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
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
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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 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
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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 optical fiber?
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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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