Physical Layer: Topology, Media, Standards. CompSci 275, Intro. to Networks following chapter 3 of Meyers

Transcription

1 Physical Layer: Topology, Media, Standards CompSci 275, Intro. to Networks following chapter 3 of Meyers

2 Topology Topology is the pattern in which network nodes are connected to each other - Physical topology refers to the actual cabling - Logical topology refers to how the cables are used» also called signaling topology Basic multiple-node topologies: - Bus - Ring - Star

3 Bus Topology Single medium, shared by all nodes - Wireless everybody on the same channel - True shared cable» Nodes electrically connected to the cabling Transmissions flood the medium, so all nodes see them Nodes must take turns transmitting May include repeaters to maintain signal strength over longer distances - A hub is a multi-port repeater

4 Bus Topology a 1990 s example All these nodes are electrically connected to the same cable The repeater / hub connects both wire runs and retransmits signals copied from m

5 Ring Topology IBM s preferred network topology Nodes connect one-to-another - No end nodes Nodes take turns transmitting frames Transmitted frames are passed from each node to the next, in one direction only - Each node touches the frame, in turn - Frame is done when it returns to sender

6 Star Topology Each node has a single connection to a central connecting point or hub Node-node communications are controlled by the central device A cable break only isolates one node; the rest of the network continues to operate - the central connection is still a weak point Star topology was originally more expensive and complicated to implement than bus or ring, so not widely used at first

7 Modern Topologies Newer networks have requirements beyond what a single bus, ring, or star segment can provide Alternatives: - hybrid - mesh - point-to-point - point-to-multipoint

8 Hybrid Topologies (why how it works isn t what it looks like) Logical or signaling topology operating over a different physical topology (usually a star) is called a hybrid - Physical star topology offers fault tolerance, easy reconfiguration - Logical ring and bus topologies are familiar and easy to design for - Physical bus or ring is shrunk to fit within hub, with long taps to individual nodes

9 10Base-T Physical Star, Logical Bus Nodes and hubs form a physical star - Hubs have a physical bus internally - Hubs can be ganged together Ethernet protocol determines the logical bus

10 Mesh Topology Nodes maintain multiple connections, allowing for more than one path between nodes Fully meshed topology: every node has connects to every other node - Best connectivity, throughput - Most complex and expensive Partially meshed topology: at least two nodes have redundant connections How many connections for 5 nodes in a fully meshed network? Answer: (5-1) + (5-2) + (5-3) + (5-4) = 5 * (5-1) / 2 = 10

11 Point-to-point Very simple topology Applies to both wired, wireless situations - Twisted-pair: the cross-over cable - Wireless: bluetooth - Logical topology for cell-phone networks

12 Point-to-Multipoint Central node transmits to all clients - Clients must filter out what isn't intended for them Clients transmit to central node Satellite-based telecommunications Wireless Access Points (WAPs) are logically like this

13 Topologies - Summary Physical vs. Logical topology the difference between wiring and usage Today's wired LANs usually physical star, logical bus Most WiFi networks physical bus, logical point-to-multipoint Internet a partial mesh Basic topologies - bus - ring - star - mesh - point-point - point-multipoint Hybrid - star - bus - star - ring - bus - point-tomultipoint

14 Electrical Cables, Optical Fibers Media

15 Media Physical/electrical/optical properties of cables affect the performance specifications of the networks that use them Cabling standards specify electrical/optical, and non-electrical properties Ethernet protocols specify many different kinds of cables

16 Coaxial cables Central conductor and return/shield conductor share a central axis (in cross section) Original Ethernet: 10Base5 cable 50 Ohms impedance RG-8/u, RG-11 are also suitable 9mm 16mm (~½") in diameter a.k.a. thicknet next gen : 10Base2 cable also 50 Ohms impedance 5mm diameter a.k.a. thinnet or cheapernet RG-58a/u is standard cable cable TV uses similar 75-Ohm RG-59 10Base5 10Base5 plenum

17 Twisted-Pair A pair of wires carry a signal in one direction at a time. Wires are twisted around each other to provide electrical shielding. Cables contain 4 twisted pairs, although some networks only use 2 pairs Pairs are color-coded and standardized.

18 UTP and STP UTP Unshielded Twisted Pair; four pairs are bundled in a plastic jacket as a cable Inexpensive; most commonly used STP Shielded Twisted Pair; pairs are wrapped in a conductive shield for additional electrical isolation, then bundled in the jacket Pricier, used where required

19 Bulk Twisted-Pair Cable Twisted-pair cable often sold in 1000-foot rolls - various colors available Suited for larger wiring installations

20 Twisted-Pair Categories Twisted-pair cables are rated into Categories with different specifications, including the number of twists per inch. - Category 1, Cat1, is only suitable for telephones Cat3 widely installed for 10Base-T Cat5/5e required for Fast Ethernet Cat6/6a required for 1+Gbps networks Cat7, Cat7a, Cat8 require shielding around each pair

21 Bandwidth of Twisted-Pair Types This diagram illustrates 3 properties of twisted-pair wires that affect their data-carrying capacity, and compares different categories of twisted-pair cables

22 Twisted-Pairs and RJ-45 Connectors Ethernet uses RJ-45 connectors for UTP and STP - Ethernet cables often called twisted-pair cables or RJ-45 cables Standard EIA / TIA 568 specifies the wiring patterns - EIA, TIA organizations collaborated to create the standard Cat7 requires shielded connectors

23 Optical Fibers Very small lightpipes - can carry a light beam over 70km (~40 miles) or more Very high data rates are possible, using pulses of light from LEDs or small lasers bits/second (Terabits/second) possible. Expensive generally used in long-distance, highperformance, and specialized situations - Overkill for a typical desktop connection.

24 Size of an Optical Fiber - from the Corning Museum of Glass

25 Fiber-Optic Cables: Structure and Operation A fiber consists of a fine glass core surrounded in cladding - Core diameters of 9-10µm (micrometers, or microns), 50µm, and 62.5µm are common - Cladding is 125µm in diameter Light travels through the core, reflecting at the core/cladding interface A buffer around the cladding provides protection Cables include more protection, Kevlar cords for pulling strength, and an outside jacket - Multiple fibers may be bundled together in a cable - Dual cables containing two fibers are common

26 Fiber-Optic Cables: Structure and Operation Kevlar cords for pulling strength, and an outside jacket - Single cables may contain multiple fibers - Dual cables containing two fibers are common Fine glass core with cladding - Cladding is 125µm in diameter Core diameters: - 50µm, and 62.5µm support multimode usage µm supports singlemode usage A buffer provides protection

27 What Limits Performance? LED Light pulses enter a multimode fiber at multiple angles, or modes Modes travel different distances, dispersing the pulse - This limits usable length and/or bandwidth

28 Multimode vs. Singlemode Singlemode fiber has a narrower core than multimode fiber A laser light source generates zero-order-mode pulses The lone reflection mode suffers much less dispersion, so can travel farther and support higher bandwidth Fiber crosssection Index of refraction profile Input waveform Mode propagation Output waveform

29 Fiber-Optic Uses Fiber-optic cables are specified for the same uses as twisted-pair and coax, as well as for long-distance and high-capacity uses - Many copper wires can be replaced with one fiberoptic cable in confined spaces Multimode fiber is cheaper, easier to work with, and has been around longer than singlemode - A lot of it is installed - Suitable for shorter distances Singlemode fiber is preferred for new installations

30 Optical Fiber versus Copper Cable This copper cable is about 6½ feet in diameter

31 Other ports, other cables Some computers have other ports, which can be used for network interfaces - USB, Firewire can be used to transmit frames directly Older computers and networks often used serial and parallel ports for simple networks - Connecting peripheral and remote I/O devices to the mainframe - Modem connections

32 Firewire, USB Standard RFC 2734 defines protocols for IPv4 over IEEE 1394 (Firewire) Most Internet standards are in the form of RFCs, Requests For Comments IEEE 1394 specifies the FireWire device connection scheme IP over FireWire is supported on Linux, Mac OS, and Windows 2000/XP Microsoft removed support for it in Vista Ethernet over USB is provided by various commercial products Linux supports it directly 3 rd -party Windows drivers are available

33 Serial and Parallel ports Standard on early personal computers Parallel ports were sometimes called printer ports or Centronics ports because the Centronics printer was connected over a parallel port. Some networks, and networking hardware, used these ports and associated cables to make connections. This IBM Thinkpad 380D included (from left to right) a monitor port, parallel port, serial port, the power-supply jack, and a PS/2 mouse port. Newer computers seldom include the old ports anymore.

34 Serial Ports Terminology RS-232 serial protocol connect dumb peripheral and smart device Peripheral: DCE, Data Communications Equipment Computer: DTE, Data Terminal Equipment

35 "Classic" Serial and Parallel Cables Serial RS-232: serial cables using DB-25 or DB-9 connectors - the larger DB-25 connector wasn't necessary - this cable has a female DB-9 and a male DB-25 Parallel IEEE-1284: parallel cables, also using DB-25 connectors - all pins used - this cable has two male DB-25 connectors; devices generally had female connectors

36 Capacity Comparisons Fill in this table: Is rated speed the same as real world performance?

37 Capacity Comparisons Fill in this table. Is rated speed the same as real world performance? What is the current record for optical fiber?

38 Fire Ratings Cables burnt in an electrical fire can produce smoke, and noxious or toxic fumes. Underwriters Laboratories (UL) and National Electrical Code (NEC) define ratings for wiring used in buildings. Many municipal building codes require that installed cabling be properly rated. These ratings apply to coax, twisted-pair, optical fiber, electrical wiring, etc. Three ratings: PVC (PolyVinyl Chloride) cables have no significant fire protection. Not permitted in building installations Typically found in patch cables and other non-permanent uses Riser cables are flame-retardant, and rated for vertical runs between floors of a building. Often replaced by plenum cable Plenum cables are rated for use in ceiling installations, etc. ( Plenum is the airspace between the acoustical ceiling tile and the actual ceiling.) Most fire protection, but 3 5 times more expensive than PVC Used for riser installation as well

39 Cabling Standards Organizations Various aspects of network cabling are defined by industry standards groups Underwriters Laboratories UL Electronic Industries Alliance EIA Telecommunications Industry Association TIA USB Implementers Forum USB-IF Institute for Electrical and Electronics Engineering IEEE I-triple-E offers many standards for computers and networks, including parallel-port cables, Firewire, floating-point numbers, and Ethernet

40 IEEE 802 Committee Develops standards for computer networks Datalink layer split into upper Logical-Link Control (LLC) and lower Media Access Control (MAC) sublayers Working groups specify standards, protocols for separate aspects of networking Some protocol development ongoing, other protocols finished or obsolete

41 Some IEEE 802 Working Groups [Inactive] - Defines Logical Link Control (LLC) sublayer 802.3» a services interface between Network-layer protocols and Media Access Control (MAC) protocols - CSMA/CD Access Method (Ethernet) - Defines Media Access Control (MAC) sublayer - Many subgroups specify different physical media Wireless Local Area Networks (WLAN) - Alternative MAC to Subgroups specify modulations, frequency bands, data rates, etc Wireless Personal Area Networks (WPAN), a.k.a. Bluetooth - (Future Bluetooth versions will not be IEEE standards)

42 Finally Good and Bad Wiring