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Physical Layer: Cabling

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Physical Layer: Cabling 2017-02-09 Physical Layer: Topology, Media, Standards CompSci 275, Intro. to Networks following chapter 3 of Meyers 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 1 2017-02-09 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” 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 www.rff.com/Bus_Topology.htm 2 2017-02-09 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 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 3 2017-02-09 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 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 4 2017-02-09 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 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 Answer: for 5 nodes in a (5-1) + (5-2) + (5-3) + (5-4) fully meshed network? = 5 * (5-1) / 2 = 10 5 2017-02-09 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 Point-to-Multipoint Looks like a star, but the central switching point is more intelligent Acts as a controller Logically, Wireless Access Points (WAPs) do this 6 2017-02-09 Topologies - Summary Physical vs. Logical Basic topologies topology – the bus difference between ring wiring and usage star mesh Today's LANs – usually physical star, logical bus point-point point-multipoint Internet – a partial mesh Hybrid star-bus star-ring Electrical Cables, Optical Fibers Media 7 2017-02-09 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 Coaxial cables Central conductor and return/shield conductor share a central axis (in cross section) Original Ethernet: 10Base5 10Base5 cable plenum 50 Ohms impedance 10Base5 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 8 2017-02-09 Twisted-Pair A pair of wires carry a signal in one direction. Wires are twisted around each other to provide electrical shielding for each other. Cables contain 4 twisted pairs, although some networks only use 2 pairs Pairs are color-coded and standardized Twisted-Pair 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 Cat5, Cat5e, or Cat6 are required for networks of 100 Mbps, Cat7, Cat7a, Cat8 require shielding 1 Gbps, 2.5 Gbps, around each pair 5Gbps Cat7/7a not recognized for 802.3 standards? 9 2017-02-09 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 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 10 2017-02-09 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 Optical Fibers Very small “lightpipes” can carry a light beam for long distances – 70km (~40 miles) or more Very high data rates are possible, using pulses of light from LEDs or small lasers. 1012 bits/second (Terabits/second) possible. Expensive – generally used in long-distance, high-performance, and specialized situations “Overkill” for a typical desktop connection. 11 2017-02-09 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 Cables include more protection, core, reflecting at the Kevlar cords for pulling strength, core/cladding interface and an outside jacket A buffer around the Multiple fibers may be bundled cladding provides together in a cable protection Dual cables containing two fibers are common 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 12 2017-02-09 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 Index of cross- refraction Input Output Mode propagation section profile waveform waveform 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 fiber-optic 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 13 2017-02-09 Optical Fiber versus Copper Cable This copper cable is about 6½ feet in diameter 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 14 2017-02-09 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 ‣ 3rd-party Windows drivers are available 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. 15 2017-02-09 Serial Ports – Terminology ✦ RS-232 serial protocol – connect “dumb” peripheral and “smart” device ✦ Peripheral: DCE, “Data Communications Equipment” ✦ Computer: DTE, “Data Terminal Equipment” "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 16 2017-02-09 Capacity Comparisons Fill in this table: Is rated speed the same as “real world performance”? Capacity Comparisons Fill in this table. Is rated speed the same as “real world performance”? What is the current record for optical fiber? 17 2017-02-09 Fire Ratings ✦ Cables burnt in an electrical fire can produce smoke, and noxious or toxic fumes. ✦ Underwriters Laboratories (UL) and National Electrical Code ✦ Three ratings: (NEC) define ratings for wiring ‣ PVC (PolyVinyl Chloride) cables used in buildings.
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