caller moves out of range.

14-81Cellular System

Cellular Telephony

14-82

Frequency – Reuse Principle

•A frequency reuse pattern is a configuration of N cells, N being the reuse factor, in which each cell uses a unique set of frequencies.

•When the pattern is repeated, the frequencies can be reused.

•Cells with same no. in a pattern can use the same set of frequencies.

Cellular Telephony

• Transmitting

- setup channel

- voice channel

• Receiving

- paging

• Handoff

- Hard Handoff (break before make)

- Soft Handoff (make before break)

• Roaming – user can have access to communication

14-83

Generations of Cellular TelephonyFirst Generation

•Voice communication using analog signals

•AMPS (Advanced Mobile Phone System)

– It is an analog cellular phone system using FDMA

– 21 channels for control

– Frequency reuse factor: 7

– North American Standard

14-84Cellular bands for AMPS

Generations of Cellular Telephony

14-85

AMPS reverse communication band


Second Generation

•Digitized voice

14-86

2nd Generation cellular phone systems


Second Generation

D-AMPS – Digital AMPS

•backward compatible with AMPS.

•Defined by IS-54 (Interim Standard 54) and later revised by IS-136

•Same bands and channels as AMPS

•D-AMPS, or IS-136 is a digital cellular phone system using TDMA and FDMA.

14-87


14-88D-AMPS

Generations of Cellular TelephonySecond Generation

GSM (Global System for Mobile Communication)

•European standard

•Aimed to replace a no. of incompatible 1st generation technologies.

•Reuse factor: as low as 3

•GSM is a digital cellular phone system using TDMA and FDMA.

14-89GSM bands


14-90GSM


14-91Multiframe Components

Generations of Cellular TelephonySecond Generation

IS - 95•IS – 95 is a digital cellular phone system using CDMA/DSSS and FDMA.

•North American standard

•Based on CDMA and DSSS.

•All base channels need to be synchronized to use CDMA.

•Uses two bands for duplex communication

•Bands can be the ISM 800-MHz band or the ISM 1900-MHz band.

•Each band is divided into 20 channels of 1.228 MHz separated by guard bands.

•Frequency reuse factor 1

14-92

Generations of Cellular Telephony• One analog channel creates 64 digital channels

14-93IS-95 forward transmission


• Reverse channels use DSSS.

14-94IS-95 reverse transmission

Generations of Cellular TelephonyPCS (Personal Communications System)

•Does not refer to a single technology such as GSM, IS-136, or IS-95.

•Common features of these systems can be summarized:

14-95

Generations of Cellular TelephonyThird Generation

•Started in 1992 when ITU issued a blueprint called Internet Mobile Communiction(IMT-2000)

•Criteria for 3rd generation technology as outlined below:

14-96

14-97

IMT Radio Interfaces

Satellite Networks

• A satellite network is a combination of nodes.

• A node can be a satellite, an earth station, or an end-user terminal or telephone.

• A satellite needs to have an orbit, the path in which it travels around the earth.

14-98

Satellite Orbits

Satellite Networks

• The period of a satellite, the time required for a satellite to make a complete trip

around the earth, is determined by Kepler’s law, which defines the period as a

function of the distance of the satellite from the center of the earth.

• In satellites, line-of-sight(LoS) propagation is used.

• The signal from a satellite is normally aimed at a specific area called the footprint.

• The signal power at the center of the footprint is maximum.

• Power decreases as we move out from the footprint center.

14-99

Satellite Networks

14-100

Satellite Categories Satellite Orbit altitudes

Satellite Frequency Bands

Satellite Networks

14-101

Satellite Networks

14-102

Satellite Networks

14-103

12-1 multiple access random access controlled access channelization

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