Networks Exam 2

LAN topologies - Star

- all computers attach to a central point, center of a star network is often called a hub - typical hub consists of an electronic device that accepts data from a sending computer and delivers it to the appropriate destination - in practice, star networks seldom have symmetric shape (hub is located an equal distance from all computers) - instead, a hub often resides in a location separate from the computers attached to it

LAN topologies - Ring

-arranges for computers to be connected in a closed loop - name ring arises because one can imagine the computers and the cables connecting them arranged in a circle (in practice they run along hallways or rise vertically from one floor of a building to another) -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

LAN topologies - Mesh

-network that uses a mesh topology provides a direct connection between each pair of computers -chief disadvantage - arises from the cost connections in a mesh network = n!/(n-2)!2! = n^2 - n / 2 - important point is that 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 certain circumstances

Ethernet frame format

-refers to the way a packet is organized -frame format has remained constant since the DIX standard was created in the 1970s, so older versions of ethernet have remained compatible with newer versions

LAN topologies - Bus

-usually consists of a single cable to which computers attach -ends of a bus network must be terminated to prevent electrical signals from reflecting back along the bus -any computer attached to a bus can send on the cable and all computers receive the signal -computers attached to a bus network must coordinate to ensure that only one computer sends a signal at any time

Circuit Switched networks

3 properties: provide point to point communication; a dedicated path between sender and receiver separate steps: circuit creation, use, and termination performance equivalent to an isolated physical path Circuit can be permanent/provisioned (left in place for long periods) or switched (created on demand) Must have at least N/2 independent paths

How many satellites are orbiting the Earth in 2015?

4077

Byte = ? bits

8 bits

Twisted Pair Ethernet Wiring and Hubs

A hub emulates a physical cable, making the entire system operate like a conventional Ethernet Twisted pair Ethernet retains the same frame format as the previous versions In fact, software on a computer cannot distinguish between thick Ethernet, thin Ethernet, and twisted pair Ethernet (NIC handles the details and hides any differences)

GEO Satellites

Adv: orbital period is exactly the same as the rate at which the earth rotates, remains in exactly the same location, once a ground station has been aligned with the satellite, the equipment never needs to move

Standards organizations

American National Standards Institute (ANSI) Telecommunications Industry Association (TIA) Electronic Industries Alliance (EIA)

Transmission order

Big endian - system that sends the MSB first Little endian - system that sends the LSB first Either form can be used but the sender and receiver must agree

Wiring Standards and Data Rates

CAT 1 - unshielded twisted pair used for telephones - < 0.1 mbps CAT 2 - unshielded twisted pair used for T1 data - 2 mbps CAT 3 - improved CAT 2 used for computer networks - 10 mbps CAT 4 - improved CAT3 used for Token Ring Networks - 10 mbps CAT 5 - unshielded twisted pair used for networks - 100 mbps CAT 5E - extended CAT5 for more noise immunity - 125 mbps CAT 6 - unshielded twisted pair tested for 200 mbps - 200 mbps CAT 7 - shielded twisted pair with a foil shield around the entire cable plus a shield around each twisted pair - 600 mbps

Data flow

Communications channel is classified as: - simplex - data flows in one direction - full duplex - data flows simultaneously in both directions half-duplex - data flows both directions, but only one at a time

Nyquist's Theorem

D = 2Blog2K Provides an absolute max that cannot be achieved in practice

Wired LAN technologies

Explosion of technologies and products during 1980s, consolidation during the 1990s, currently: one de facto wired LAN standard - ETHERNET

Thinnet Ethernet Wiring

Formally named 10base2 and informally known as Thinwire Ethernet or Thinnet Flexible coaxial cable Connections run point to point among computers

Frames and Framing

Frame - Layer 2 (Data Link) packet - header - contains metadata, such as an address - payload - contains data being sent Network only examines the frame header, which usually has fixed fields, each technology imposes a maximum payload size Use ASCII character set, start of header (SOH), and end of transmission (EOT) characters to mark the start and end of a frame

Ethernet frame format

Header - 6 byte destination address, 6 byte source address, 2 byte type 46 - 1500 bytes of payload 4-byte CRC

Variants of Twisted Pair Ethernet and Speeds

Higher speed ethernet technologies use an electronic device known as a switch rather than a hub To remain backward compatible - standards for the higher speed versions specify that interfaces automatically sense the speed at which a connection can operate and slow down to accomodate older devices

Physical and Logical Ethernet Topology

Hub emulates a physical cable, topology is a physical star but logical bus, hub is known as a bus in a box

IEEE standard for addressing

IEEE media access control address (MAC address) Each address is 48 bits long - 3 byte organizationally unique id (OUI) - 3 byte network interface controller (NIC) Assigned to Network Interface Card (NIC) when device manufactured

Byte and Bit Stuffing

In the ASCII character set, SOH has hexadecimal value 01 or 0x01, EOT has the hexadecimal value of 04 or 0x04

IEEE

Institute of Electrical and Electronics Engineers - professional society - standardizes vendor-independent technologies

Ethernet

Interesting aspect: newer versions of Ethernet remain backward compatible

Packet switched networks examples

LAN - Local Area Network - least expensive; spans a single room or a single building MAN - Metropolitan Area Network - medium expense; spans a major city or metroplex WAN - Wide area network - most expensive; spans sites in multiple cities PAN - Personal Area Network - spans the area around an individual used for earphones SAN - Storage Area Network - spans the distance between a disk farm and processors in a data center CAN - Chip Area Network - spans a single chip and connects processor, memories, etc

Which layer is the network interface in?

Layer 2 - Data Link

Satellite Communication

Low Earth Orbit (LEO) - has advantage of low delay, disadvantage that from an observer's pov on the earth, the satellite appears to move across the sky - 100 to 500 mi Medium Earth Orbit (MEO) - an elliptical (rather than circular) orbit primarily used to provide communication at the North and South Poles - 6000 to 12000 mi Geostationary Earth Orbit (GEO) - has the advantage that satellite remains at a fixed position with respect to a location on the earth's surface, but the disadvantage of being farther away 22240 mi

Visible Spectrum Classification

Low frequency - < 2 MHz - wave follows earth's curvature, but can be blocked by unlevel terrain Medium frequency - 2 to 30 MHz - wave can reflect from layers of the atmosphere, especially the ionosphere High frequency - > 30 MHz - wave travels in a direct line, and will be blocked by obstructions

Electromagnetic (Radio) Communication

Most common form of unguided communication Use electromagnetic energy in the radio frequency range Has a distinct advantage over light - can traverse long distances and penetrate objects such as the walls of a building Properties depend on the frequency FCC sets rules for how frequencies are allocated

3 forms of optical fibers

Multimode, step index - least expensive and used when performance is unimportant, the boundary between the fiber and the cladding is abrupt, which causes light to reflect frequently, dispersion is high Multimode, graded index - slightly more expensive than the step index fiber, density of the fiber increase near the edge, which reduces reflection and lowers dispersion Single mode - most expensive, and provides the least dispersion, smaller diameter and other properties that help reduce reflection, used for long distances and higher bit rates

LAN connections and Network Interface Cards

NIC handles address recognition, frame recognition, sending/receiving frames NIC consists of a circuit board with a plug on one side, NIC is independent from the rest of the computer

What do these standards define?

Network topology (shape) Endpoint addressing scheme Frame (packet) format Media Access Mechanism Physical layer aspects and wiring

Optical fiber vs. Copper Wiring

Optical fiber - immune to electrical noise, less signal attenuation, higher bandwidth Copper wiring - lower overall cost, less expertise/equipment needed, less easily broken

Ethernet Evolution and Thicknet Wiring

Original Ethernet Wiring - thick wire Ethernet or Thicknet, medium consisted of a heavy coaxial cable: 10base5 2 major parts of the hardware - NIC handled the digital aspects of communication, transceiver connected to the Ethernet cable

Ethernet Evolution and Thicknet Wiring

Physical cable known as an Attachment Unit Interface (AUI) connected a transceiver to a NIC in a computer Heavy coaxial cable typically in the ceiling Each computer attached to the cable

Advantages/Disadvantages of Thinnet

Primary advantages: lower overall cost and ease of installation, no external transceivers were needed, thinnet cable could be installed in a convenient path Chief disadvantage: user can disconnect the network, if a user unplugged a segment of the network to relocate wired or move a computer, the entire network would stop working

IEEE 802 Model

Project 802 - LAN/MAN standards committee - organized in 1980 - focuses on layer 1 and layer 2 standards -divides layer 2 into 2 sublayers: Logical Link Control (LLC) - addressing and demultiplexing, Media Access Control (MAC) - access to shared media

Measuring Transmission Media

Propagation delay - time required for a signal to traverse the medium Channel capacity - max data rate that the medium can support

Advantages and Disadvantages of LAN Topologies

Ring - easy for computers to coordinate access, entire network is disabled if one cable is cut Star - protect the network from damage to a single cable as each cable connects only one machine, need more wires/connections Bus - requires fewer wires than a star, has same disadvantage as a ring

Variants of Twisted Pair Ethernet and Speeds

Several variants of twisted pair ethernet have been created, variants differ in data rate and wiring required 10BaseT - Twisted Pair Ethernet - 10mbps data rate, category 5 cable used 100BaseT - Fast Ethernet - 100mbps, category 5E 1000BaseT - Gigabit Ethernet - 1gbps, category 6

Twisted Pair Connectors and Cables

Twisted pair ethernet uses rj45 connectors Straight cable - used between a computer and a switch, connects each pin of the rj45 attached to one end of the cable directly to the corresponding pin on the rj45 at the other end Crossed cable - which is used to connect 2 switches, connects a pin on one end to a different pin on the other end

Address Types

Unicast - destination is a single computer; only that computer should receive a copy of the packet Broadcast - destination is all computers on a network; they should each receive a copy of the packet Multicast - subset of the computers on a network should receive a copy of the packet

How can we reduce the effect of noise on copper wiring?

Unshielded Twisted Pair (UTP) Coaxial Cable Shielded Twisted Pair (STP)

Twisted Pair Ethernet Wiring and Hubs

Unshielded or shielded twisted pairs using RJ45 connectors Multiple pairs allows full-duplex operation Each computer connects to central hub - hubs were available in a variety of sizes, with the cost proportional to size, but recently replaced with switches

Addressing

Used to direct messages from source to destination, addresses are assigned in various ways, exist at different layers, may be translated from one layer to another Application Layer - URL Internet Layer - IP Address Data Link Layer - MAC Address

Packet switched networks

all data divided into packets multiplex communication over shared media (after sending one packet, sender allows others a chance to transmit before sending a second packet properties: arbitrary, asynchronous communication, no set-up required before communication begins concept: underlying infrastructure is shared among users multiple sources compete for the use of shared media *forms the basis for the internet

Multi Access Networks - LAN

allow multiple computers to share a medium in such a way that any computer on the LAN can communicate with any other provide direct connection among communicating entities

Bit stuffing

also available for systems that transfer a stream of bits

Analog Modulation techniques

amplitude modulation frequency modulation phase shift modulation

Frequency Modulation (FM)

amplitude of the carrier remains fixed frequency changes according to the signal - when signal is stronger, the carrier frequency increases slightly - when signal is weaker, carrier frequency decreases slightly

Timing of serial transmission

asynchronous - can occur at any time synchronous - occurs continuously isochronous - regular intervals

Carrier

basic oscillating electromagnetic wave transmitted through a circuit

Infrared (IR) Communication

behaves like visible light but falls outside the range that is visible to a human eye disperse quickly, can reflect from a smooth, hard surface, easily blocked by an opaque object commonly used to connect to a nearby peripheral

Coaxial Cable

better protection: wrap a metal shield around the wire price - inexpensive (more costly than UTP) distance - up to 2km Use - cable TV/Internet in the past

2 approaches to guided and unguided transmission:

by type of path by form of energy

Ethernet tech specifies that data is sent

byte big endian and bit little endian

Modulation

changes made in a carrier, modification of a carrier wave's fundamental characteristics in order to encode information input: carrier, signal output: modulated carrier

Packet identification

each station on a LAN is assigned a unique address LAN hardware uses the address in a packet to determine which station(s) receive a copy

3 energy types

electrical light electromagnetic (radio)

Shannon's Theorem

gives the max channel capacity, C, in the presence of noise C = Blog2(1 + S/N)

Demodulator

hardware mechanism that accepts a modulated carrier wave and recreates the sequence of data bits that was used to modulate the carrier

Modulator

hardware mechanism that accepts a sequence of data bits and applies modulation to a carrier wave

Broadband modem

high data rate - normally > 1mbps

Advantages to parallel transmission

high speed - it can send N bits at the same time, parallel interface can operate N times faster than an equivalent serial interface match to underlying hardware - internally, computer and communication hardware uses parallel circuitry

Media using light energy

infrared transmission - short range and low data rate point to point lasers - useful between buildings optical fiber - high data rate and long distance

GEO Coverage of the Earth

limited amount of space available in GEO orbit above the equator - must be separated from one another to avoid interference - min separation depends on power of the transmitters

Advantages of packet switching

lower cost that arises from sharing scalability - to connect N computers with packet switching, a network must have a connection for each computer, but only requires one path that is shared

Transmission mode

manner in which data is sent over the underlying medium serial - one bit sent at a time parallel - multiple bits are sent at the same time

UTP

more current is generated in the first wire the interference hits - price - inexpensive - distance - typically up to 100m - use - telephones, LANs

Shift keying

operates similar to analog modulation except signal is digital - digital shift keying has a fixed set

Light energy types

optical fiber infrared laser

Point to point laser communication

pair of devices with a beam that follows the line-of-sight requires a clear, unobstructed path between the communicating sites - beam is only a few centimeters wide - sending and receiving equip must be aligned precisely - used outdoors, can span great distances

Byte stuffing

payload of a frame includes one or more bytes with value 01 or 04

Noise

random electromagnetic radiation

Serial transmission

sends one bit at a time most communication systems use serial mode can be extended over long distances at much less cost no timing problem since it uses one wire

LEO satellites

short delays, 1-4 min Orbit does not match rotation of earth, tracking is difficult

When electromagnetic radiation encounters metal

small electrical current is induced that can interfere with signals being carried on the wire

IEEE's version of ethernet (802.3)

standardized in 1983 as IEEE standard 802.3, adds 8 bytes of payload occupied by LLC/SNAP header - logical link control/sub-network attachment point (LLC/SNAP) header

Wiring in an office building

styles of wiring used for LANs make little difference in a machine room or laboratory Type of wiring makes a major difference in terms of: type, number of wires needed, distance spanned, cost 3 versions of Ethernet wiring illustrate the 3 principal forms that LANs use *Twisted pair ethernet requires many individual cables to go between offices and a central point (wiring closet), so twisted pair ethernet requires careful labeling of cables

Electromagnetic (radio) energy types

terrestrial radio satellite

STP

thinner, more flexible metal shield surrounding one or more twisted pairs of wires

Parallel transmission

transfers of multiple data bits at the same time over separate media used with a wired medium that uses multiple, independent wires signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same time

Guided (wired) media

transmission flows along physical medium

Unguided (wireless/radiated) media

transmission flows through the air

Electrical energy types

twisted pair coaxial cable

Ethernet type field and demultiplexing

type field provides multiplexing and demultiplexing set by sender to identify content of frame used by receiver to determine how to process the frame formed when frame arrives, usually handled by protocol software, frame type field examined and frame passed to appropriate protocol module: unrecognized types are discarded

Phase Modulation (PM)

use changes in a phase to represent a signal

Optical Fiber

used for communication in a single direction one end of the fiber connects to a laser or LED used to transmit light the other end connects to a photosensitive device to detect incoming light price - expensive distance - 500 m - 100 km Use - trunk line/backbone, long distance circuits

Dialup Modems

uses data to modulate an audible carrier (audible tone instead of electrical signal)

Amplitude Modulation (AM)

varies the amplitude of a carrier in proportion to the information being sent continues oscillating at a fixed frequency, but amplitude of the wave varies