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Fall 2005
LAN Technologies and Network Topology
Qutaibah MalluhiComputer Science and Engineering
DepartmentQatar University
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Network Classification Terminology
Network technologies classified into three broad categories– Local Area Network (LAN)
– Metropolitan Area Network (MAN)
– Wide Area Network (WAN)
LAN and WAN most widely deployed
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Point-to-Point Networks
Computers connected by communication channels that each connect exactly two computers
Point-to-point network – Allows flexibility in communication hardware, packet
formats, etc. – Provides security and privacy because communication
channel is not shared Number of wires grows as square
of number of computers For N hosts,
No. Connections = (N2-N)/2 Adding a new computer requires
N - 1 new connections
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Local Area Network Developed in the 60’s and 70’s Interconnect a wide range of devices over short
distances, e.g., within the same floor, building or campus (typically up to 10 km diameter).
Key idea - reduce number of connections by sharing connections among many computers – Computers take turns – TDM– Reduce cost but ... attached computers compete for
use of shared connection– Must include techniques for synchronizing use
In practice – Local communication almost exclusively LAN – Long distance almost exclusively point-to-point
» E.g., ATM
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LAN (Cont.)
Typically, all hosts on a LAN share a common medium.– operate on a broadcast mode
High throughput, low delay Many LAN technologies and standards exist
– E.g., Ethernet and FDDI are popular ones– LAN standards are collectively known as the IEEE 802
standards– Must include techniques for synchronizing hosts
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Significance of LAN and Locality
LANs are most popular types of networks WHY?– Economical– Principle of locality
Principle of locality of reference helps predict computer communication patterns: – Spatial (or physical) locality of reference
» computers likely to communicate with other computers that are located nearby
– Temporal locality of reference » computers are likely to communicate with the same
computers repeatedly
Thus - LANs are effective because of spatial locality of reference, and temporal locality of reference
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Different LAN Types
LAN parameters– topology– shared medium (twisted pair, coaxial, fiber)– medium access control technique
» governs the access to the LAN transmission medium.
Topology– Specifies general “shape” of a network– Handful of broad categories– Three most popular:
» Star » Ring » Bus
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Star Topology
Central component of network known as hub Hub: repeats incoming signal to all outgoing links Each computer has separate connection to hub
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Ring Topology
No central facility
Connections go directly from one computer to another
In practice, there is a short connector cable from the computer to the ring
Fault tolerance with two rings
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Bus Topology
Single cable connects all computers Each computer has connector to shared cable Computers must synchronize and allow only one
computer to transmit at a time
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Why Multiple Topologies?
Each has advantages and disadvantages: – Ring ease synchronization; may be disabled if any
cable is cut – Star easier to manage and more robust; requires more
cables – Bus requires fewer cables; may be disabled if cable is
cut Industry is settling on star topology as most
widely used
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Ethernet Widely used LAN technology
– Invented at Xerox PARC (Palo Alto Research Center) in 1970s – Defined in a standard by Xerox, Intel and Digital - DIX
standard – Standard now managed by IEEE - defines formats, voltages,
cable lengths, ... Uses bus topology
– Single coax cable - the ether – Multiple computers connect to the ether
One Ethernet cable is sometimes called a segment – Limited to 500 meters in length – Minimum separation between connections is 3 meters
Speed– Originally 3Mbps (Obsolete)– Popular standard is 10Mbps (Classic Ethernet)– Fast Ethernet operates at 100Mbps – Now Gigabit and 10 Gigabit
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Ethernet Operation
One station transmits at a time Signal propagates across entire cable All stations receive transmission
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Manchester Encoding
Ethernet standard uses Manchester encoding Uses rising and falling edges to encode data
– Edge triggered hardware Falling edge to encode 0, rising edge to encode 1 Use preamble for synchronization Preamble consists of 64 alternating 1’s and 0’s
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Characteristics of Good Encoding Scheme
Self synchronization– Digital signal includes information about bit boundaries– Transitions at the beginning, middle or end of the signal are
used– A signal with the same voltage level for a long period of time
is bad No DC Component
– DC Component: Useless extra energy residing on the line– Signal with only positive voltage (unipolar) has a DC
component– Signal with positive and negative voltages (polar) reduce the
DC component Bit rate versus pulse rate (baud rate)
The first two (Self-Synch and No DC Comp) are provided by Manchester encoding
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Manchester Encoding Efficiency
Classical Ethernet uses Manchester encoding– Up to two signal transitions per-bit– 1 Gbps Requires baud rate of 2 G– Waste of bandwidth
Newer Faster networks use a more efficient block encoding schemes (e.g. 4b/5b encoding for fast Ethernet and 8b/10b encoding for Gb Ethernet)
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Careful Selection of Valid Codes Consider 4b/5b coding Select a subset of the 5-bit
codes such that no more than three consecutive zeros are sent.
Use a coding scheme that does not waste bandwidth (unlike Manchester Encoding)
Advantages– Synchronization
» No same voltage level for long period of time
– Error detection» Errors may generate a non-
valid 5 bit code– Higher bit rate (than
Manchester)
1101011010110001010010100100
1101111011110101011010110101
1110011100111001110011100110
1110111101111101111011110111
1011110111101110101101010011
1010010100
0100101001
1111011110
Code
101101011010100010
100111001110010001
10010100101000 0000
CodeDataData
1101011010110001010010100100
1101111011110101011010110101
1110011100111001110011100110
1110111101111101111011110111
1011110111101110101101010011
1010010100
0100101001
1111011110
Code
101101011010100010
100111001110010001
10010100101000 0000
CodeDataData
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CSMA/CD
No central control managing when computers transmit on ether
Ethernet employs CSMA to coordinate transmission among multiple attached computers
Carrier Sense with Multiple Access – Multiple access
» multiple computers are attached to shared media» each uses same access algorithm
– Carrier sense » computer wanting to transmit tests the media for carrier
before transmitting
Simultaneous transmission possible Collision
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CSMA/CD (cont’d) Even with CSMA, two computers may transmit simultaneously
– Both check ether at same time, find it idle, and begin transmitting – Window for transmission depends on speed of propagation in ether
Signals from two computers will interfere with each other – Overlapping frames is called a collision – Data from both frames is garbled
Ethernet uses CSMA + Collision Detection (CD) to coordinate transmission– Ethernet interfaces include hardware to detect transmission
» Monitor outgoing signal » Garbled signal is interpreted as a collision
– Listen to medium during transmission– Detect whether another station’s signal interferes– Back off from interference (wait random amount of time) and try
again
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Recovery from Collision
Computer that detects a collision sends special signal to force all other interfaces to detect collision
Computer then waits for ether to be idle before transmitting – If both computers wait same length of time, frames will
collide again – Standard specifies maximum delay, and both
computers choose random delay less than maximum After waiting, computers use carrier sense to
avoid subsequent collision
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Exponential Backoff
Even with random delays, collisions may occur Especially likely with busy segments Computers double delay with each subsequent
collision Reduces likelihood of sequence of collisions Also called binary backoff
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Wireless LAN
Use radio signals at 900 MHz Data rate of 2 Mbps Shared medium - radio instead of coax In contrast with wired LAN, not all participants
may be able to reach each other – Low signal strength – Propagation blocked by walls, etc.
Can't depend on CD; not all participants may hear
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CSMA/CA
CSMA: Sense before sending. Only send if idle for IFS (Inter-Frame Space). O/W backoff.
Wireless uses collision avoidance rather than collision detection – Transmitting computer sends very short message to receiver – Receiver responds with short message reserving slot for
transmitter Response from receiver is broadcast so all potential
transmitters receive reservation
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Handling Collisions
Receiver may receive simultaneous requests – Results in collision at receiver – Both requests are lost – Neither transmitter receives reservation; both use
backoff and retry Receiver may receive closely spaced requests
– Selects one – Selected transmitter sends message – Transmitter not selected uses backoff and retries
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Token Ring
Many LAN technologies that use ring topology use token passing for synchronized access to the ring
Ring itself is treated as a single, shared communication medium
Bits pass from transmitter, past other computers and are copied by destination
Hardware must be designed to pass token even if attached computer is powered down
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Token Passing Synchronization
Used with ring topology Guarantees fair access: IEEE 802.5 standards Token: Special small (a few bits) reserved (can
not appear in data) message Only computer holding the token can transmit
– Because there is only one token, only one computer will transmit at a time
– Hardware must regenerate token if lost Token gives computer permission to send one
frame – If all ready to transmit, enforces ``round-robin'' access – If none ready to transmit, token circulates around ring
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Token Ring Transmission
Station waits for token before sending Signal travels around entire ring Sender receives its own transmission
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Token Release Mechanisms
Release After Reception (RAR): – Each station reissues the free token only after
it receives the transmitted frame.– Used on lower speed token rings ( <=
4Mbps). Release After Transmission (RAT):
– Each station attaches a free token at the end of its frame
– Possible multiple frames propagate in a ring– Used on higher speed token rings (>=
16Mbps)
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Token Passing Ring Technologies
IBM token ring– Very widely used – Originally 4mbps, now 16Mbps
Fiber Distributed Data Interface (FDDI)– Operates at 100 Mbps
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FDDI
Fiber Distributed Data Interconnect (FDDI) – Uses ring token passing synchronization (RAT token release)– Uses fiber as transmission media – Transmits data at 100Mbps – Also suitable for MAN– Can attach 1000 stations, can be up to 200 km– Uses pairs of fibers to form two concentric rings
FDDI uses counter-rotating rings in which data flows in opposite directions ==> Reliability
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In case of fiber or station failure, remaining stations loop back and reroute data through the spare ring
All stations automatically configure loop back by monitoring the data ring
FDDI Self-healing
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ATM
Asynchronous Transfer Mode technology consists of electronic packet switches to which computers can connect
ATM switches form hub into which computers connect in a star topology
Computers get point-to-point connections - data from transmitter is routed directly through hub switches to destination
Transmits data at over 100Mbps Uses fiber optics to connect computer to switch Each connection includes two fibers
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IEEE 802.x LAN Standards
10 Mbps CSMA/CD (802.3) 100 Mbps CSMA/CD (802.3u) 1000 Mbps CSMA/CD (802.3z) Token Bus (802.4) Token Ring (802.5) Wireless LAN (IEEE 802.11) And many more……
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Summary Local Area Networks
– Designed for short distance– Use shared media
» Transmitting computer has exclusive use of communication medium
» computers must synchronize transmission– Many technologies exist
» Ethernet, Wireless, IBM Token Ring, FDDI, ATM Topology refers to general shape
– Bus– Ring– Star
Ethernet– CSMA/CD – Manchester encoding– Exponential Binary backoff
Synchronization by token passing in a ring– IBM token ring and FDDI