Part 3: The Medium Access Control Sublayer More Contents on the Engineering Side of Ethernet
4b Ethernet_Medium Access Control Sublayer
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The Medium Access control(MAC) Sublayer
4b : ETHERNET
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IEEE standards
802.3: Ethernet
802.11: Wireless LAN
802.15: Bluetooth 802.16: Wireless MAN
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Ethernet Cabling
Four TypesType Cable Max Seg. Nodes/seg Remark
10Base5: Thick coax 500m 100 Original cable/ obsolate
10Base2: Thin Coax 185 m 30 No hub Needed 10BaseT: Twisted Pair 100 m 1024 cheapest system
10BaseF: Fiber Optics 2000m 1024 Best between bldgs
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Ethernet Cabling
10Base5 (Thick Ethernet) 10Mbps Base: Baseband transmission For Coax medium its length is given in round number
to multiples of 100m Its max segment length = 500 m
Signal-regenerating repeaters Thick Coax
Advantages: Low attenuation, excellent noiseimmunity, superior mechanical strength
Disadvantages: Bulky, difficult to pull, transceiverboxes too expensive
* Wiring represented a significant part of total installedcost.
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MAU device is physically hooked on main cable.
50 meter AUI cable from MAU to station.
10Base5 (Thick Ethernet)
AUI: Attachment unit interface cable
MAU Connectors: Multistation Access Unit
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Ethernet Cabling
10Base2 (Thin Ethernet) Cheapernet 10 Mbps
185 meter segment length
Signal-regenerating repeaters
Transceiver was integrated onto the adapter
Thin Coax (coax thinner and lighter)
Advantages: Easier to install, reduced hardwarecost, BNC connectors widely deployed lower
installation costs. Disadvantages: Attenuation not as good, could
not support as many stations due to signalreflection caused by BNC Tee Connector.
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BNC connector : Bayonet Navy Connector.
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(a)
(b)
transceivers
Figure 6.55
Thick Ethernet Cable
Thin Ethernet Cable
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10BASET (Twisted pair Ethernet)
10 Mbps
100 meter segment length
Signal-regenerating repeaters Transceiver integrated onto adapter
Two pairs of UTP
Hub-and-spoke topology {Hub in the closet}
Advantages: could be done without pulling newwires. Each hub amplifies and restores incomingsignal.
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Hub Concept
Separate transmit and receive pair ofwires.
The repeater in the hub retransmitsthe signal received on any input paironto ALL output pairs.
Essentially the hub emulates a
broadcast channel with collisionsdetected by receiving nodes.
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Hub Concept
10Base T
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Single collision domain
Twisted Pair Ethernet
hub
Hub Concept
10BaseT
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Switched Ethernet
High-Speed Backplane or
Interconnection fabricswitch
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Switched Ethernet
* Basic idea: improve on the Hub concept
The switch learns destination locations byremembering the ports of the associated
source address in a table.The switch may not have to broadcast to all
output ports. It may be able to send theframe onlyto the destination port.
a big performance advantage over a hub,if more than one frame transfer can go
through the switch concurrently.
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Switched Ethernet
The advantage comes when the switchedEthernetbackplane is able to repeat morethan one frame in parallel (a separatebackplane bus line for each node). The frame is relayed onto the required output
port via the ports own backplane bus line.
Under this scheme collisions are stillpossible when two concurrently arriving
frames are destined for the same station.Note each parallel transmission can
take place at 10Mbps!!
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Figure 4-20.A simple example of switchedEthernet.
Switched Ethernet
Note: Tanenbaums discussionconsiders a more powerful switch
that reduces collisions evenfurther!!
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Switched Ethernet Hub
Since servers are often shared bymultiple nodes, one can employ aswitching hubwith a port whichoperates at a higher rate than theother ports.
Extra buffering inside hub to handle
speed mismatches. Can be further enhancedby higher
rated port full-duplex.
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Ethernet
Switch
Ethernet
Switch
Server
100 Mbps links
10 Mbps links
Figure 6.57
Fast Ethernet
Switch
Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks
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Features of Ethernet
1-persistent, CSMA-CD
Binary Exponential Backoff.
Manchester encoding.
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Features of Ethernet
Operational Description Ethernet stations sense the channel. When the channel is free the station
transmits a frame. Stations monitor the ether during the
transmission. If a collision is detected by any station, the
transmission is terminated immediately and
a jam signal is sent. Upon collision, stations backoff using a local
counter and then retransmit.
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A begins to
transmit at
t=0A B
B begins to
transmit att= tprop-
B detects
collision at
t= tprop
A B
A B
A detects
collision at
t= 2 tprop-It takes 2tpropto find out if channel has been captured
Figure 6.22
Collision Detection [worst case]
Leon-Garcia & Widjaja: Communication NetworksCopyright 2000 The McGraw Hill Companies
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frame contention frame
Figure 6.23
Frame seizes the channel after 2 tprop
On 1 km Ethernet, tpropis approximately 5microseconds.
Contention interval = 2 tprop Interframe gap =9.6 microsecondsModeled as slotted schemewith slot = 2 tprop
Ethernet
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Binary Exponental Backoff
Upon a collision, the sending stationsincrement a localcounter K. The backoff interval is randomly selectedusing a uniform distribution over the L = 2K slots.
K is initially set to 0.
Thus upon collision, the value of L is doubled locally for
each sending station.
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Binary Exponential Backoff (BEB)
Slotted ALOHA shown to be unstable when
p > 1/n
Since Ethernet permits up to 1024 stations, backoff
continues until K = 10, L = 2
10
, and p = 1/2
10
Normally K is incremented up to 10, but BEB is set for 16retries. After 16 retries, MAC gives up trying to send
frame.
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Preamble SDDestination
Address
Source
AddressLength Information Pad chksum
7 1 2 or 6 2 or 6 2 4
Total size of Ethernet frame : 64 to 1518 bytesSync Start
frame
0 Single address
1 Group address
Destination address is either single address
or group address (broadcast = 111...111)
Group address : Multicast
Addresses are defined on local or universal
basis
246 possible global addresses
0 Local address
1 Global address
802.3 MAC Frame
Figure 6.52Leon-Garcia & Widjaja: Communication Networks
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Preamble SD
Destination
Address
Source
Address Type Information Pad chksum
7 1 2 or 6 2 or 6 2 4
64 to 1518 bytesSync Start
frame
Ethernet Frame
Figure 6.53Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks
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FastEthernetandGigabit Ethernet
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Fast Ethernet (100BASE-T)
How to achieve 100 Mbps capacity?
Media Independent Interface provides threechoices.
LLC
MACConvergence Sublayer
Media Dependent Sublayer
Media Independent Interface
Data Link
Layer
Physical
LayerMII
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Fast Ethernet [IEEE 802.3u]
Three Choices
Figure 4-21.The original fast Ethernet
cabling.*Concept facilitated by 10Mbps/100Mbps Adapter Cards
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100 BASE T
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FastEthernet Details
UTP Cable has a 30 MHz limit
Not feasible to use clock encoding (i.e.,NO Manchester encoding)
Instead use bit encoding schemes withsufficient transitions for receiver tomaintain clock synchronization.
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100 BASE T4
Can use four separate twisted pairs of Cat3 UTP
Utilize three pair in both directions (at 331/3 Mbps) with other pair for carriersense/collision detection.
Three-level ternary code is used 8B/6T.
Prior to transmission each set of 8 bits isconverted into 6 ternary symbols.
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100 BASE T4
The signaling rate becomes
100 x 6/8
------------ = 25 MHz
3
Three signal levels : +V, 0, -V
Codewords are selected such that line is
d.c.balancedAll codewords have a combined weight of 0
or 1.
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100 BASE T4
36 = 729 possible codewords.
Only 256 codewords are requires, hencethey are selected:
To achieve d.c. balance Assuming all codewords have at least two signal
transitions within them (for receiver clocksynchronization).
To solve d.c. wander, whenever a string of
codewords with +1 are sent, alternatecodewords (inverted before transmission)are used.
To reduce latency, ternary symbols are
sent staggered on the three lines.
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100 BASE T4
Ethernet Interframe gap of 9.6microseconds becomes 960
nanoseconds in Fast Ethernet.100 m. max distance to hub; 200
meters between stations.
Maximum of two Class II repeaters.
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100 Base TX
Uses two pair of twisted pair, onepair for transmission and one pair forreception.
Uses either STP or Cat 5 UTP.Uses MTL-3 signaling scheme that
involves three voltages.
Uses 4B/5B encoding.
There is a guaranteed signaltransition at least every two bits.
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100 BASE FX
Uses two optical fibers, one fortransmission and one for reception.
Uses FDDI technology of converting4B/5B to NRZI code group streamsinto optical signals.
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Fast Ethernet Repeaters andSwitches
Class I Repeater supports unlike physicalmedia segments (only one per collisiondomain)
Class II Repeater limited to single physicalmedia type (there may be two repeaters percollision domain)
Switches to improve performance can addfull-duplexand have autonegotiation forspeed mismatches.
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Collision Domains
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Gigabit Ethernet History
In February 1997 the Gigabit Ethernet Allianceannounced that IEEE802.3z Task Force met toreview the first draft of the Gigabit Ethernet
StandardAccording to IDC by the end of 1997 85% of allnetwork connections used Ethernet.
Higher capacity Ethernet was appealing becausenetwork managers can leverage theirinvestment in staff skills and training.
1000 BASE X (IEEE802.3z)was ratified in June1998.
G b h ( 000 S
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Gigabit Ethernet (1000 BASEX)
Provides speeds of 1000 Mbps (i.e., one billionbits per second capacity) for half-duplex andfull-duplex operation.
Uses Ethernet frame format and MAC technology
CSMA/CD access method with support for onerepeater per collision domain.
Backward compatible with 10 BASE-T and 100 BASE-T.
Uses 802.3 full-duplex Ethernet technology.Uses 802.3x flow control.
All Gigabit Ethernet configurations arepoint-to-point!
Gigabit Ethernet Architecture Standard
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Gigabit Media Independent Interface (GMII)
(optional)
Media Access Control (MAC)
full duplex and/or half duplex
1000 Base T
PMA
transceiver
1000 Base X PHY
8B/10B auto-negotiation 1000 Base TPCS
Unshielded twisted pair
IEEE 802.3ab
1000 Base-LXFiber optic
transceiver
1000 Base-SXFiber optic
transceiver
1000 Base-CXCopper
transceiver
MultimodeFiber
ShieledCopper Cable
Single Mode orMultimode Fiber
IEEE 802.3z
g
Source - IEEE
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Gigabit Ethernet Technology
Figure 4-23.Gigabit Ethernetcabling.1000 BASE SX fiber - short wavelength
1000 BASE LX fiber - long wavelength
1000 BASE CXcopper - shielded twisted pair
1000 BASE T copper - unshielded twisted pair
* Based on Fiber Channel physical signaling technology.
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Gigabit Ethernet (1000 BASE-T)
LLC
MAC
Medium
Physical Layer
Data Link
Layer
GMII Gigabit Media Independent Interface
Media Dependent Interface
Gigabit Media Independent
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Gigabit Media IndependentInterface
GMIIAllows any physical layer to be used
with a given MAC.
Namely, Fiber Channel physical layercan be used with CSMA/CD.
Permits both full-duplex and half-duplex.
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1000 BASE SXShort wavelength
Supports duplex links up to 275meters.
770-860 nm range; 850 nm laserwavelength
(FC) Fiber Channeltechnology
PCS (Physical Code Sublayer)includes8B/10B encoding with 1.25 Gbpsline.
Only multimode fiber
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8B/10B Encoder
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8B/10B Encoding Issues
When the encoder has a choice for codewords, it alwayschooses the codeword that moves in the direction of balancingthe number of 0s and1s. This keeps the DC component of thesignal as low as possible.
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1000 BASE LXLong wavelength
Supports duplex links up to 550meters.
1270-1355 nm range; 1300 nmwavelength using lasers.
Fiber Channel technology
PCS (Physical Code Sublayer)includes8B/10B encoding with 1.25 Gbpsline.
Either single mode or multimode fiber.
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1000 BASE CXShort haul copper jumpers
Shielded twisted pair.
25 meters or less typically withinwiring closet.
PCS (Physical Code Sublayer)includes8B/10B encoding with 1.25 Gbpsline.
Each link is composed of a separateshielded twisted pair running in eachdirection.
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1000 BASE TTwisted Pair
Four pairs of Category 5 UTP.
IEEE 802.3ab ratified in June 1999.Category 5, 6 and 7 copper up to 100
meters.
This requires extensive signalprocessing.
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Gigabit Ethernet compared toFiber Channel
Since Fiber Channel (FC) alreadyexisted, the idea was to immediatelyleverage physical layer ofFC into
Gigabit Ethernet.
The difference is that fiber channelwas viewed as specializedfor high-
speed I/O lines. Gigabit Ethernetisgeneral purpose and can be used as ahigh-capacity switch.
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Gigabit Ethernet
Viewed as LAN solution while ATM is WANsolution.
Gigabit Ethernet can be shared (hub) or
switched.Shared Hub
Half duplex: CSMA/CD with MAC changes:
Carrier Extension
Frame BurstingSwitch
Full duplex: Buffered repeater called{BufferedDistributor}
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Gigabit Ethernet
Figure 4-22. (a) A two-station Ethernet. (b) A multistationEthernet.
Carrier Extension
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Carrier Extension
Based on Raj Jains slide
RRRRRRRRRRRRRFrame
Carrier Extension
512 bytes
For10BaseT : 2.5 km max; slot time = 64 bytes
For1000BaseT: 200 m max;slot time = 512 bytes Carrier Extension :: continue transmitting control
characters [R] to fill collision interval.
This permits minimum 64-byte frame to be handled.
Control characters discarded at destination.
For small frames net throughput is only slightly betterthan Fast Ethernet.
F B i
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Frame Bursting
Based on Raj Jains slide
512 bytes
ExtensionFrame Frame Frame Frame
Frame burst
Source sends out burst of frames without relinquishingcontrol of the network.
Uses Ethernet Interframe gap filled with extension bits
(96 bits) Maximum frame burst is 8192 bytes
Three times more throughput for small frames.
Buffered Distributor
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Buffered Distributor
A buffered distributor is a new type of 802.3
hub where incoming frames are buffered inFIFOs.
CSMA/CD arbitration is inside the distributorto transfer frames from an incoming FIFO to
all outgoing FIFOs.802.3x frame-based flow control is used to
handle congestion.
All links are full-duplex.
Hub