Lecture 4 6-Apr-2016

Local Networks

Roadmap of the Course So far Basic telecom concepts General study of LANs LAN topologies Flow and error control

Today we continue the general study of LANs Medium access methods Logical link control

Medium Access Control (MAC) Methods

Medium Access Methods In LANs all stations are connected to a shared

transmission medium Simultaneous attempts to access the medium by

more than 1 station possible (=> collision) To prevent the above situation, each protocol has a

MAC method (Medium Access Control) Station follows some procedure when it needs to send

frames No conflicts between stations thus ensured

Types of methods

Random access methods No station controls other stations Also named contention methods Stations compete with each other

A station with data to send Applies a protocol-related procedure for deciding

whether or not to transmit Decision may depend on medium state (idle or

busy)

Collision When more than one station tries to transmit =>

access conflict => frames will be destroyed or modified

This access conflict is called collision

Procedures issues When should station access the medium? What should station do if medium is busy? How should station determine success or failure of

a transmission? What should station do when there is an access

conflict? Note: Broadcasting provides feedback

A sender can always find out whether its transmitted frame was destroyed, by listening to the channel, as all the other stations do

LAN – immediate feedback, satellite system – about 270 ms delay

Random access methods

Multiple access (MA) methods

ALOHA-type methods Earliest and simplest, least efficient Designed in early 70s, to be used in communication

at University of Hawaii Wireless (radio) LAN

ALOHA Originally: ground-based radio broadcasting, on 2

frequencies one for sending data, one for receiving ACKs

Basic idea works for any system with uncoordinated users competing for a single shared channel

Two versions: pure and slotted

ALOHA rules Multiple access No carrier sense No checking for collision Acknowledgement (explicit or implicit) Ack should arrive in the allotted time (two times the

maximum propagation delay) Otherwise frame has to be resent After waiting a random amount of time

Pure ALOHA

Users transmit whenever they have data to send There will be collisions, collided frames damaged Sender finds out if transmission was successful With feedback property With acknowledgements

If frame collided, sender waits random amount of time and sends again Called backoff strategy

Efficiency of such channel allocation: ≈ 18%

Pure ALOHA procedure

Pure ALOHA’s vulnerable time Time interval during which collision can occur

Slotted ALOHA

Time divided into slots, each slot corresponds to one frame

Agreement on slot boundaries needed (synchronization) One special station can emit a “beep” at the start of

each slot Users send at the beginning of slots only Vulnerable period is halved when compared

to pure ALOHA => double efficiency: ≈ 37%

Slotted ALOHA illustration

Slotted ALOHA’s vulnerable time

Carrier Sense Multiple Access (CSMA) methods

The medium is sensed by each station before trying to use it Chance of collision reduced => performance

increased Collisions can still appear Due to propagation delays

Vulnerable time in CSMA = propagation time Tprop Time needed for a signal to propagate from one end

of the medium to the other

Collisions in CSMA

CSMA methods

Several types Persistent (and p-persistent) Non-persistent CSMA with collision detection (CSMA/CD) CSMA with collision avoidance (CSMA/CA)

Non-persistent CSMA Station senses the channel If channel idle station starts transmitting If channel busy, station waits a random amount of time

and then it senses the channel again Better efficiency than persistent CSMA’s Unlikely that two stations wait the same amount of

random time Reduces the network’s efficiency Idle medium while stations have frames to send

Non-persistent CSMA illustration

(1-)Persistent CSMA When a station has to send, it first listens to

the channel If channel busy, station waits until it becomes

idle If channel idle, station transmits frame

(probability 1) If collision occurs, a random amount of time

passes and everything starts over again Better than ALOHAs; used in Ethernet

(1-)Persistent CSMA illustration

p-Persistent CSMA Applies to slotted channels Station senses channel and if idle: Transmits with probability p Defers transmission until next slot with probability q=1-p At next slot, if idle, same thing Algorithm repeated until frame transmitted or another

station begins transmission If channel busy, station waits a random time and senses

again Better performances for small p

p-Persistent CSMA illustration

Channel utilization versus load

CSMA / CD Different improvement when compared to ALOHA Stations abort transmission as soon as they detect a

collision => saves time and bandwidth Widely used on LANs’ MAC (basis of Ethernet) Model At t0 some station finished transmission Any other station may start transmitting => collisions may

occur After a detected collision, stations wait a random time and

start sending again, sensing the channel first When does a frame know it sent successfully?

CSMA/CD 2 Minimum time to detect a collision Time needed for a signal to propagate from a station to

another Worst case scenario τ – the time needed for a signal to propagate between

farthest stations It takes 2τ until a station is certain it seized the channel (if

transmits for 2τ without hearing a collision, then it is sure) A sending station must continually monitor the

channel To listen for noise bursts indicating a collision

CSMA/CD inherently a half-duplex system

Collision detection

CSMA/CD illustration

(Truncated) binary exponential backoff algorithm

Used by all collision-detecting stations to calculate their individual retransmission delay (backoff delay) after 1st collision each station waits 0 or 1 slot times before trying again;

if each station picks the same random number, they will collide again after 2nd collision each station picks 0,1,2, or 3 slot times before trying

again, and waits that number of slot times if a 3rd collision occurs, the next time number of slots to wait is chosen

randomly by each station from the interval 0 to 23-1 after i collisions, a random number between 0 to 2i-1 is chosen, and that

number of slots is skipped after 10 collisions the randomization interval is frozen at a maximum of

1023 slots after 16 collisions a failure is reported and recovery is up to higher

layers

(Truncated) binary exponential backoff algorithm 2

The algorithm dynamically adapts to the number of stations trying to send

If randomization interval for all collisions was 1023 the chance for two stations to collide for a second time: negligible the average wait after a collision: hundreds of slot times => delay

If each station always delayed for 0 or 1 slots stations would collide again and again

The algorithm ensures a low delay when only few stations collide

Also ensures collision is resolved in reasonable interval when many stations collide

Truncating the backoff at 1023 keeps the bound from growing too large

CSMA/CA No collision, collisions are avoided If line idle, it waits an Inter Frame Gap (IFG) time

interval, then a random amount of time, then sends frame and sets timer

Station waits for ACK from receiver If received before timer expires => success If not => failed; increases backoff parameter, waits

a backoff time interval and senses line Used in wireless LANs

CSMA/CA illustration

Controlled access Stations consult each other to see which has the

right to send One station then sends Being authorized by the rest

Example: token passing

Token passing

Token passing illustration (simplified)

A Bitmap protocol

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Collision-free protocols avoid collisions entirely Senders must know when it is their turn to send

The basic bit-map protocol: Sender set a bit in contention slot if they have data Senders send in turn; everyone knows who has

data

Binary Countdown protocol

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Binary countdown improves on the bitmap protocol

Stations send their address in contention slot (log N bits instead of N bits)

Medium ORs bits; stations give up when they send a “0” but see a “1”

Station that sees its full address is next to send

Limited-Contention Protocols Performance measures

delay at low load channel efficiency at high load

Contention strategies - Light load: contention preferred

- it has low delay and collisions are rare - High load: contention less atractive

- overhead associated with channel arbitration Collision-free

- Light load - relatively high delay

- High load - channel effciency improves, as overheads are fixed

⇒ it would be nice to combine the advantages of both

⇒ contention at low load to get low delay ⇒ collision free at high load for good channel efficiency

Limited-Contention Protocols

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Idea is to divide stations into groups within which only a very small number are likely to want to send Avoids wastage due to idle periods and collisions

Already too many contenders for a good chance of one winner

Limited-Contention Protocols: how? To increase probability of success for station i Decrease competition amount

Divide stations into groups Can be non-disjoint

Only members of group 0 can compete for slot 0 If one succeeds, it transmits If not, or nothing to submit, group 1 contends for slot 1,

etc Key: appropriate division into groups Dynamic is better many stations per slot when load is low Few (even one) stations per slot when load is high

Limited Contention – Adaptive Tree Walk

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Tree divides stations into groups (nodes) to poll Depth first search under nodes with poll collisions Start search at lower levels if >1 station expected

Level 0

Level 1

Level 2

Wireless LAN Protocols (1)

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Wireless has complications compared to wired Cannot detect collisions acknowledgements used

A station may not be able to transmit to/to receive from all other stations limited radio range

Wireless LANs (2) – Hidden terminals

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Hidden terminals are senders that cannot sense each other but nonetheless collide at intended receiver Want to prevent; loss of efficiency A and C are hidden terminals when sending to B

Wireless LANs (3) – Exposed terminals

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Exposed terminals are senders who can sense each other but still transmit safely (to different receivers) Desirably concurrency; improves performance B A and C D are exposed terminals

Wireless LANs (4) – MACA

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

MACA protocol grants access for A to send to B: A sends RTS to B [left]; B replies with CTS [right] C can send while the data frame is being sent if not interfering;

D and E must be silent until data frame sent completely

A sends RTS to B; C and E hear and defer for CTS

B replies with CTS; D and E hear and defer for data

Logical Link Control

IEEE 802 Project 1985: IEEE Computer Society started the 802

Project Standard to enable LAN intercommunication from a

variety of manufacturers Specifies functions of PhL and DLL of major LAN

Protocols

IEEE 802 working groups Number Topic

802.1 Overview and architecture of LANs 802.2 ↓ Logical link control 802.3 * Ethernet 802.4 ↓ Token bus (was briefly used in manufacturing plants) 802.5 Token ring (IBM's entry into the LAN world) 802.6 ↓ Dual queue dual bus (early metropolitan area network) 802.7 ↓ Technical advisory group on broadband technologies 802.8 † Technical advisory group on fiber optic technologies 802.9 ↓ Isochronous LANs (for real-time applications) 802.10 ↓ Virtual LANs and security 802.11 * Wireless LANs (WiFi) 802.12 ↓ Demand priority (Hewlett-Packard's AnyLAN) 802.13 Unlucky number; nobody wanted it 802.14 ↓ Cable modems (defunct: an industry consortium got there first) 802.15 * Personal area networks (Bluetooth, Zigbee) 802.16 * Broadband wireless (WiMAX) 802.17 Resilient packet ring 802.18 Technical advisory group on radio regulatory issues 802.19 Technical advisory group on coexistence of all these standards 802.20 Mobile broadband wireless (similar to 802.16e) 802.21 Media independent handoff (for roaming over technologies) 802.22 Wireless regional area network

IEEE Standard and OSI model

IEEE Standards for LANs

Logical Link Control (LLC) - 802.2 Generic sublayer, part of the data link layer Allows interoperability between different LAN

protocols

LLC functions LLC connects together LANs with different

protocols LLC hides the LAN-specific aspects to upper

layers Access method, encoding, signaling, transmission

media LLC adds reliability by supervising MAC

frames MAC sublayer provides a virtual unreliable link

LLC can be present or not in a network

Multiplexing using LLC When stations use multiple upper layer protocols

(e.g. IP and IPX)

LLC services LLC supplies services to the user of the LAN (typically

the network layer)

LLC service types

Service primitives (message exchanges)

Unacknowledged connectionless service

User passes data unit without making any connection nor expecting any ACK

Acknowledged connectionless service

User gets ACK for data transfer

Connection-oriented service Connection established, data exchange,

connection released

Connection-oriented service, data transfer

Connection-oriented service, disconnection

Connection-oriented service, other primitives

Resetting the connection If either user believes something is wrong (e.g.,

synchronization) Flow control LLC user can indicate how much data should be

passed between entities in the next data transfer

LLC protocol Defines the format of data units sent between 2

LLCs Data unit: PDU (protocol data unit) Information PDU: I-PDU Transport user data in connection-oriented services

Supervisory PDU: S-PDU Flow and error control

Unnumbered PDU: U-PDU Carry data in connectionless services and management

information in connection-oriented services

PDUs

PDU format 4 fields: destination service access point (DSAP),

source service access point (SSAP), control, and information

DSAP and SSAP Addresses used by LLC Identify protocol stacks on receiving and sending

machine

Control field First one/two bits define the PDU type

I-PDU N(S) specifies number of PDU being sent Its own identifying number

N(R) specifies number of PDU being expected Implies two-way exchange (with piggybacking) ACK field referring to a previous correct reception of

PDUs until the next expected one NAK field referring to a previous incorrect reception of

the respective PDU

S-PDU N(R) is used when receiver has no data of its own

to send S-PDUs do not transmit data Hence do not need N(S) to identify them

“Code” refers to some coded flow and error control information

U-PDU Has neither N(S) nor N(R) Has 2 code fields, one 2-bit long, another 3-bit long

P/F bit Stands for poll/final, single bit with dual purpose Has meaning only when set (bit=1) It means poll when a PDU is sent by a primary station to a

secondary one Address field contains the receiver’s address

It means final when a PDU is sent by a secondary station to a primary one Address field contains the sender’s address

In some protocols is used to force the other machine to send a Supervisory frame immediately, rather than waiting for reverse traffic and piggyback

Information field Used to carry The data sent from an upper layer Management information needed for LLC operation

Possible to include flow/error/other control information in I-PDUs

Combining data to be sent with control information: piggybacking

More on PDUs I-PDU designed for user information transport and

piggybacked ACKs S-PDU Used for ACKs, flow control, error control when

piggybacking is not appropriate No information in the information field, but still carry

messages to receiver Messages based on S-PDU type and transmission

context

More on S-PDUs Receive ready (RR,00): receiving station returns

ACK of a received I-PDU In this case receiver has no data of its own to send N(R) contains the sequence number of next expected PDU

Receive not ready (RNR,01): receiver returns ACK for all PDUs except the one in N(R) Also requests that no more PDUs be sent until a RR S-PDU is issued

Reject (REJ,10): NAK returned by a receiver in a go-back-n ARQ system When receiver has no data on which to piggyback the response N(R) contains the number of damaged PDU => that PDU and all that

follow need to be retransmitted Selective reject (SRJ,11): requires the

retransmission of specified frame

More on U-PDUs Unnumbered PDUs used for exchanging User information Management and control information between connected

devices Much of the carried information contained in the control

field codes

PDUs in unacknowledged connectionless service

This service uses 3 unnumbered PDUs UI, XID, TEST XID: exchange IDs; TEST: loopback test

PDUs in acknowledged connectionless service

This service uses only one unnumbered PDU AC: acknowledged connectionless information

PDUs in connection-oriented service

This service uses all 3 types of PDUs

Data transfer

Piggybacking

Disconnection