Network Media and Layer 1 Functionality

Transcription

1 Network Media and Layer 1 Functionality BSAD 146 Dave Novak Dean, Chapter 3, pp Objectives Introduction to transmission media Basic cabling Coaxial Twisted pair Optical fiber Basic wireless (NIC) Basic Physical Layer (layer 1) functionality Background Different parts of the electromagnetic frequency spectrum can be used for data transmission depending on the medium used and the communications standards being followed Properties of specific media affect Bandwidth Attenuation Noise Distortion Background Bandwidth range of frequencies occupied or used by a carrier wave Attenuation strength of signal decreases as it propagates Noise unwanted electromagnetic energy that degrades the signal (crosstalk, background interference) Distortion original shape or characteristic of waveform is altered Transmission Computer communication at basic level involves the transfer of signals over media Layer 1 functionality Examples Electrical current over wire Radio waves through air Microwave through air Infrared through air Light rays through glass fibers Source: 1

2 Transmission Media Directed Coaxial cable Twisted pair Fiber-optic Undirected Radio Frequency (RF) Coaxial Cable At one time, coax was the most widely used copper networking cable Coaxial Cable Legacy Ethernet LAN implementation supports two IEEE standards RG 58 cable Thinnet supports 10Base2 RG 8 cable Thicknet supports 10Base5 Coaxial Cable RG 8 cable Thicknet supports 10Base5 Transceiver AUI Vampire taps Coaxial Cable RG 58 cable Thinnet supports 10Base2 Currently the most widely used LAN cabling Two physical copper circuits (wires) are twisted together to filter out EMI and reduce cross talk Each wire carries an equal, but opposite signal 2

3 1) UTP - unshielded 2) STP - shielded Source: Source: Cable reduces interference and supports higher bandwidth through twisting and isolation Cat 6 UTP has more twists/cm and has thicker sheathing than Cat 5 UTP Source: Source: Different types of cable have different performance characteristics Network Cable Specification Comparison Specification CAT5 CAT5e CAT6 CAT6a CAT7 Frequency 100 MHz 100 MHz 250 MHz 500 MHz 600 MHz Attenuation (Insertion Loss)* 24 db 24 db 19.8 db 18.4 db 20.8 db Characteristic Impedence 100 ohms ± ohms ± ohms ± ohms ± ohms ±15 NEXT * 27.1 db 30.1 db 44.3 db 59 db 62.1 db PS-NEXT * N/A 27.1 db 42.3 db 59.1 db 59.1 db ELFEXT * 17.9 db 17.4 db 27.8 db 43.1 db Not Specified PS-ELFEXT * 14.4 db 14.4 db 24.8 db 41.8 db Not Specified Different types of cable may be needed to support different Ethernet standards 10BASE-T, 100BASE-TX, 100BASE-FX, 1000BASE-T 10GBASE-T Cat5e supports most common standards One could potentially use Cat 6, 6a, or 7 why wouldn t you? Return Loss * 16 db 20.1 db 20.1 db 32 db 14.1 db Delay Skew * 50 ns 45 ns 45 ns 45 ns 20 ns 3

4 Refers to copper conductor in wire pairs Solid uses single piece copper strand Not flexible, but more durable Most common UTP and STP cables use 8 pin RJ45 connectors Stranded uses multiple smaller copper strands woven together Flexible Optical Fiber Cable Supports high bandwidth, long distance transmission by sending pulses of light over very thin, pure glass (or plastic) strands Optical Fiber Cable Two types of cable / transmission 1) Multi-mode 2) Single-mode Source: YFg&sqi=2&ved=0CDwQsAQ&biw=1129&bih=842/ Source: ed=0cfaqsaq&biw=1129&bih=842/ Multi-Mode Fiber Mostly used for communication over short distances within building, around campus High bandwidth, but higher dispersion Bandwidth-distance product limit is lower than single mode Has larger core diameter (and larger wave length) than single mode Less complex equipment than single mode LED versus laser Single-Mode Fiber Has higher bandwidth than multi-mode and is effective over very large distances High bandwidth, low dispersion Carries only a single ray Equipment is complex and expensive Specific types of lasers 4

5 Optical Fiber Connectors Optical Fiber Connectors Source: Source: Wired Cable Design A WLAN (wireless LAN) is a LAN that does not rely on physical cabling to transmit data between devices WLANs use radio frequencies (RF) to transmit data The goal of wireless communication is to transmit large amounts of data as fast as possible without wires Wireless communication follows the IEEE specification which defines halfduplex transmission using the same frequency for send and receive Let s talk about this a bit RF transmission does not require licensing but it is regulated by the government! The regulating agencies are the FCC in the US and the ETSI in Europe provides standards for RF operation within FCC rules (frequencies and power levels) 5

6 There tend to be more difficulties achieving high speed transmission with RF than with wired technologies for a number of reasons To send an RF signal, the use of a modulation technique is required How did we define modulation? Spread Spectrum Examples Spread Spectrum technique used to modulate data by spreading the bandwidth available for transmission Signal is transmitted on bandwidth larger than the frequency content of the transmission Security; increased resistance to noise, interference, and jamming Lower power per frequency band Redundancy Direct Sequence Spread Spectrum (DSSS) An RF modulation technique where data are multiplied by a pseudo noise (PN) signal to spread the signal, the modified signal is transmitted over various frequencies according to the PN sequence, original signal reconstructed by receiver As long as PN sequence is different than the PN sequence of other transmissions the signal to noise ratio is positive Spread Spectrum Examples Frequency Hopping Spread Spectrum (FHSS) Subdivides bandwidth on a particular channel (say channel 1) into separate 1 MHz channels Switch rapidly (pseudo-randomly) between the different 1 MHz frequency bands (min of 75 frequencies every 30 sec) broadcasting for only a few milliseconds over any particular band FCC controls the available spectrum and WLANs can operate in only 3 ranges: 1) 900 MHz, 2) 2.4 GHz, or 3) 5 GHz 6

7 900-MHz band ( MHz) 2.4 GHz band ( GHz) Most widely used frequency range used in WLANs Used by (base, b, g, n) standards 11 channels, each 22 MHz wide 5 GHz band (complicated regs vary by country) 23 non-overlapping channels Used by (a, h, j, n, ac) standards If you are interested see GHz band 11 overlapping channels 1, 6, and 11 non overlapping 7

8 Potential drawbacks to wireless Reliability IEEE technologies have historically been slower than wire-based technologies Limited range Security concerns Provides a physical connection between a networked device and the network medium Also provides an interface between layers 1 and 2 of the OSI Separate cards are needed to support different layer 2 networking technologies (Ethernet, Wi-Fi, dialup, etc.) Generally plugs right into PCI (Peripheral Component Interconnect) slots Most modern NICs are PCI Have different network cable connectors depending on types of cable supported RJ-45 connector most common Basics of data transfer Data move through devices along paths called buses Several paths side by side (parallel) Data moves in lateral groups as opposed to single (serial) data stream 32-bit bus means 32 bits moving along side by side Data move over network channel in a single streams called serial transmission One lane highway with data ALWAYS traveling in same direction 1) Data transmission and reception Primary function of NIC is to generate and transmit signals of the appropriate type over network and to receive incoming signals Signal type depends on network medium and Data Link Layer (2) protocol used On legacy Ethernet LAN, every computer receives all packets transmitted over network NIC must examine MAC destination address in each packet to see if it is intended for the local host 8

9 2) Signal encoding and decoding Converts binary data encapsulated in a frame into electrical voltage, light pulses, or other signal 3) Data buffering Since NIC transmits and receives data in serial manner, it must be able to store data until outgoing frames can be sent and incoming frames can be processed by CPU 4) Data encapsulation Building frame around data in preparation for transmission Read contents of incoming frames, strips off frame, and pass data up to Network Layer (3) 5) Serial / parallel conversion Usually 64 bits at a time 6) Media access control Regulates how device access network medium Layer 1 hardware functionality Recognize / work with basic signals Little to no management capabilities Examples Repeaters Hubs Does not understand frames or MAC addresses Does not understand network level addresses and routing Layer 1 hardware functionality Hubs vs- repeaters I ll discuss this from a functionality / historical perspective as no one really makes or uses layer 1 hubs anymore The concept of how a layer 1 hub works is important with respect to our discussion of legacy versus switched Ethernet though Layer 1 hardware functionality Center discussion on hubs Used to connect devices to a star or ring LAN Send copies of electrical signals to next segment, do not reconstruct frames Regenerate and forward ALL electrical signals Multiple ports Do not understand frame formats Do not understand physical address Do not understand higher level network addresses Token Ring MAU Look like Ethernet hubs/switches, but are NOT the same Passive device Acts like a ring, even if topology is a star Do not retransmit incoming traffic out other ports simultaneously Transmits to each connected device in roundrobin manner 9

10 Token Ring MAU Summary PC on port 5 transmits packet Hub receives packet and forwards a copy out each port MAU receives incoming packet from port 5 and transmits out port 6 MAU waits for packet to return through port 6 before transmitting packet out port 7 Introduction to transmission media Basic cabling Coaxial Twisted pair Optical fiber Basic wireless (NIC) Basic Physical Layer (layer 1) functionality 10