computer network andrew s. tanenbaum. outline the mobile telephone system cable television wireless...

Computer Computer NetworkNetwork

Andrew S. TanenbaumAndrew S. Tanenbaum

OutlineOutline• The mobile telephone system• Cable television• Wireless LANS• Broadband wireless• Bluetooth• Data Link layer switching• Quality of service

Wireless telephonesWireless telephones• Cordless phones -Never used for networking• Mobile phones – through three

generations with different technologies:

-Analog voice -Digital voice -Digital voice and data

The mobile telephone The mobile telephone systemsystem

• First-generation mobile phones: analog voice -IMTS -AMPS• Second-generation mobile phones: digital voice -D-AMPS -GSM -CDMA• Third-generation mobile phones: digital voice and data -W-CDMA -CDMA2000• 2.5G scheme -GPRS

First-generation mobile First-generation mobile phones: analog voicephones: analog voice

• 1946 push-to-talk system -A single channel for both sending and

receiving

• 1960 IMTS (Improved Mobile Telephone System)

-High-powered transmitter -Two frequencies (sending/receiving) -23 channels spread out from 150 MHz to 450 MHz

IMTS drawbacksIMTS drawbacks• Due to the small number of channels,

users often had to wait a long time before getting a dial tone.

• Due to the lager power of the hilltop transmitter, adjacent systems had to be several hundred kilometers apart to avoid interference.

AMTS (Advanced Mobile AMTS (Advanced Mobile Phone System)Phone System)

• In all mobile phone systems, a geographic region is divided up into cells.

• Key idea: -increases the system capacity (reuse

of transmission frequencies) -less power is needed.

The idea of frequency reuseThe idea of frequency reuse

MTSO (Mobile Telephone MTSO (Mobile Telephone Switching Office)Switching Office)

• Handoff - soft handoff -no loss of continuity -telephone needs to be able to tune to two frequencies at the same time - hard handoff

AMTS ChannelsAMTS Channels• The AMPS system uses 832 full-duplex

channels, each consisting of a pair of simplex channels

-832 simplex transmission channels from 824 to 849 MHz -832 simplex receive channels from 869 to 894 MHz• AMPS uses FDM to separate the channels

AMPS call managementAMPS call management• When a phone switch on • When a caller makes a call

Second-generation mobile Second-generation mobile phones: digital voicephones: digital voice

• D-AMPS is fully digital• D-AMPS is designed to co-exist with

AMPS• Upstream channels are in the 1850-1910 MHz • Downstream channels are in the

1930-1990 MHz

D-AMPSD-AMPS• The voice signal is digitized and

compressed • Users can share a single frequency

pair using TDM

A D-AMPS channel with A D-AMPS channel with usersusers

Difference between AMPS Difference between AMPS and D-AMPSand D-AMPS

• How handoff is handled

GSM (The Global System for GSM (The Global System for Mobile Communications)Mobile Communications)

• GSM versus D-AMPS: -FDM is used with each mobile

transmitting on one frequency receiving on a higher frequency

-A single frequency pair is split by TDM into time slot shared by multiple mobiles

-GSM has a much higher data rate per user than D-AMPS

GSM uses 124 frequency GSM uses 124 frequency channels, each of which uses channels, each of which uses

an eight-slot TDM systeman eight-slot TDM system

A partition of the GSM A partition of the GSM framing structureframing structure

CDMA (Code Division CDMA (Code Division Multiple Access)Multiple Access)

• D-AMPS , GSM use both FDM and TDM.

• CDMA allows each station to transmit over the entire frequency spectrum all the time.

• Multiple simultaneous transmissions are separated using coding theory.

CDMA coding theoryCDMA coding theory• Each bit time is subdivided into m short

intervals called chips (There are 64 or 128 chips per bit).

• Each station is assigned a unique m-bit code called a chip sequence. - To transmit 1 bit , a station sends its chip sequence - To transmit 0 bit , a station sends the one’s complement of its chip sequence

Example (1/2)Example (1/2)

Example (2/2)Example (2/2)

PropertiesProperties

Third-generation mobile Third-generation mobile phones: digital voice and dataphones: digital voice and data

• WCDMA (Wideband CDMA) -uses direct sequence spread spectrum -runs in a 5 MHz bandwidth -has been designed to interwork with GSM• CDMA2000 -not be designed to interwork with GSM -has the differences between WCDMA : chip rate,

frame time, spectrum used, the way to do time synchronization

2.5 G schema GPRS (General 2.5 G schema GPRS (General Packet Radio Service)Packet Radio Service)

• Is an overlay packet network on top of D-AMPS or GSM.

• Allows mobile stations to send and receive IP packets in a cell running a voice system

Cable televisionCable television• Community antenna television• Internet over cable• Spectrum allocation• Cable modems• ADSL versus cable

An early cable television An early cable television systemsystem

Part of a modern HFC Part of a modern HFC systemsystem

The fixed telephone systemThe fixed telephone system

Spectrum allocationSpectrum allocation

Cable modemsCable modems• Internet access requires a cable

modem• Cable modem is always on• Cable operators do not charge for

connect time

What happens when a cable What happens when a cable modem is plugged in and modem is plugged in and

powered up? (1/2)powered up? (1/2)• The modem scans the downstream channels l

ooking for a special packet periodically put out by the headend to provide system parameters to modems.

• Modem announces its presence on one of the upstream channels

• The headend responds by assigning the modem to its upstream and downstream channels

What happens when a cable What happens when a cable modem is plugged in and modem is plugged in and

powered up? (2/2)powered up? (2/2)• The modem determines its distance fro

m the headend –ranging• During initialization, the headend also a

ssigns each modem to a minislot to use for requesting upstream bandwidth

• What happens when a computer wants to send a packet?

Typical details of the upstream Typical details of the upstream and downstream channelsand downstream channels

ADSL versus cableADSL versus cable• Both use fiber in the backbone, but

they differ on the edge• The increasing numbers have

different effects on existing users on the two system

• Availability and security and reliability are issues on which ADSL and cable differ

Wireless LANSWireless LANS• The 802.11 protocol stack• The 802.11 physical layer• The 802.11 MAC sublayer protocol• The 802.11 frame structure• services

The 802.11 protocol stackThe 802.11 protocol stack• MAC sublayer determines how the channel is a

llocated, that is, who gets to transmit next• LLC sublayer hides the differences between th

e different 802 variants• 802.11 specifies three transmission techniques

allowed in the physical layer -infrared method -short-range radio (FHSS/DSSS)

Part of the 802.11 Part of the 802.11 protocol stackprotocol stack

The 802.11 The 802.11 physical layer(1/7)physical layer(1/7)

• Infrared option -uses diffused transmission at 0.85 or 0.95 microns -two speeds are permitted: 1 Mbps, 2Mbps -infrared signals can’t penetrate walls


• FHSS (Frequency Hopping Spread Spectrum)

-uses 79 channels, each 1 MHz wide, starting at the low end of the 2.4GHz ISM band -A pseudorandom number generator is used to produce the sequence of frequencies hopped to


-The amount of time spent at each frequency—dwell time -advantages: 1.a fair way to allocate spectrum 2.security 3.good resistance to multipath fading 4.relatively insensitive to radio interference -disadvantage: low bandwidth


• DSSS (Direct Sequence Spread Spectrum)

-restricts to 1 or 2 Mbps -has some similarities to the CDMA system -each bit is transmitted at 11 chips, using Barker sequence -uses phase shift modulation


• High-speed wireless LANs, 802.11a, uses OFDM (Orthogonal Frequency Division Multiplexing)

-deliver up to 54 Mbps in the wider 5GHz ISM band -advantages: 1.good immunity to multipath fading 2.using noncontiguous bands (good spectrum efficiency)


• 802.11b uses HR-DSSS (High Rate Direct Sequence Spread Spectrum)

-uses 11 million chips/sec to achieve 11Mbps in the 2.4GHz band -data rate 1,2 Mbps use phase shift modulation (compatibility with DSSS) -data rate 5.5,11Mbps use Walsh/Hadamard codes


• Although 802.11b is slower than 802.11a, its range is about 7 times greater.

• 802.11g uses OFDM modulation of 802.11a, but operates in the narrow 2.4GHz ISM band along with 802.11b

The 802.11 MACThe 802.11 MAC sublayer protocol sublayer protocol

• The 802.11 MAC sublayer protocol is quite different from that of Ethernet due to the inherent complexity of the wireless environment compared to that of a wired system

Two ProblemsTwo Problems

802.11 supports two modes of 802.11 supports two modes of operation to deal with the problemoperation to deal with the problem

• DCF (Distributed Coordination Function)

-uses a protocol CSMA-CA 1.physical channel sensing 2.virtual channel sensing (based on MACAW)• PCF (Point Coordination Function)

The use of virtual channel The use of virtual channel sensing using CSMA/CAsensing using CSMA/CA

Fragment frame Fragment frame • The probability of a frame making it

through successfully decreases with frame length

• To deal with the problem of noisy channels, 802.11 allows frames to be fragmented into smaller pieces, each with its own checksum

A fragment burstA fragment burst

PCFPCF• The base station polls the other

stations, asking them if they have any frames to send

• Transmission order is completely controlled by the base station in PCF mode, no collisions ever occur

• The basic mechanism is for the base station to broadcast a beacon frame periodically

Four different intervals are defined, Four different intervals are defined, each for a specific purposeeach for a specific purpose

The 802.11 frame structureThe 802.11 frame structure

Services (1/2)Services (1/2)• Distribution services-manage cell

membership and interact with stations outside the cell

1.association 2.disassociation 3.reassociation 4.distribution 5.integration

Services (2/2)Services (2/2)• Station services- activity within a

single cell 1.authentication 2.deauthentication 3.privacy 4.data delivery

Broadband wirelessBroadband wireless• Comparison of 802.11 with 802.16• The 802.16 protocol stack• The 802.16 physical layer• The 802.16 MAC sublayer protocol• The 802.16 frame structure

Comparison of 802.11 Comparison of 802.11 with 802.16 (1/2)with 802.16 (1/2)

• 802.11 and 802.16 are very different as they try to solve different problems

-similar :they were designed to provide high-bandwidth wireless communications

-differ:802.16 provides service to buildings 1.buildings are not mobile 2.buildings can have more than one computer in them

Comparison of 802.11 Comparison of 802.11 with 802.16 (2/2)with 802.16 (2/2)

802.16(wireless MAN) properties: -Because of distances, the perceived power at the base

station vary widely from station to station (affects the signal-to-noise ratio)

-802.16 operate in the much higher 10-to-66 GHz frequency range

-These millimeter waves have different physical properties than the longer waves in the ISM bands (requires a completely different physical layer).

-802.16 provide QoS.

The 802.16 protocol stackThe 802.16 protocol stack

The 802.16 physical layerThe 802.16 physical layer• Because signal strength in the millimeter

band falls off sharply with distance from the base station, the signal-to-noise ratio also drops with distance from the base station

• 802.16 employs three different modulation schemes, depending on how far the subscriber station is from the base station

-the farther the subscriber is from the base station, the low the data rate

802.16 transmission 802.16 transmission environmentenvironment

802.16 provides a more flexible 802.16 provides a more flexible way to allocate the bandwidthway to allocate the bandwidth

• Two schemes: -FDD (Frequency Division Duplexing) -TDD (Time Division Duplexing)

TDDTDD

The 802.16 MAC The 802.16 MAC sublayer protocolsublayer protocol

• Downstream and upstream maps -tell what is in which time slot and which time sl

ots are free• Downstream channel -base station simply decides what to put in whi

ch subframe• Upstream channel -there are competing uncoordinated subscribe

rs that need access to it -Its allocation is tied closely to the QoS issue

Four classes of serviceFour classes of service• Constant bit rate service• Real-time variable bit rate service• Non-real-time variable bit rate

service• Best-efforts service

The 802.16 frame structureThe 802.16 frame structure

BluetoothBluetooth• Bluetooth architecture• Bluetooth application• The Bluetooth protocol stack• The Bluetooth radio layer• The Bluetooth baseband layer• The Bluetooth L2CAP layer• The Bluetooth frame structure

BluetoothBluetooth• A wireless standard for interconnecting

computing and communication devices and accessories using short-range, low-power, inexpensive wireless radios

• Bluetooth specification is for a complete system, from the physical layer to the application layer

Bluetooth architectureBluetooth architecture• The basic unit of a Bluetooth system is a

piconet, which consists of a master node and up to seven active slave nodes within a distance of 10 meters

• An interconnected collection of piconets is called a scatternet

Two piconets can be connected tTwo piconets can be connected to form a scatterneto form a scatternet

Master/slave design Master/slave design • The reason is that the designers intended to fa

cilitate the implementation of complete Bluetooth chips for under $5

• Slaves are fairly dumb, doing whatever the master tells them to do

• A piconet is a centralized TDM system, with master controlling the clock and determining which device gets to communicate in which time slot

The Bluetooth applicationThe Bluetooth application

The Bluetooth protocol The Bluetooth protocol stackstack

The Bluetooth The Bluetooth radio layer (1/2)radio layer (1/2)

• The radio layer moves the bits from master to slave, or vice versa

• Bluetooth is a low-power system with a range of 10 meters operating in the 2.4-GHz ISM band

• The band is divided into 79 channels of 1MHz each

• To allocate the channels fairly, frequency hopping spread spectrum is used with 1600 hops/sec and a dwell time of 623μsec

The Bluetooth The Bluetooth radio layer (2/2)radio layer (2/2)

• All the nodes in a piconet hop simultaneously, with the master dictating the hop sequence

• 802.11 and Bluetooth operate in the 2.4GHz ISM band on the same 79 channels, they interfere with each other

The Bluetooth The Bluetooth baseband layer (1/2)baseband layer (1/2)

• The baseband layer turns the raw bit stream into frames and defines some key formats

• Longer frames are much more efficient then single-slot frames

• Each frame is transmitted over a logical channel, called a link, between the master and a slave

The Bluetooth The Bluetooth baseband layer (2/2)baseband layer (2/2)

• Two kinds of links : -ACL (Asynchronous Connection-Less) 1.It is used for packet-switched data available at irregular intervals 2.traffic is delivered on a best- efforts basis -SCO (Synchronous Connection Oriented) 1.It is used for real-time data 2.the type of channel is allocated a fixed slot in each direction

The Bluetooth L2CAP layerThe Bluetooth L2CAP layer• It accepts packets of up to 64KB from th

e upper layers and breaks them into frames for transmission

• It handles the multiplexing and demultiplexing of multiple packet sources

• It handles the quality of service requirements

The Bluetooth frame The Bluetooth frame structurestructure

Data link layer switchingData link layer switching• Bridges from 802.x to 802.y• Local internetworking• Spanning tree bridges• Remote bridges• Repeaters, hubs, bridges, switches,

routers, gateways• Virtual LANs

Why a single organization may Why a single organization may end up with multiple LANs?(1/2)end up with multiple LANs?(1/2)

• The goal of the various of departments differ, different departments choose different LANs.

• The organization may be geographically spread over several buildings separates by considerable distances.

• It may be necessary to split what is logically a single LAN into separate LANs to accommodate the load.

Multiple LANs connected by Multiple LANs connected by bridges are used bridges are used

Why a single organization may Why a single organization may end up with multiple LANs?(2/2)end up with multiple LANs?(2/2)

• A single LAN would be adequate in terms of load, but the physical distance between the most distant machines is too great

• There is the matter of reliability• Bridges can contribute to the

organization’s security

Operation of a LAN bridge Operation of a LAN bridge from 802.11 to 802.3from 802.11 to 802.3

some difficulties when trying to some difficulties when trying to build a bridge between the various build a bridge between the various

802 LANs802 LANs

• Each of the LANs uses a different frame format

• Interconnected LANs don’t necessarily run at the same data rate

• Different 802 LANs have different maximum frame lengths

• Another problems are security and quality of service

The IEEE 802 frame formatsThe IEEE 802 frame formats

Local internetworkingLocal internetworking• The bridges should be completely

transparent • When a frame arrives, a bridge must

decide whether to discard or forward it• The decision is made by looking up the

destination address in a hash table inside the bridge

• The algorithm used by the transparent bridges is backward learning

The routing procedure for The routing procedure for an incoming framean incoming frame

• If destination and source LANs are the same, discard the frame

• If the destination and source LANs are different, forward the frame

• If the destination LAN is unknown, use flooding

A configuration with four A configuration with four LANs and two bridgesLANs and two bridges

Two parallel Two parallel transparent bridgestransparent bridges

Spanning tree bridgesSpanning tree bridges• To increase reliability, some sites use two

or more bridges in parallel between pairs of LANs, however, introduce some problems because it creates loops in the topology

• The solution to this difficulty is for the bridges to communicate with each other and overlay the actual topology with a spanning tree that reached every LAN

ExampleExample

Remote bridges can be used Remote bridges can be used to interconnect distant LANsto interconnect distant LANs

Devices operate in Devices operate in different layersdifferent layers

RepeatersRepeaters• There are analog devices that are

connected to two cable segments• A signal appearing on one of them is

amplified and put out on the other• Repeaters understand volts

HubsHubs• Frames arriving on any of the lines

are sent out on all the others• If two frames arrive at the same

time, they will collide

BridgesBridges• A bridge connects two or more LANs• When a frame arrives, software in

the bridge extracts the destination address from the frame header and looks it up in a table to see where to send the frame

SwitchesSwitches• Switches are similar to bridges in

that both route on frame addresses• The main difference is that a switch

is most often to connect individual computers

• Since each port is its own collision domain, switches never lose frames to collisions

RoutersRouters• When a packet comes into a router,

the frame header and trailer are stripped off and the packet located in the frame’s payload field is passed to the routing software

• The software uses the packet header to choose an output line

Transport gatewaysTransport gateways• These connect two computers that

use different connection-oriented transport protocols

Application gatewaysApplication gateways• Application gateways understand the

format and contents of the data and translate messages from one format to another

A building with centralized A building with centralized wiring using hubs and a switchwiring using hubs and a switch

Virtual LANsVirtual LANs• With hubbed (switched) Ethernet, it was

often possible to configure LANS logically rather than physically

Why use virtual LANs?Why use virtual LANs?• Security• Load• Broadcasting• In response to user requests for more

flexibility-rewire building entirely in software

VLANVLAN• VLANs are based on specially-designed V

LAN-aware switches• To make the VLANs function correctly, c

onfiguration tables have to be set up in the bridges or switches

• These tables tell which VLANs are accessible via which ports (lines)

ExampleExample

The IEEE 802.1Q The IEEE 802.1Q standard (1/3)standard (1/3)

• What actually matters is the VLAN of the frame itself, not the VLAN of the sending machine

• If there were some way to identify the VLAN in the frame header, them the need to inspect the payload would vanish

• How about 802.11, 802.16, Ethernet?


• The new format (change the Ethernet header) was Published in IEEE standard 802.1Q

• The key to the solution is to realize that the VLAN fields actually used by bridges and switches and not by the user machines


• The first VLAN-aware bridge or switch to touch a frame adds the VLAN fields, and the last one down the road removes them

• Switches have to know which VLANs are reachable on each port

Transition from legacy Ethernet to Transition from legacy Ethernet to VLAN-aware EthernetVLAN-aware Ethernet

The 802.3 and 802.1Q The 802.3 and 802.1Q Ethernet frame formatsEthernet frame formats

ConclusionConclusion• To use VLANs property, each frame carri

es a new special identifier that is used as an index into a table inside the switch to look up where the frame is supposed to be sent

• That is precisely what happens in connection-oriented networks

Quality of serviceQuality of service• Requirements• Techniques for achieving good

quality of service• Integrated services• Differentiated services• Label switching and MPLS

RequirementsRequirements• The needs of each flow can be characteri

zed by four primary parameters: reliability, delay, jitter, and bandwidth.

• Together these determine the QoS the flow requires.

How stringent the quality-of-How stringent the quality-of-service requirements are service requirements are

Techniques for achieving Techniques for achieving good quality of servicegood quality of service

• Overprovisioning• Buffering• Traffic shaping• The leaky bucket algorithm• The token bucket algorithm• Resource reservation• Admission control• Proportional routing• Packet sheduling

OverprovisioningOverprovisioning• Theorem: An easy solution is to provide

so much router capacity, buffer space, and bandwidth that the packets just fly through easily.

• Disadvantage : It is expensive.• Application: The telephone system is ov

erprovisioned.

BufferingBuffering• Flows can be buffered on the receiving

side before being delivered.• Buffering them -does not affect the reliability or

bandwidth. -increases the delay. -smoothes out the jitter.• The source outputs the packets with a

uniform spacing between them.

Smoothing the output Smoothing the output stream by buffering packetsstream by buffering packets

Traffic shapingTraffic shaping• How about if the server is handling

many streams at once? -No uniform output is common.• Traffic shaping smoothes out the

traffic on the server side. (server transmits at a uniform rate)

The leaky bucket algorithmThe leaky bucket algorithm• It is a single server queueing system wit

h constant service time and finite queue.• The leaky bucket algorithm enforces a ri

gid output pattern at the average rate, no matter how bursty the traffic is.

A leaky bucketA leaky bucket

The token bucket The token bucket algorithm(1/2)algorithm(1/2)

• It is better to allow the output to speed up somewhat when large bursts arrive.

• The token bucket algorithm: -For a packet to be transmitted, it

must capture and destroy one token. -It provides a different kind of traffic

shaping.

The token bucket The token bucket algorithm(2/2)algorithm(2/2)

-The token bucket algorithm throws away tokens when the bucket fills up but never discards packets.

-A packet can only be transmitted if enough tokens are available to cover its length in bytes.

The token bucket algorithmThe token bucket algorithm

Resource reservationResource reservation• A virtual circuit has to be set up from

the source to the destination, and all the packets that belong the flow must follow this route.

• Three kinds of resources can be potential be reserved:

-bandwidth -buffer space -CPU cycles

Admission controlAdmission control• A router has to decide whether to

admit or reject the flow based on its capacity and how many commitments it has already made for other flows.

• The sender, receiver, and all the routers along the path between them may be involved in the flow negotiation.

An example flow An example flow specificationspecification

Proportional routingProportional routing• To provide a higher quality of service

by splitting the traffic for each destination over multiple paths.

Packet shedulingPacket sheduling• Fair queueing algorithm -routers have separate queues for each o

utput line, one for each flow. -the round robin is done in such a way as

to simulate a byte-by-byte round robin.• Weighted fair queueing algorithm -the weight is equal to the number of flo

ws coming out of a machine.

Fair queueing algorithm byte-Fair queueing algorithm byte-by-byte round robinby-byte round robin

Integrated services-RSVP (the Integrated services-RSVP (the resource reservation protocol)resource reservation protocol)

• RSVP (Flow-based algorithm) -It offer good quality of service to one

or more flows by reserving whatever resources are needed along the path.

-It requires an advance setup to establish each flow. (not scalable)

RSVP uses multicast routing RSVP uses multicast routing using spanning treeusing spanning tree

An exampleAn example

Differentiated servicesDifferentiated services• Differentiated services (Class based QoS) -It defines a set of service classes with correspo

nding forwarding rules. -It requires no advance setup, no resource reser

vation, and no time-consuming end-to-end negotiation for each flow.

• Expedited forwarding• Assured forwarding

Expedited forwardingExpedited forwarding

Assured forwardingAssured forwarding

Label switching and MPLSLabel switching and MPLS• This work focused on adding a label in fr

ont of each packet and doing the routing based on the label rather than on the destination address.

Transmitting a TCP segment Transmitting a TCP segment using IP, MPLS, and PPPusing IP, MPLS, and PPP

Thank you for your attention~

computer network andrew s. tanenbaum. outline the mobile telephone system cable television wireless...

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