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Transcript
Cisco Academy – Chapter 5 Physical Layer Physical Layer - 1 • defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems • Media • Devices • Topologies • Collision Domains Media • STP • UTP Shielded Twisted Pair Unshielded Twisted Pair » 10-100 Mbps, Inexpensive, 100 meters (333’), 4pairs • Coaxial Coaxial Cable » 10-100 Mbps, Inexpensive, 500 meters, Not used • Fiber Glass filament » >100 Mbps, Expensive, 2 km, single and multi-mode • Wireless Electromagnetic waves Standards • Sets of rules or procedures that are either widely used, or officially specified, and that serve as the gauge or model of excellence • ISO International Standards Organization • IEEE Institute of Electrical & Electronic • TIA/EIA Telecommunications or Electrical Industry Association • UL Underwriters Laboratory TIA/EIA Standards • • • • • • Horizontal cabling Telecommunications closets Backbone cabling Equipment rooms Work areas Entrance facilities LAN Technologies • Ethernet – 10-Base T • Patch Panels, Cables, Jacks, Wiring Closets, Plugs • Transceivers, repeaters, hubs • Token Ring • FDDI RJ-45 Jack and Connector • Registered standard • Reduces noise, reflection and stability problems • Layer 1 component • 8 conductor pins • Punch down separates wires & forces into slots (Layer 1 component) – Rack mounted, 12 or 24 or 48 ports • Wiring Standard 568A or 568B Cat 5 Cable • 4 Twisted Pairs (8 wires) – Reduces noise problems (crosstalk) • Inexpensive • Easy to install • Layer 1 component – Carries the bits Transceiver • Combines transmitter and receiver • Convert signals from one form to another – Converts AUI ports to RJ-45 ports • Layer 1 devices • Can be built into NICs – Called signaling components • Encode signals onto the physical medium Repeaters • • • • • • Retime and Regenerate Signals Deal with packets ONLY at bit level Layer 1 devices Extend length of network Extend collision domain Can’t filter traffic Hubs • • • • • • Multi-port repeaters Amplify and retime signal Contain many ports Layer 1 Device Extend the collision domain Provides a central collection point for the network Networks • Shared – Many hosts have access to same medium • Extended Shared – Extends environment for multiple access • Point to Point (2 units, 1-1 connection) – Shared, connected to only other other device • Indirectly connected – Circuit Switched Collisions • When two bits propagate (travel along the same media) at the same time • Bits’ are actually packets containing many bits • Result of too much network traffic (the more nodes the more work being done the more traffic) • Domain – area within network where data packets originated and collided • All of layer 1 connections are part of collision domain Signals and Collisions • Contention – managing competition for the system resources • Collision results in destruction of data packets – Collision magnifies the signal strength • A JAM Signal is sent to all nodes • Algorithm takes over to determine when retransmission can occur (Back off algorithm) • Segment is the collision domain Collision Domain and Devices • Layer 1 devices that extend the network result in larger collision domains – Includes repeaters and hubs • A network segment includes all wires, repeaters, and hubs as part of one collision domain • Too many collisions result in poor network performance Four Repeater Rule • • • • • • A.K.A. as 5-4-3-2-1 rule 5 network segments 4 repeaters 3 network segments with hosts 2 link segments 1 collision domain Segmentation • • • • At Layer 2 breaks up the collision domains Use Bridges and Switches (layer 2 devices) DECREASES the SIZE of the collision domains INCREASES the NUMBER of collision domains created • Both filter the traffic – Keep local traffic on same segment – Allow traffic on other segments to pass through • Segments are still part of the same broadcast domain Segmentation cont. • Try to design the LAN to keep 80% of the traffic local • All nodes on a segment before, the bridge/switch will see and evaluate the datagram • If the destination node is on the same segment the nodes processes the datagram • The bridge/switch will evaluate the datagram but drop it • Traffic on the LAN is isolated and reduced Latency • All devices add a latency component to the network • It takes time for packets to travel through devices • Bridges and switches have to take time to check MAC address to filter the frame • Router has to take time to check IP address to route and switch the packet Network Topology • • • • The study of location Two types - Physical and Logical Physical – describes the wiring schema Logical – describes how data flows through the network • Network can have different physical and logical topologies Network Types • 10-Base T Ethernet – Logical Topology is a bus – Physical Topology is a star or extended star • Token Ring – Logical Topology is a ring – Physical Topology is star • FDDI – Logical and Physical Ring Physical Topologies -1 • • • • • • Bus Star Extended Star Ring Hierarchical Mesh Physical Topologies - 2 • Bus – Logical – all devices can see all communications – Physical – each device is on the same wire • Ring – Logical –each station passes data to adjacent station – Physical –devices wired in a daisy chain Physical - 3 • Star – Physical – all nodes connected to center node – Logical – all data passes through center node • Extended Star – Physical – all center nodes from extensions connected to center node – Logical – hierarchical – information encouraged to stay local Physical - 4 • Tree – Logical – hierarchical – Physical – trunk has several layers or branches • Mesh – Logical – depends on exact wiring – Physical – all nodes connected directly to each other Physical - 5 • Cellular – Physical – geographic cells; electromagnetic waves – Logical – communicate within cell or to adjacent cells