chapter 13 and 14 – multiple access, local area networks
TRANSCRIPT
Chapter 13 and 14 – Multiple Access, Local Area Networks
Spring 2006 Computer Networks 2
A Link and Types of Links
A link is a communication channel established through some kind of physical medium (guided or unguided)
Type of links with concern to line configuration Point-to-point link
Consists of a single sender on one end and a single receiver on the other end.
Broadcast link - Multipoint Multiple sending and receiving nodes all connected to
the same shared broadcast channel When one node transmits, the channel broadcast the
frame and each of the other nodes receives a copy
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Multiple Access Protocols and LANs
In case of a broadcast link, there must be rules for sharing the channel. Otherwise no-one can “hear” no-one.
Media access protocols are protocols that resolve the multiple access problem
Broadcast links and media access protocols are used in computer networks that are local (span a single or several buildings)
LAN (Local Area Network)
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Examples for Multiple Access Channels
Sharing a room(Conversation of many persons)
Sharing the wire
Sharing satellite channel
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Media Access Control Protocols
Three types of sharing a single channel Channel partitioning or using some kind of
multiplexing techniques Multiple access by allowing contention
Collisions are allowed Recover from collision Ethernet is an example
Taking turns in accessing the channel The access is controlled by a circulating token Token ring is an example
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Evolution of Contention Protocols
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
Aloha
SlottedAloha
CSMA
CSMA/CD
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CSMA Operation
Carrier sensing the ability to detect idle/busy channel quickly
The logic followed: There is no point of transmitting if the channel is busy (another station is transmitting)
The chances for collisions are reduced The possibility of collision still exists
because of the propagation time The signal sent by a single device is sent
immediately by the closest devices. For the others to sense it, some time is needed
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Persistent vs. Non-persistent CSMA
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Normal Operation and Collisions
A
B
D
Data
C
Address mismatchpacket discarded
Address mismatchpacket discarded
Address matchpacket processed
Send datato node D
Transmitted packet seenby all stations on the LAN
(broadcast medium)The packet carries the
address of the destination
A
B C
D
Collision
Data transmission for A Data transmission for C
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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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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
Spring 2006 Computer Networks 13
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 802.3 and ANSI 8802/3.
Structure of Ethernet frame
(Length)
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Structure of Ethernet Frame Preamble:
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
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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Annimation for Better Understanding
The following link will lead you to several annimations that explain important issues in the area of networking.
Play annimation 6.1 and 6.2 to understand how Ethernet works.
http://www.netbook.cs.purdue.edu/othrpags/page15.htm
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Network Interface Card (NIC)
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
02-11-02-2C-4D-1B (example address)
NIC for a desktop
NIC for a laptop
Spring 2006 Computer Networks 17
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
Spring 2006 Computer Networks 18
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 (10base2 802.3a) - obsolete RG58 coaxial cable 185 meter max length, bus physical topology
Twisted Pair Ethernet (10baseT 802.3i) 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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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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10Base5 (Thick Ethernet)
transceiver
Resistor terminator
Resistor terminator Ethernet medium (cable)
Drop cable
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10Base2 (Thin Ethernet)
Terminator
BNC T connector
BNC connector
To next equipment or terminator
Thin coaxial cable
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10BaseT (Twisted Pair Ethernet)
High-Speed Backplane or Interconnection fabric
hubswitch
The central device can be a hub or a switch
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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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Ethernet Evolution
Introducing bridges 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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Bridged vs. Switched Ethernet
Bridge
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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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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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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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