Autumn 2000John Kristoff1 Computer Networks Other Network Protocols.
Spring 2000John Kristoff1 Data-link Layer Computer Networks.
-
date post
21-Dec-2015 -
Category
Documents
-
view
215 -
download
1
Transcript of Spring 2000John Kristoff1 Data-link Layer Computer Networks.
Spring 2000 John Kristoff 1
Data-link Layer
Computer Networks
Spring 2000 John Kristoff 2
Where are we?
Spring 2000 John Kristoff 3
The Data Link Interface
Spring 2000 John Kristoff 4
The Local Area Network
Popular (most data links are LANs)High ThroughputLow CostShort DistancesOften shared medium access
Spring 2000 John Kristoff 5
Shared Medium Access
A Shared Medium Used by AllOnly One Station Transmits at a
TimeStations “Take Turns”MAC Protocol defines fairness policy
Spring 2000 John Kristoff 6
Topology Review
Spring 2000 John Kristoff 7
Data Link Bit Encoding
Spring 2000 John Kristoff 8
Example Bus: Ethernet
Most Popular LANIEEE Standardized as 802.3Several Generations
Same frame format (mostly) Changing data rates Different physical layer requirements
The book: Gigabit Ethernet, Rich Seifert
Spring 2000 John Kristoff 9
Ethernet Transmission
Only one station transmits at a time Signal propagates entire cable length All stations receive all transmissions CSMA/CD medium access control scheme
Spring 2000 John Kristoff 10
CSMA/CD
Carrier Sense (CS) Wait until medium is idle Begin to transmit frame
Multiple Access (MA) Multiple stations attached to shared media Each station uses the same access algorithm
Simultaneous Transmission is Possible
Spring 2000 John Kristoff 11
CSMA/CD [continued]
Simultaneous Transmission: Interfere with each other Known as a collision
CSMA with Collision Detect (CD) Listen to media during transmission Detect whether another station’s signal
interferes Back off from interference and try again
Spring 2000 John Kristoff 12
Transmission Logic
1. If media is idle, transmit.2. Else, continue to listen to the media and
when it is available, transmit.3. Listen to media while transmitting.4. If collision is detected while transmitting,
send jam and back-off5. Go to step 1 until max-try counter is
reached.
Spring 2000 John Kristoff 13
Exponential Back-off Algorithm
Let 1 Slot Time = 512 bit times Upon 1st collision, randomly choose among {0,1} slot delay Upon 2nd collision, randomly choose among {0,1,2,3} slot
delay Up to a maximum of 16 transmission attempts with a range
of delay from {0 to 1024} bit times
0 <= r < 2k-1
Where r is the random number generated, where k = MIN(n,10) and where n is the n-th retransmission attempt
Spring 2000 John Kristoff 14
The Collision Domain
Minimum Length Frame Must Be >= Maximum RTT of the Ethernet segment
Minimum Frame is 512 bits Requires 46 bytes of data whether the
upper layer has them or notDistances decrease as speed increasesFull-duplex mode eliminates the
collision domain
Spring 2000 John Kristoff 15
An Aside - Collisions
They are NOT bad, unless they’re lateCollision statistics are mostly
meaninglessMonitor utilizationDistance MattersBecoming irrelevant with switchingThe Name “Collision” is misleading
Spring 2000 John Kristoff 16
Ethernet Addressing
Standardized by IEEE Each station assigned a unique 48-bit address
First 24-bits are the OUI Second 24-bits are vendor assigned
Usually set when NIC is manufactured Canonical address format
Spring 2000 John Kristoff 17
Ethernet Address Recognition
Each Frame Contains a Destination Address
All Stations Receive All TransmissionsStation Discards Any Frame Not Destined
for ItImportant: interface hardware, not
software, checks address
Spring 2000 John Kristoff 18
Possible Destinations
1. Single destination (unicast)2. All stations on the Ethernet
(broadcast)3. Subset of stations on the Ethernet
(multicast)
MAC address is used to distinguish between the destinations
Spring 2000 John Kristoff 19
Ethernet Destination Addresses
Spring 2000 John Kristoff 20
Promiscuous Mode
Designed for testing/debuggingAllows interface to accept all framesAvailable on most Ethernet hardware
Spring 2000 John Kristoff 21
IEEE 802.3 Frame Format
Sender fills in: Sender’s source
address Recipient’s
destination address Type of data in the
frame type field Cyclic Redundancy
in FCS field
Spring 2000 John Kristoff 22
Demultiplexing on Frame Type Field
Network Interface Hardware Receives a copy of each transmitted frame Examines address and either accepts or
discards Passes accepted frame to system software
Network device software Examines frame type Passes frame to correct software module
Spring 2000 John Kristoff 23
Ethernet Wiring - 10BASE5
Thick Ethernet (Thicknet)Heavy coaxial cable
Spring 2000 John Kristoff 24
Ethernet Wiring - 10BASE2
Thin Ethernet (Thinnet)Smaller coaxial cable
Spring 2000 John Kristoff 25
Ethernet Wiring - 10BASE-T
Uses a hubTwisted-pair wiring
Spring 2000 John Kristoff 26
Ethernet Office Wiring
Spring 2000 John Kristoff 27
High-speed Ethernet
Fast Ethernet Operates at 100 Mb/s Standardized in IEEE 802.3 as 100BASE-
T and 100BASE-F standards 10/100 Devices available
Gigabit Ethernet Operates at 1 Gb/s Mostly fiber systems using switches
Spring 2000 John Kristoff 28
Ethernet - Final Notes
Data Link Layer Usually Implemented with Physical Layer
Link BeatInterframe Gap TimeCapture EffectModern Ethernet is a star-shaped busnntp://comp.dcom.lans.ethernet
Spring 2000 John Kristoff 29
Example Ring: Token Ring
Popular in IBM environmentsIEEE Standardized as 802.5Operates at 4Mb/s, 16Mb/s Quickly Being Abandoned
Spring 2000 John Kristoff 30
Token Ring Transmission
Station waits for token before sending Signal travels the entire ring Sender receives its own transmission
Spring 2000 John Kristoff 31
Token Passing Paradigm
Frames travel in a unidirectional fashion around the ring
Stations must wait for token to transmit
Stations can reserve the tokenToken will circle indefinitely until a
station wants to transmit
Spring 2000 John Kristoff 32
MAC Frames
Ring management and control frames Beacon, Ring purge, claim token, report
errorRing Poll every 7 seconds
Active monitor present Standby monitor present NAUN notification process
Spring 2000 John Kristoff 33
Active and Standby Monitor
Only 1 Active Monitor per ringAM is the master clock for the ringAM inserts 24-bit delay to
transmissionsAM ensures tokens/frames are presentAM removes circulating framesSMs are ready to take over if AM fails
Spring 2000 John Kristoff 34
Monitor Contention
Ring elects a new Active MonitorInitiated when:
Loss of signal is detected Active monitor not detected Time-outs of token timer, NAUN, etc.
Highest MAC address winsEveryone else is Standby Monitor
Spring 2000 John Kristoff 35
Token Ring Insertion Process
Phase 0 - Media Lobe CheckPhase 1 - Physical InsertionPhase 2 - Address VerificationPhase 3 - Participation in Ring PollPhase 4 - Request Initialization
Spring 2000 John Kristoff 36
The Token Frame
When no station is transmitting, the token frame travels continuously around the ring.
Spring 2000 John Kristoff 37
Token Ring Addressing
Standardized by IEEE Each station assigned a unique 48-bit address
First 24-bits are the OUI Second 24-bits are vendor assigned
Usually set when NIC is manufactured Non-canonical address format
Spring 2000 John Kristoff 38
Token Ring Address Recognition
Each Frame Contains a Destination Address
All Stations Receive and Repeat All Transmissions
Stations Copy Any Frame Destined for It Important: interface hardware, not
software, checks address
Spring 2000 John Kristoff 39
Token Ring Destination Addresses
Spring 2000 John Kristoff 40
Token Ring Frame Format
Sender fills in: Sender’s source
address Recipient’s destination
address Cyclic Redundancy in
FCS field
Other stations may change: Frame Status
Spring 2000 John Kristoff 41
High-speed Token Ring
HSTR Operates at 100 Mb/s 1 Gb/s being worked on Standardized in IEEE 802.5 Some 4/16/100 devices
Spring 2000 John Kristoff 42
Why Token Ring Lost
IBM was the only systems manufacturer that promoted it
CostComplexitySupport throughout the industryOnly one vendor left to develop
product!
Spring 2000 John Kristoff 43
Token Ring - Final Notes
JitterEarly Token ReleaseBackup PathToken Transmission TimerNeeds LLC - we haven’t talked about
it yetnttp://comp.dcom.lans.token-ring
Spring 2000 John Kristoff 44
Example Ring: FDDI
Uses Optical Fiber cablingHigh reliability (dual rings)Immune to interferenceStandardized by ANSITransmission rate of 100 Mb/s
Spring 2000 John Kristoff 45
FDDI Dual Ring Operation
Spring 2000 John Kristoff 46
Logical Link Control
Standardized by IEEE 802.2Often used for MACs that don’t use type field
Spring 2000 John Kristoff 47
What else is there?
ATMWireless (802.11)Fiber ChannelHIPPIToken Bus (802.4)