220 Exam2

Frequency Modulation (FM)

-Signal amplitude can be easily affected by the environment. -Signal frequency, however, is quite stable. -In FM, frequency changes according to the signal.

Metropolitan Area Network (MAN)

Medium expensive; spans a major city or a metroplex

Wide Area Network (WAN)

Most expensive; spans sites in multiple cities

Demodulation

Recreating the low-frequency signal from the high frequency signal.

Digital Subscriber Line (DSL)

High-speed data communication services over a local loop

Quadrature Amplitude Modulation (QAM)

How can the data rate be increased further -Combine amplitude modulation and phase modulation at the same time. (ASK + PSK) -QAM is also used with dialup modems as a way to maximize the rate at which data can be sent

Wavelength Division Multiplexing (WDM)

What are they: FDM in optical fiber How do they work: -Prism: multiplexing and demultiplexing device -a multiplexor: combines beams of light of various wavelengths into a single beam -a demultiplexor: separate a single beam into beams with different wavelengths.

Demultiplexing

Separating a combined stream back into individual streams

Amplitude Modulation (AM)

The amplitude of a carrier is modified in proportion to the meaningful signal. -The frequency of the carrier is fixed. -The carrier continues oscillating at a fixed frequency, but the amplitude of the carrier signal is modified

Statistical TDM

-Also called asynchronous TDM Eliminating unused slots Statistical TDM takes less time to send the same amount of data -Extra overhead ID of the receiver in each slot.

Bus Topology

-Bus topology usually consists of a single cable to which computers are attached. -The ends of a bus network must be terminated. >Preventing electrical signals from reflecting back along the bus. -All computers attach directly to the cable. >Any computer attached to a bus can send data to any other computer attached. >Any computer can receive the signal. -The computers attached to a bus network must coordinate >to ensure that only one computer sends a signal at any time

PSK

Data communications systems often use phase shifting keying to send more bits. -PSK changes the phase of the carrier wave abruptly. -Each such change is called a phase shift -The smaller a phase shift is, the easier to be interfered or distorted. -In theory, it is possible to increase the data rate by increasing the range of phase shifts

Cable Modem VS. DSL

Speed -Usually cable modem is better than DSL. -FiOS and U-Verse have changed the game. Availability -In US, cable is more popular. -Outside US, DSLs dominate. Installation -Cable model and regular DSL. -VDSL, HDSL are more complicated than cable modem. Security -Cable < DSL

Use of an FM Radio Channel_FDM

-200 kHz -Usually divided the range of frequencies into multiple carriers. -Sub-channels have different purposes. >Increase the data rate >Increase immunity/reliability to interference -High data rate (전송속도) >Each carrier for different piece of information. -High reliability >Each carrier for the same piece of information.

Packet switching

-A packet switching system requires a sender to divide each message into blocks of data that are known as packets the size of a packet varies each packet switching technology defines a maximum packet size -One of the chief advantages of packet switching is the lower cost because of sharing a connection.

CDM 2

-A sender is assigned a vector, chip sequence, that is orthogonal to all other senders' chip sequences. -Information from this sender (digitized voice) is processed with this vector. -The processed information from all senders are added up together and transmitted through the same medium and at the same time. -Only the information sent by the sender can be extracted because of the use of orthogonal vectors. -Information cannot be intercepted by others.

Adsl2

-ADSL uses Discrete Multi Tone modulation that combines FDM+DMT -An FDM (splitter) Low frequencies to phone High frequencies to modem -FDM in DMT is implemented by dividing the bandwidth into 286 separate frequencies >255 sub-channels allocated for downstream data transmission >31 allocated for upstream data transmission >Not using the bandwidth below 26 kHz.

ADSL

-ADSL uses FDM to divide the bandwidth of the local loop into three regions -One of the regions corresponds to traditional analog phone service, which is known as Plain Old Telephone Service (POTS) -Two regions provide data communication -ADSL can only guarantee to do as well as line conditions allow Downstream rate: 8 Mbps Upstream rate: 640 kbps

FSK and ASK

-Amplitude shift keying and frequency shift keying are NOT efficient. -Require at least one cycle of a carrier wave. >Fewer bits in each cycle. -Must capture the amplitude or frequency information of the carrier Require at least one cycle of a carrier wave. -Transmission capacity is limited by the carrier frequency! >One bit per cycle.

Mesh Topology

-Direct link between each pair of computers. -Very inefficient: n x (n-1) /2 >The number of connections needed for a mesh network grows faster than the number of computers. -Because connections are expensive >Few LANs employ a mesh topology in special circumstances

Why Different Topologies?

-Each topology has advantages and disadvantages -Ring: >Easy for computers to coordinate access and to detect whether the network is operating correctly >An entire ring network is disabled if one of the cables is cut -Star: >Protecting the network from damage to a single cable because each cable connects only one machine >Requiring more wires -Bus: >Fewer wires >A network is disabled if someone accidentally cuts the main cable >Difficult to add new devices if no tap exists. >>No tap/connector existing means cutting into the line. >As such, this topology is losing popularity >Plenty still around.

Circuit Switching

-Establishing a physical path between a sender and a receiver -Guaranteed channels for pairs of senders and receivers -Circuit switching networks use electronic devices to establish circuit multiple circuits are multiplexed over shared media Virtual circuits

Integrated Services Digital Network (ISDN)

-ISDN offers three separate digital channels -Designated B, B, and D (usually written 2B + D) -B channels: each 64 kbps - data channels -D channel: 16 kbps - a control channel -Typical bandwidth: 128 kbps -ISDN is retiring.

LAN Topologies

-Many LAN technologies have been invented -Each network is classified into a category according to its topology or general shape -Four basic topologies Bus Topology >Tree topology Ring Topology Mesh Topology Star Topology

Three general properties define a circuit switched paradigm:

-Point-to-point communication means that a circuit is formed between exactly two endpoints -Separate steps for circuit creation, use, and termination distinguishes circuits that are switched (i.e., established when needed) from circuits that are permanent -Performance equivalent to an isolated physical path >communication between two parties is not affected in any way by >communication among other parties >circuit switching must provide the illusion of an isolated path for each pair of communicating entities

CSMA/CD-Carrier Sense Multiple Access with Collision Detection

-The computers attached to a bus network must coordinate >Ensuring that only one computer sends a signal at any time -A sender sends a brief transmission (Request to send) before sending data. -A receiver responds by sending a control message (clear to send) to indicate that it is ready. -A sender begins sending data. Collision > back-off mechanism -Not common anymore. >Star topology based on switch becomes more common. >No collision at all.

Synchronous TDM

-The multiplexor's clock is twice as faster as the demultiplexor's. -Need a technique to indicate where a slot begins and ends.

Packet switching 2

-To provide communication among N computers -A circuit-switched network: at least N/2 independent paths -A packet-switching network: only requires one path that is shared -A packet switching system uses statistical multiplexing

Cable modem

-Use FDM to deliver TV signals over coaxial cable -Coaxial cable has high bandwidth and is less susceptible to electromagnetic interference than twisted pair. -Data Rate of Cable Modems: In theory DS:52 Mbps/US:512 kbps In practice 6 Mbps -Shared among a set of N subscribers

Code Division Multiplexing (CDM)

-While FDM and TDM have been used in landline phone systems (rely on physical properties of signals (time, frequency)), CDM does not rely on any physical property of signals -Uses an interesting mathematical idea >Values from orthogonal vector spaces can be combined and separated without interference

Local Area Network (LAN)

-least expensive; spans a single room or a single building -LAN technologies allow multiple computers to share a medium in communication. -Multi-access -A LAN is a multi-access network connecting communicating entities (e.g.Computers, printers, ...)

Why do we need modulation

1) Transmit and receive low frequency signals easily. 2) Allow different signals to be transmitted simultaneously. 3) A way to combine the meaningful signal and the carrier signal Three most often seen methods -Amplitude modulation (AM) -Frequency modulation (FM)

Wireless LAN/Wifi

A LAN using wireless transmission. -Signals transmitted in open space with publicly available channels. Challenges -Crowded frequency spectrum >Diagram >Interference from many sources >Allocation: auctioned -Mobility of computers -Open to anyone with a wifi receiver.

Hub

A hub emulates a physical cable -Making the entire system operate like a conventional Ethernet A system that uses a hub in CSMA/CD receives a copy of each frame and uses the address to determine -whether to process or ignore the frame

A variety of wireless LAN technologies

A variety of wireless LAN technologies: various frequencies various modulation techniques various data rates

FDM 2

Advantage of FDM -One transmission medium for multiple pairs of entities A dedicated frequency channel for each pair. Limitation -Frequency interference >Requiring adequate spacing between channels. (Guard band)

Star Topology

All computers attach to a central point. -Hub, switch The central point is an electronic device to accept data from a sending computer and deliver it to the appropriate destination

CDMA

CDMA: Code Division Multiple Access A CDM version for cellular phones CDM has also been used in some satellite communication before being used in cellular phone systems.

Multiplexing

Combining information streams from multiple sources for transmission over a shared medium

MAC Addresses

Composition -Vendor ID + NIC ID VV:VV:VV:NN:NN:NN -IEEE assigns a vendor ID -Organizationally Unique ID (OUI) -The vendor assigns a unique value to each device

IP Address vs. MAC Address

Different layers IP: Internet layer MAC: Network layer Both are required to deliver a pack correctly.

ISM Wireless Bands

Electromagnetic spectrum reserved for use by Industrial, Scientific, and Medical (ISM) groups The frequencies are not licensed to specific carriers

FDM, TDM- hierarchy implementation

Hierarchical FDM -FM signals can be further modulated to a different frequency band! -FDM hierarchy is good enough to determine which frequency band to use in multiplexing. -The difference is that each successive stage of a TDM hierarchy uses N times the bit rate, whereas each successive stage of an FDM hierarchy uses N times the frequencies.

Pros and cons of Cable modem

PROS The FDM hardware in existing cable boxes and cable modems guarantees that data and entertainment channels will not interfere with one another

Choice of Topology

Reliability 확실성 Scalability 확장성 Performance 실적 Consistency 지속성 >Medium, wiring layout, access control Star topology becomes more and more common. >Based on switches.

Ring Topology

Ring topology arranges for computers to be connected in a closed loop. -A cable connects the first computer to a second computer, another cable connects the second computer to a third, and so on, until a cable connects the final computer back to the first -Ring: the computers and the cables connecting them are arranged in a circle conceptually. >In practice, the cables in a ring network do not form a circle >Instead, they run along hallways or rise vertically from one floor of a building to another -Ring topology requires a computer to connect to a small device that forms the ring >This is needed for the ring to continue operation even if some of the computers are disconnected

Types of DSL

SDSL -Equal downstream and upstream HDSL -Addressing the distance problem of ADSL. VDSL/VDSL2 -FiOS and U-Vers -No longer limited to copper wires. DSL: only means that the service still relies on the phone infrastructures.

Why do we need demodulation

Separating the meaningful signal from the carrier

MAC Address 2

Some devices allow the modification of MAC address. -MAC CAN be modified. >MAC spoofing. -As long as the changed MAC address does not conflict with another NIC card in the same LAN. MAC address is only relevant at Layer 2 (Network).

Circuit Switching 2

Switched circuits use a three-step process analogous to placing a phone call =a circuit is established between two parties =the two parties use the circuit to communicate =the two parties terminate use

Time Division Multiplexing (TDM)

TDM does not rely on special properties of electromagnetic energy. A simple trick: an item from one source per unit time slot How do they work: -Framing Used in the Telephone System Version of TDM -Synchronous TDM -Statistical TDM -Inverse Multiplexing : Single hs input > Multi ls connection > single hs output

Phase Shift and a Constellation Diagram

To encode data > phase shift -A sender and receiver can agree on the number of bits per second Use phase shifts to denote the change of bits. -A constellation diagram is used to express the exact assignment of data bits to specific phase changes

802.11 Frame Format

Under 802.11, a wireless host can only associate with a single AP 802.11 frame format Multiple addresses: MAC addresses Source, destination, access point.

Modulation/Demodulation

Using a high frequency signal to carry information about a low-frequency signal.

Frequency Division Multiplexing (FDM)

What are they Each pair of sender and receiver use a particular carrier frequency. Multiple carrier waves sent over the same medium (e.g., a single copper wire, air). How do they work :radio stations -Each station has particular frequency band, regarded as a channel -Channels do not interfere with each other.

Shift keying

Instead of a continuum of possible values, digital shift keying has a fixed set -Mapping to the power levels of a digital signal