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

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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

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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

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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

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10Base-T – Physical Star, Logical Bus

 Nodes and hubs form a physical star

 Hubs have a physical bus internally

 Hubs can be “ganged” together

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

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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

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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

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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

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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?

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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 ; 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

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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.

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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

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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

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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

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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.

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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

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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 ?

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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. have no significant fire protection. ‣ Many municipal building codes • Not permitted in building require that installed cabling be installations properly rated. • Typically found in “patch” cables and other non-permanent uses ✦ These ratings apply to coax, twisted-pair, optical fiber, ‣ “Riser” cables are flame-retardant, electrical wiring, etc. 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

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

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IEEE 802 Committee

 Develops standards for computer networks

 Working groups specify standards, protocols for separate aspects of networking

 Datalink layer split into upper Logical-Link Control (LLC) and lower Media Access Control (MAC) sublayers

 MAC sublayer specifies NIC hardware addresses, hence "MAC addresses"

 MAC overlaps OSI Physical layer

 802.2 specifies a common LLC interface for layer-3 protocols

 802.3, 802.4, 802.5, 802.11 all specify MAC protocols for various physical media

Some IEEE 802 Working Groups

 802.2 [Inactive]  802.11

 Defines Logical Link  Wireless Local Area Control (LLC) sublayer Networks (WLAN) . a services interface  Alternative MAC to 802.3 between Network-layer protocols and Media  Subgroups specify Access Control (MAC) modulations, frequency protocols bands, data rates, etc.

 802.3  802.15

 CSMA/CD Access Method  Wireless Personal Area (Ethernet) Networks (WPAN), a.k.a.  Defines Media Access Bluetooth Control (MAC) sublayer  (Future Bluetooth versions  Many subgroups specify will not be IEEE standards) different physical media

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Finally – Good and Bad Wiring

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