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Datornätverk A – lektion 10
Kapitel 13: Multiple access control. Local Are Networks. (CSMA/CD,Token Bus, Token Ring, Logical Link Control) Kapitel 14: Ethernet (Kapitel 15: Wireless LANs översiktligt.)
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Chapter 13 Multiple Access
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Figure 13.1 Multiple-access protocols
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Figure 13.2 Evolution of random-access methods
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Evolution of Contention Protocols
Aloha Developed in 1970 to be used on radio LAN on Hawaiian islands. The access to the channel is random Improvement to Aloha: Start transmission only at fixed time slots Carrier Sense Multiple Access: Start transmission only if no transmission is ongoing CD=Collision Detection: Stop ongoing transmission if collision is detected Slotted Aloha CSMA CSMA/CD
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Figure 13.5 Collision in CSMA
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Animeringar Animeringar som illustrerar tystnadsdetektering i CSMA:
Animering som illustrerar kollisionshantering i CSMA/CD:
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Figure 13.6 Persistence strategies
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CSMA/CD procedure
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Figure 13.8 CSMA/CA procedure
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CSMA/CD Sense for carrier.
If carrier present, wait until carrier ends. Send packet and sense for collision. If no collision detected, consider packet delivered. Otherwise, abort immediately, perform “exponential back off” and send packet again. CSMA/CD is used in traditional Ethernet LAN
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Exponential Back-off When a sender detects a collision, it sends a “jam signal”. Jam signal is necessary to make sure that all nodes are aware of the collision Length of the jam signal 48 bits When collision is detected, the sender resends the signal after a random time The random time is picked from an interval of 0 to 2N x maximum propagation time N is the number of attempted retransmission Length of the interval increases with every retransmission
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Channelization FDMA TDMA CDMA
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In FDMA, the bandwidth is divided into channels.
Note: In FDMA, the bandwidth is divided into channels.
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In TDMA, the bandwidth is just one channel that is timeshared.
Note: In TDMA, the bandwidth is just one channel that is timeshared.
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In CDMA, one channel carries all transmissions simultaneously.
Note: In CDMA, one channel carries all transmissions simultaneously.
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Figure Chip sequences
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Figure Encoding rules
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Figure 13.16 CDMA multiplexer
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Figure 13.17 CDMA demultiplexer
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Local Area Networks: Ethernet
Chapter 14 Local Area Networks: Ethernet
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Local Area Networks (LANs)
A computer network in a limited geographical area, a single building or several close to each other buildings LANs are privately owned and built by the companies Generally less expensive than WAN for comparable speed LAN technologies use multiple access channels Ethernet is the most common LAN technology
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Figure 14.1 Three generations of Ethernet
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Traditional Ethernet Work started back in 1973 by Bob Metcalfe and David Boggs from Xerox Palo Alto Research Center, as an improvement of the ALOHA Experimental Ethernet implemented in 1975. Cooperative effort between Digital, Intel, and Xerox produced Ethernet Version 1.0 in 1980. Ethernet was adopted with modifications by the standards committees IEEE and ANSI 8802/3. Structure of Ethernet frame (Length)
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Structure of Ethernet Frame
Preamble: 7 bytes with pattern followed by one byte with pattern Used to synchronize receiver, sender clock rates Addresses: 6 bytes, the frame is received by all adapters on a LAN and dropped if address does not match Type: 2 bytes, is actually a length field in 802.3 CRC: 4 bytes, checked at receiver, if error is detected, the frame is simply dropped Data payload: maximum 1500 bytes, minimum 46 bytes. If data is less than 46 bytes, pad with zeros to 46 bytes
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Figure MAC frame
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Figure 14.3 Minimum and maximum length
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Network Interface Card (NIC)
NIC for a desktop Each device on Ethernet network has its own interface card (NIC) to connect to the network The NIC is usually plugged into the device and has a 6 bytes (48 bits) physical address The physical address is normally written in hexadecimal notation C-4D-1B (example address) NIC for a laptop
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Ethernet Addressing Each station recognizes three classes of addresses. Own address Broadcast address (all 1's) Optionally, one or more multicast addresses Major reason for broadcast is address discovery. Brodcast Ethernet address is all 1s, or in hexadecimal FF : FF : FF : FF : FF :FF Multicast addresses are used for specialized link layer functions. Ethernet addresses are unique First three bytes assigned to manufacturer by IEEE, the other three bytes assigned by the manufacturer
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Figure 14.5 Unicast and multicast addresses
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Physical Layer of the Ethernet
PLS (Physical Layer Signaling) encodes and decodes data Ethernet uses Manchester encoding AUI (Attachment Unit Interface) – interface between PLS and medium dependent interface MAU (Medium Attachment Unit) or transceiver MDI (Medium Dependent Interface) is a piece of hardware connecting the transceiver to the medium
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Figure Physical layer
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Figure PLS
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Figure AUI
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Figure 14.9 MAU (transceiver)
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Figure 14.10 Categories of traditional Ethernet
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Classic 10Mbps Ethernet Four different implementation at the physical layer for the baseband 10Mbps Ethernet Thick Ethernet (10base5) – obsolete Thick coaxial cable (0.5” diameter) 500meter max length, bus physical topology Thin Ethernet (10base a) - obsolete RG58 coaxial cable 185 meter max length, bus physical topology Twisted Pair Ethernet (10baseT i) 4 pair UTP (unshielded twisted pair) cable 100 meter max length, star physical topology Fiber-link Ethernet (10Base-FL) Fiber cable connected to external transceiver Star topology is used
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Figure 14.11 Connection of a station to the medium using 10Base5
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Figure 14.12 Connection of stations to the medium using 10Base2
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Reflektioner Animering:
Se
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Figure 14.13 Connection of stations to the medium using 10Base-T
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Figure 14.14 Connection of stations to the medium using 10Base-FL
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Hub Concept Separate transmit and receive pair of wires.
The hub retransmits the signal received on any input pair onto all output pairs. Essentially the hub emulates a broadcast channel with collisions detected by receiving nodes.
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Figure 14.15 Sharing bandwidth
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Figure 14.16 A network with and without a bridge
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Figure 14.17 Collision domains in a nonbridged and bridged network
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Ethernet Evolution Introducing bridges Introducing switches
Unlike a hub, a bridge is capable of filtering frames Each port of the bridge is connected to a single segment of LAN Capable of learning which the stations are connected to which ports Separates collision domains and therefore increases bandwidth Introducing switches Similar function as bridges Contain bigger number of ports A single device can be attached to a port
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Figure 14.18 Switched Ethernet
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Figure 14.19 Full-duplex switched Ethernet
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Bridged vs. Switched Ethernet
A B C D E F Switch
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Fast Ethernet Go from 10mbit/s to 100mbit/s 3 competing standards:
100Base-TX 100Base-T4 100VG-Anylan 100Base-T4 and 100VG-Anylan are the losers (were not very well accepted). 100Base TX is the winner. It is almost a standard everywhere.
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100Base - TX 100 Mbps over 2 pairs of wire (just like 10base-T)
Requires Category 5 UTP wiring or STP De facto standard today Very small price difference with 10Mbps-only equipment Has clearly won over 100baseT4 and 100VG-Anylan by now
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Figure 14.22 Fast Ethernet implementations
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100Base-FX Fast Ethernet with fiber optic cables
Uses two optical fibers, one for transmission and one for reception
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Gigabit Ethernet Provides speeds of 1000 Mbps (i.e., one billion bits per second capacity) for half-duplex and full-duplex operation. Uses Ethernet frame format and MAC technology CSMA/CD access method Backward compatible with 10Base-T,100Base-T and 100BaseTX Can be shared (hub) or switched
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Figure 14.29 Physical layer in Gigabit Ethernet
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Gigabit Ethernet Implementations
Fiber 1000 Base – SX Short wavelengths, two fiber-optic cables 1000 Base – LX Long wavelengths, two fiber-optic cables Copper 1000 Base – CX Uses shielded twisted pair copper jumpers 1000 Base – TX Uses category 5 twisted pair copper cable
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Figure 14.30 Gigabit Ethernet implementations
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1000Base - T Four pairs of Category 5 UTP
IEEE 802.3ab ratified in June 1999. Category 5, 6 and 7 copper up to 100 meters Uses encoding scheme 4D-PAM5 Five level of pulse amplitude modulation are used Complicated technique
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