data link control - medium access control rudra dutta csc 401- fall 2011, section 001

Data Link Control - Medium Data Link Control - Medium Access ControlAccess Control

Rudra DuttaCSC 401- Fall 2011, Section 001

Copyright Rudra Dutta, NCSU, Fall, 2011

PositioningPositioning Local Area Networks

– Small size; room, floor, building– Small number of computers, dozens, hundreds

Many different approaches, standards Ethernet has become de facto standard for

smaller size LANs


StandardizationStandardization By IEEE 802 working group Sub-layering of DLC

– MAC – Regulate access to media (possibly shared) (next)

– LLC – error/flow control, frame multiplexing (read)

802.1 – General concerns

Throughput and Efficiency Latency Wiring Type and Distances Topology Security Reliability


CharacterizationCharacterization


MAC - The Channel Allocation ProblemMAC - The Channel Allocation Problem There is only one channel

– No out-of-band to control multiple access– How do you know if it is your turn?

Static Channel Allocation – FDM-like– Problem: not flexible, also not really one channel

Dynamic allocation– In-band channel allocation methods

Contention to decide allocaiton

– Pseudo out-of-band Scheduling, reservation

– Truly out-of-band may be available – control plane


Categorization by CollisionCategorization by Collision Collision is what occurs when more than one

station transmits on the same medium– Here we assume data cannot be used after collision– May not be true in some cases, CDMA etc. useful

Collision must be detected– May or may not be avoided

Collision avoidance may be partial– “Limited contention”

Or complete– “Collision free”


Modeling the MediumModeling the Medium Station Model Single Channel Assumption Collision Assumption Temporal sequence

– Continuous Time– Slotted Time

Idle medium Assumption– Carrier Sense– No Carrier Sense


Examining a ProtocolExamining a Protocol

Central problem – medium access control– We shall examine control methods

Other concerns– Protocol details

Headers, formats, standardizationEthernet (various), 802.11, 802.16

– Familiarize from standards, references, etc.


ALOHAALOHA Granddaddy of all shared MACs Renewed interest in wireless field Straightforward and elementary

– Transmit when you want– Collision may occur – conclude from no

acknowledgement– Retransmit after delay

Important observation– Traffic offered to the medium is different from– Traffic offered to the stations, due to retransmission


Vulnerability PeriodVulnerability Period• Period during which another transmission collision


Slotted ALOHASlotted ALOHA

Time is slotted Can start transmitting only at beginning of next

slot, when frame arrives Vulnerability period is halved Efficiency goes up

– What is the tradeoff ?


CSMA/CDCSMA/CD Sophistication added to ALOHA

– Test the water – carrier sense– Don’t throw good money after bad – collision detect– Backoff algorithm

Reduce the chances of further collision

Quantities involved– τ – frame transmission time– ρ – maximum propagation time– Time slots of 2ρ – not transmission time!

Much smaller than transmission time

– Need to be careful about reconciling notation


CSMA/CDCSMA/CD A station does not transmit on busy carrier

– Uninterrupted packet transmissions can occur, even with high traffic arrival rate

A station stops transmission on sensing collision– Time wasted in contention is reduced


Persistence – the Role of Persistence – the Role of pp Non-Persistence

– If channel busy, wait random time before re-testing– Otherwise transmit

Persistence (1-persistence)– If channel busy, re-test very next slot– p-persistence: On idle channel, probably transmit

Ready


ALOHA CharacteristicsALOHA Characteristics


Random Access CharacteristicsRandom Access Characteristics

EthernetEthernet

Classic Ethernet Type/Length duality OUI in addresses Maximum and minimum wire lengths Binary exponential backoff Reception by noting destination address


Source address

Dest. address

Type

...

Payload

FCS

SFD

Preamble

10101011

10101010 (7 times)

Switched EthernetSwitched Ethernet

“Hub” contracted long wire into a box “Switch” replaces short wire in box with a

switchable network– No collisions – no MAC!

Supports simultaneous transmissions– But needs buffering


Fast EthernetFast Ethernet 100Base With Cat5 cables or multimode fiber, full duplex

100 Mbps– 100m for Cat5, 2km for fiber

Auto-negotiation of speed in protocol Fiber can only connect to switch

– Otherwise collision detection not possible– Sender would not still be sending when collision finds

its way back to it


Gigabit EthernetGigabit Ethernet

Hubs still allowed – but impractical– Needs half-duplex mode– Frame padded to 512 bytes by hardware– Bursts can be formed by hardware to help

Fiber or STP – later UTP Signaling and symbol translation complex 10GBase-T uses low-density parity check Faster ones on drawing board



Collision-Free SchedulingCollision-Free Scheduling

Eliminates collision completely– Not contention

Based on numbering and synchronization– Synchronization is required for slotting anyway– It is assumed that numbering is available

out-of-band– Number and address may be related, or same– May be hardware

Could also be result of negotiation


Bit-map ProtocolBit-map Protocol Set of short reservation (contention) slots Collision is eliminated in contention period by

allowing transmission by strict numbering No collisions in data transmission phase

– Behavior in low load? High load?


Limited Contention ProtocolsLimited Contention Protocols Remember, contention and collision are

different– Collisions can be eliminated– Contention cannot be eliminated on shared medium

Maximum contention will arise when all stations are always allowed to contend

Contention may be reduced by forbidding some stations to contend, at any given time

Key idea: form contention groups– Bitmap and ALOHA may be considered extremes


Optical Optical LANsLANs Significant work in research context

– Comparatively less development/standardization– Now considered more suited to WANs

Specific characteristic – τ/ρ ratio– Collision detection impractical– Scheduling is necessary

Specific characteristic – several shared media in one– From scheduling to multi-scheduling– Out-of-band control channel is natural


Wireless LANsWireless LANs Significant work in development context

– Standardization has followed, but divergent standards

– Active research area as well– Seems to be well suited for LAN context

Specific characteristic – medium is shared, but not completely or predictably– Typically sender is unable to draw strong conclusions

about collision or successful transmission

Copyright Fall 2011, Rudra Dutta, NCSU 26

CDMA – No MACCDMA – No MAC Use “chips” for each bit Each transmitter has

unique chip sequence Chip sequences are

orthogonal– Total power received by

mismatched Tx/Rx is zero– Other chip sequences sound like

noise

Frequency of bits transmitted is much less than frequency of symbols

Any user takes up whole band, but uses only a part of its information capacity

– Spread spectrum

“Asynchronous” CDMA – chip sequences (nearly) orthogonal even if they are offset

Power equalization at receiver


Wireless ProblemsWireless Problems Range of station may be much less than area

covered by LAN– Complicated by changing radio characteristics


The MACA SolutionThe MACA Solution Basic idea: ask the receiver to resolve collision Timed waits based on RTS and CTS

– Minimizes collisions– MACAW: refinements such as MAC layer ACK

802.11802.11

LLC to hide difference between variants


CSMA / CACSMA / CA

Like CSMA/CD, but sender starts with a backoff– Backoff countdown is frozen during other transmission

Collision (no ACK) doubles backoff for retry



RTS / CTSRTS / CTS B and C (but not D) can hear A D can hear B Timed waits on the part of C and D enable A

and B to communicate


BridgingBridging Interconnecting LANs

– Possibly running different protocols

Is this really DLC layer?– Historical reasons– Nevertheless, not clear in which layer

Issues in interconnecting different DLCs Issues in interconnecting multiple same DLCs Taxonomy


Interconnecting Different DLCsInterconnecting Different DLCs May need to connect for developmental reasons Main problem – frames have different formats Decapsulation and reencapsulation is required


Interconnecting Different DLCsInterconnecting Different DLCs

Other issues Some header information may not be available Data rate may be different

– Buffering may or may not be a good option

Allowed frame lengths may be different Security may also be an issue


Interconnecting Same DLCsInterconnecting Same DLCs Bridges should not flood frames unnecessarily

– Need to store some mappings– Looks more like internetworking

Bridges detect which destination on which LAN– Store in tables– Persist for short time – periodically old entries are purged

For Ethernet – “switches”


Backward Learning BridgesBackward Learning Bridges Initially, each bridges repeats all frames on all interfaces (other than

incoming one) As frames are sent, bridges remember incoming LAN using A sends frame to D

– B1: “A LAN1”, B2: “A LAN2”

D sends frame to A– B2: “D LAN3”, B1: “D LAN2”

Consider A sending frame to B


Spanning Tree BridgesSpanning Tree Bridges Above method works well with tree topology

– Path (sequence of bridges) between each node pair is unique– Also prone to complete disconnection due to any one bridge

failure

Multiple bridges may improve reliability– Will also introduce loops !

Bridges must intelligently decide to operate or not– Inoperative bridges “remove” themselves from network– Results in a tree (loop-free) topology– Must be spanning tree to avoid disconnecting the topology


Spanning Tree – A SampleSpanning Tree – A Sample


Loose TaxonomyLoose Taxonomy Hubs and repeaters are more like wires Bridges and switches connect collision domains

– With switches usually collision domain is single computer– Not clearly in DLC layer

Higher layer forwarding – clearly not DLC

VLAN – IEEE 802.1QVLAN – IEEE 802.1Q

Sometimes reverse can be useful– Making multiple logical LANs out of a single one – or multiple

ones connected by switches

VLAN tags identify which stations are “on the same LAN” (logically)

Switches must know what ports belong to which VLAN ID or IDs, and limit forwarding accordingly


VLAN TagsVLAN Tags Need new tags

– VLAN-aware switches only

Switch by VLAN tags

– Can use learning

Introduce tag in frame without one

– Use default, or configuration

Remove tag when delivering to VLAN-unaware port



SummarySummary DLC may serve the function of arbitrating

shared medium access– Collision, Contention, Reservation, Token passing

Wireless medium - overlapping but not identical collision domains– Hidden and Exposed Stations, RTS/CTS

Bridging– Interconnects DLCs– May translate protocols– Verges into Network layer in some cases

data link control - medium access control rudra dutta csc 401- fall 2011, section 001

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