S3_Underlying Protocols Part II

Underlying protocols (Part II) ©2013 Pitipatana Sakarindr

Underlying Protocols: Physical-Layer Protocols and

Data Link-Layer Protocols

Dr. Pitipatana Sakarindr ECE, NJIT

ECE637


Click to edit Master title style

Wireless LANs (IEEE 802.11a/b/g/n)

ECE 637 Internet and Higher-Layer Protocols 2


Click to edit Master title style WLAN (cont’d)

• 802.11 defines the basic service set (BSS) (or also known as cell) as the building block of a WLAN – A BSS consists of stationary or mobile wireless stations and,

optionally, a central base station – The central base station is also known as the access point (AP) – Theoretically, a wireless LAN can contain just one cell with one

AP. But, most WLANs have several cells connected through multiple APs, which may connect to the backbone, i.e., Internet or other internetworking systems


IBSS = Independent BSS)


Click to edit Master title style WLAN (cont’d) • 802.11 evolutions

– 1999, 802.11b • 2.4GHz ISM (Industrial, Scientific and Medical) bands • 5.5 and 11 Mbps

– 2001, 802.11a • 5.8GHz ISM bands • 6-54 Mbps

– 2003, 802.11g • 2.4GHz ISM bands • 5.5 and 11 Mbps

– 2009, 802.11n • 2.4 and 5.8GHz ISM bands • 54 – 600Mbps

– 2013, 802.11ac • 5.8GHz ISM bands • Up to 1.3Gbps



Click to edit Master title style WLAN (cont’d)

• An extended service set (ESS) – consists of two or more BSSs with APs – These BSSs are connected through a distribution

system, usually a wired LAN (i.e., Ethernet) – ESS uses 2 types of stations: stationary or mobile

• Mobile stations are normal stations inside a BSS • Stationary stations are APs that are part of a wired LAN



Click to edit Master title style PCF and DCF • 2 operational modes:

– DCF: Distributed coordination function (all 802.11 devices support DCF)

• Used in Ad Hoc mode • Also referred to as a contention period (CP) • Each station contends for available bandwidth • Several media access mechanisms: CSMA/CA, RTS/CTS, and

exponential backoff – PCF: Point coordination function (many 802.11 devices do

not support PCF – not used in most wireless networks) • Used in infrastructure mode • Also referred to as a contention free period (CFP) • A point coordinator located in an access point, AP (also referred to

as a base station, BS) controls the access to bandwidth by controlling which station can transmit to the AP and which station receives packet from the AP

• Method: Polling (no RTS/CTS) • HCF-Hybrid Coordination Function in 802.11e is not included in

this class



Click to edit Master title style Physical and virtual carrier sensing • Physical carrier sensing • Virtual carrier sensing:

– If two or more stations could not hear each other (they are hidden from each other), they could not perform physical carrier sense on the channel

– To reduce the collision from these far-distance stations, two short control frames, RTS and CTS, are proposed

• A station that has a data frame to transmit sends out one small control frame, RTS (Request-to-Send), that contain the source address, destination address , and the estimated duration of the transmission (for this data frame and a corresponding positive ACK frame)

• If the channel is free, the destination replies with one small control frame, CTS (Clear-to-Send), that contains similar transmission parameters

• Upon receiving either RTS, CTS, or both, all other stations set their virtual carrier sensing indicator, NAV (network allocation vector) timer, with respect to those parameters in RTS/CTS frames

• All other stations defer access and wait until the NAV timer expires before trying to sense and contend for the now-free channel



Click to edit Master title style PCF and DCF interframe spacing • PCF and DCF can coexist within a cell, as 802.11 defines the

interframe time spacing in 4 types:

– Short interframe spacing (SIFS) allows the parties in a single dialog to chance to go first (RTS, CTS, ACKs)

– SIFS is used because a station needs some time to change from the receiving mode to the sending mode

– SIFS also give the priority to ACK packets

– SIFS value varies on the physical layer protocol, i.e., SIFS on FHSS PHY is 28 μs

– Distributed interframe spacing (DIFS)

– DIFS = SIFS + (2 × slot times)

– DIFS value varies on the physical layer protocol, i.e., DIFS on FHSS PHY is 128μs (28+2×50)

– If DCF interframe spacing (DIFS) time elapses, any station may try to get the channel



Click to edit Master title style PCF and DCF interframe spacing (cont’d) – Point interframe spacing (PIFS)

– PIFS is used by an access point to access the channel before other stations

– PIFS = SIFS + 1 slot time

– PIFS varies on the physical layer protocol, i.e., PIFS on FHSS PHY is 78μs

– If the PIFS elapses without a station response (to SIFS), the base station may send a beacon

– Extended interframe spacing (EIFS)

– EIFS is only used by a station to report a received bad frame – This station usually could not read out the transmission

parameters, i.e., duration of data transfer, so it cannot properly set its NAV timer

– This station may then send its frames and cause a collision with other frames subsequent to that bad frame



Click to edit Master title style PCF and DCF interframe spacing (cont’d)

• After a SIFS period, a control frame, a data frame, or the next fragment may be sent

• After a PIFS period, PCF frames may be sent (starting with beacon bytes, following with CF-Poll/CF-ACK/CF-End/Data)

• After a DIFS period, DCF frames (RTS/CTS/Data/ACK) may be sent

• After a EIFS period, a bad frame is completely recovered


EIFS

1 Slot time

SIFS PIFS

DIFS

1 Slot time

Busy Frame Transmission …

Contention period

Backoff slots

Defer access Select slot using binary exponential backoff


Click to edit Master title style DCF: CSMA/CA and RTS/CTS(cont’d)

• RTS and CTS are short frames that are sent to notify all other stations (except the destination) to stop listening or sensing the channel for a determined period (of which the two communicating stations send their data and ACK frames) – RTS and CTS are short, so it is faster than other frame

types (i.e., data or ACK) for all other stations to receive and set their NAV timer


Contention Period

RTS

CTS

SIFS DIFS

MPDU

SIFS

ACK

Defer access

Defer access

SIFS

NAV (network allocation vector)

NAV

Contention Period

Host A

Host B

Host C

Host D

Sense the busy channel, defer

Collision occurs

1st attempt

D’s back-off

SIFS 2nd attempt

MPDU (MAC Protocol Data Unit) DIFS

RTS

CTS

SIFS

NAV

NAV

D’s back-off

C’s back-off

Back-off period not yet reached


Click to edit Master title style Contention window – After a DIFS, all stations independently choose a random

back off period between zero and contention window (CW) – The chosen backoff period is translated into the number of

slots times • The slot time is either 50 μs for 802.11 FHSS PHY or 20 μs for

802.11 DSSS PHY • The transmission of a RTS frame can begin only at the beginning

of the slot time (not in-between) – Each station waits for the randomly chosen number of slot

times before trying to carrier sensing the channel – Initially, the contention window is set to a minimum

contention window size (CWmin) – If the 1st attempt fails (e.g., there is a collision of RTS

frames from two or more stations that have independently and randomly chosen the same back-off period), the contention window size is doubled

– This exponential back-off mechanism continues ( collisions occurred at all attempts) until the contention window reaches a maximum value (CWmax)

• The randomization and exponential scaling of the contention window size are used to reduce collisions



Click to edit Master title style DCF: Exponential back-off


CWmin

CWmax

CW (unit: μs)

1st (failed)

2nd (failed)

Max # of

failed attem

pts

If the contention still fails, the station aborts the transmission (the number of back-off counter reaches the limit)

A station randomly selects the back-off period between 0-CW

At N-th attempt a station randomly selects the backoff period between 0-X μs

Xμs Yμs

At N+1-th attempt a station randomly selects the backoff period between 0-Y μs

Contention Period

Sense the busy channel, defer N-th attempt

D’s back-off

SIFS N+1-th attempt

DIFS

RTS

CTS

NAV

NAV

D’s back-off

C’s back-off

Back-off period not yet reached

Host A

Host B

Host C

Host D

At 1st attempt, C and D select the same shortest period (the fewest slot times), so their RTS frames collide, triggering another attempt. At 2nd attempt, only D selects the shortest period

1 Slot time


Click to edit Master title style DCF: CSMA/CA and RTS/CTS(cont’d)

• Host C does not yet come out from the backoff period, but it hears the RTS from host A, so it defers from accessing the channel (to allow the host A to receives the CTS frame) and increments its backoff counter – Once it hears the RTS frame, it sets its NAV timer according to the

required transmission period retrieved from the RTS frame • Host D might be hidden from host A, so it does not hear the RTS frame OR

it just joins the network and senses a busy channel – It hears the CTS frame and sets its NAV timer according to the required

transmission period retrieved from the CTS frame


RTS

CTS

SIFS DIFS

MPDU

SIFS

ACK Defer access

(waiting for RTS)

SIFS

NAV timer is set upon receiving the RTS

Contention Period

Host A

Host B

Host C

MPDU (MAC Protocol Data Unit)

NAV timer is set upon receiving the CTS Host D

Defer access (waiting for data transmission)

Carrier sense “busy channel”

Just join the network and sense the busy channel OR hidden to host A

DIFS

Contention Period

MPDU

shortest

shortest

SIFS

ACK

Defer access


Click to edit Master title style 802.11 MAC and small data frames (cont’d)

– RTS/CTS handshaking is used when the length of data frames exceed the length threshold of RTS/CTS frames

• 802.11 defines the maximum length threshold of RTS and CTS frames to 2347 bytes

– If a data frame is less than 2347 bytes, it is transmitted immediately, without triggering RTS/CTS handshaking

– RTS/CTS can eliminate the hidden station problem, but not the exposed station problem



Click to edit Master title style Small data frames

• The short MPDU is sent without RTS/CTS handshaking


RTS

CTS

SIFS DIFS

MPDU

SIFS

ACK Defer access

(waiting for RTS)

SIFS

NAV timer is set upon receiving the RTS

Contention Period

Host A

Host B

Host C

MPDU (MAC Protocol Data Unit)

NAV timer is set upon receiving the CTS Host D

Defer access (waiting for data transmission)

Carrier sense “busy channel”

Just join the network and sense the busy channel OR hidden to host A

DIFS

Contention Period

MPDU

shortest

shortest

SIFS

ACK

Defer access


Click to edit Master title style PCF

• In PCF, • AP senses if the channel is idle: if idle, it will take over medium after

waiting a certain period of time, known as PIFS (point interframe spacing) • After PIFS period, the base station broadcasts a beacon frame periodically

(10 or 100μs) • Also known as the CFP (Contention Free Period) • Operation in an Infrastructure BSS (not ad hoc) • The PCF mode starts with issue of a Beacon at the target beacon

transmission time (TBTT) to announce if it is supporting the PCF and to start the CFP period

• No Collisions take place because the channel access is controlled by the AP • The point coordinator located in an access point polls the other stations by

sending CF-Poll frames • A CF-Poll allows stations to send data frames • When no station has any data to send, the CF-End control frame is sent to

end the CFP period • The contention period (CP) immediately follows the CFP to complete the

contention-free repetition interval



Click to edit Master title style PCF and DCF


DCF (Contention Period)

Contention-Free Period (CFP)

B PCF (Contention Free Period)

DCF (Contention Period) B PCF

(Contention Free Period)

TBTT TBTT

Access Point PC (Point

Coordinator)

Channel bandwidth Channel bandwidth

Each station contends for the channel bandwidth, possible collision

The AP controls who can access the channel, no collision

PCF DCF


Click to edit Master title style PCF: polling

• Polling list: the AP maintains the polling list for all frames in its buffer • CF-Poll: Once the station receives the CF-Poll frame from the AP, along

with its data frames buffered in the AP, this station sends data frames in its buffer

• CF-End: With no more buffered frames and no station has any data frame to send, the AP sends out the CF-End frame to end the CFP and reset all stations’ NAV timer to start the DCF period


CF Poll

PIFS SIFS

AP

Host A

Host B

Host C

Contention-Free Period (CFP)

B

MPDUA CF

ACK

Data(A) Data(B) CF

ACK Data(C) CF Poll

CF Poll

CF End

PIFS SIFS SIFS SIFS SIFS

CF ACK

MPDUB CF

ACK

(A)

ACK

NAV timer is set upon receiving the beacon




Click to edit Master title style PCF: polling (AP) • All multicast and broadcast frames must be sent first before

the AP allows the polling (and before the CFP reaches the maximum allowed duration, named CFP MaxDuration)

• The polling list – The AP maintains the polling list for each CFP – In each CFP, there must be at least 1 station in the list;

otherwise, the CF-End frame is sent to end the CFP – The AP lists the order of the station in the list according to

the association ID (AID) (in ascending order) • When a station associates with the AP, an AID is assigned

– If the CFP MaxDuration ends, the next station to be polled will be polled first during the next CFP

– If all frames in the AP’s buffer have been sent and there is still times left before the CFP MaxDuration ends, the AP can poll a station multiple times

– The stations to be polled during the current CFP are explicitly in the delivery traffic indication message (DTIM)



Click to edit Master title style Association request frame (from a station)

• The AP periodically broadcasts beacons – The station uses the beacons to learn about this AP

• The station associates with the AP by sending the association request management frame – With the association request frame, the station announces

to the AP if it is CP-pollable (i.e., can receive data from the AP during the CFP)

• The AP replies with the association response management frame


Capability information Listen interval SSID Supported rates 1 byte 1 byte 2 bytes 2 bytes


Click to edit Master title style PCF: polling (AP) (cont’d)

• At the beginning of the CFP, the AP can send – Only CF-Poll: the station at the top of the polling

list (i.e., the station that is being polled) does not have a data frame buffered at the AP

– CF-Poll and Data: the station at the top of the polling list has its data frame(s) buffered at the AP

– CF-End: no data frame is buffered at the AP and the polling list is null (no requested station to transmit)



Click to edit Master title style PCF: polling (AP) (cont’d) • The AP always replies to any station with the CF-ACK

frame – The CF-ACK frame for one station can be piggybacked

with a CF-Poll frame for another station that is being polled

– The CF-ACK frame for one station can be piggybacked with a data frame for another station

• If the AP has more than 1 data frames for the same station, it sets MORE DATA bit (in the MAC header) to one

• If the AP does not receive any response (after sending out the CF-Poll frame), it retakes the control of the channel after a PIFS period – The station that receives the CF-Poll frame must

respond within a SIFS period (SIFS<PIFS)



Click to edit Master title style PCF: polling (stations) • A station can be pollable or not pollable

– A non-pollable station: it is not at the top of the polling list OR it does not support the PCF

– A non-pollable station • Receiving: Data from the AP, Replying: ACK (not CF-ACK) • Cannot send any data to any station or to the AP

– A pollable station • (Without CF-Poll) Receiving: Data/Management frames from the

AP, Replying: ACK • (With DATA) Receiving: CF-Poll/Data or CF-Poll/CF-ACK/Data from

the AP, Replying: CF-ACK or CF-ACK/Data (if any) • (Without DATA) Receiving: CF-Poll or CF-Poll/CF-ACK from the AP,

Replying: Null, Sending: only 1 data frame to any station or to the AP

– If the station needs to send more than 1 data frames, it must set the MORE DATA bit (in the MAC header) to one

• If the station does not receive the CF-ACK in response, it cannot retransmit that frame until it receives another CF-Poll frame or until the CFP ends



Click to edit Master title style PCF: polling (cont’d)

• 2 polling approaches: – The AP polls all stations to check if each of them

ahs data frames to send • Lots of bandwidth overhead (for CF-Poll frames) wasted

– A node with data to send individually requests a connection or to transmit a data frame with the AP

• Bandwidth overhead is reduced, but still quite high



Click to edit Master title style Beacon frame body


# byte

Timestamp

Beacon interval

Capability information

SSID

Supported rates

FH parameter set

DS parameter set

CF parameter set

IBSS parameter set

TIM

1

2

1

1

1

varied (generated by the station using FHSS PHY)

(generated by the station using DSSS PHY)

(generated by the AP supporting a PCF)

(generated by the station in an IBSS)

(generated by the station)

varied

varied

varied

varied

Beacon frames are used to allow stations to locate a BSS. A station continually scan all 802.11 channels and listens to beacon frames for choosing the best AP to associate with or to roam to. The AP periodically sends a beacon frame to announce the BSS presence and relay information (i.e., SSID, supported data rates, timestamp, power management options, and other parameters).


Click to edit Master title style Beacon frame (from AP)

• The point coordinator broadcasts the beacon only if the channel is idle and only after it waits for the PIFS period


Timestamp Beacon interval

Capability information SSID Supported

rates Parameter set

Information elements

ESS IBSS CF Pollable

CF Poll Request Privacy Reserved

Beacon (2 )bytes

1 bit 11 bits 1 bit 1 bit 1 bit 1 bit

0 0

0 1

1 0

1 1 Reserved

No point coordinator located at AP (CFP not supported)

Point coordinator located at AP for transmission (but not for polling)

Point coordinator located at AP for transmission and polling)




Click to edit Master title style Beacon frame (from a station)




rates Parameter set


ESS IBSS CF Pollable

CF Poll Request Privacy Reserved

Beacon (2 )bytes

1 bit 11 bits 1 bit 1 bit 1 bit 1 bit

0 0

0 1

1 0

1 1 This station is CF-pollable, but requests never to be polled

This station is not CF-pollable

This station is CF-pollable, but requests not to be currently placed in the CF-polling list

This station is CF-pollable, and requests not to be currently placed in the CF-polling list




Click to edit Master title style TIM parameter set information element frame

• Delivery traffic indication message (DTIM) – DTIM period and DTIM count:

• A DTIM period (per CFP) is the number of beacon frames that must be sent before the AP sends all buffered multicast and broadcast frames (client still has to send PS-Poll to request)

• Because each beacon frame includes the DTIM period, all stations know when to wake up and receive multicast and broadcast frames if they are on the power saving (PS) mode

• If a DTIM period is 2, multicast and broadcast frames are sent after every two beacon frames

• The DTIM count is decremented every sent beacon ECE 637 Internet and Higher-Layer Protocols 29



rates Parameter set


FH parameter set

DS parameter set

CF parameter set

IBSS parameter set

TIM parameter set

Element ID Length DTIM count DTIM period Bitmap control Partial virtual bitmap 1 byte 1 byte 1 byte 1 byte 1 byte 1-251 bytes


Click to edit Master title style 802.11 Beacon frame capture • For example,

– If the beacon period is 100ms, and the DTIM value is 2, the AP will transmit buffered multicast and broadcast frames 5 times per second

• VoIP applications usually expect frequent broadcast/multicast frames



Click to edit Master title style PCF and DCF (cont’d)

• The DCF period needs to allow at least one transaction consisting of 1 RTS frame, 1 CTS frame, 1 MPDU (maximum length) frame, and 1 ACK frame

• Most 802.11 implementations use only DCF (CSMA/CA) – CSMA/CA in DCF does not need a base station (in

Ad Hoc networks) – Polling in PCF needs a base station (in

Infrastructure networks) – CSMA/CA offers a comparable performance in

terms of collision and overhead



Click to edit Master title style Type and Subtype in frame format


Ref: http://www.sss-mag.com/pdf/802_11tut.pdf


Click to edit Master title style 802.11 MAC Fragmentation and Reassembly

• Bit error rates on wireless systems (10-5, 10-6) are substantially higher than wired systems (10-12)

• If the error rate is high (i.e., noisy channel), the probability of an error for transmission of large data frames may be close to 1, i.e., every frame could possibly fail including the retransmission

• A large data frame may be fragmented by the transmitter and reassembled by the receiver node – Con: additional overhead (duplicated headers) – Pros: the probability of an error is reduced and the

retransmission is faster, due to smaller frames, instead of a large frame

• In particular microwave signal generates interferences in pulses (due to 60Hz AC power)

• Fragmentation allow smaller frames to slip through the pulses


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S3_Underlying Protocols Part II