physical and data link layersphysical and data link layers yuzo taenaka graduate school of science...

Physical and Data Link layers

Yuzo Taenaka

Graduate School of Science and Technology,Nara Institute of Science and Technology

Today’s topic

Internet Engineering / 2018 2

Application

Presentation

Session

Transport

Network

Data Link

Physical

ES (End System)Transmission procedure for data chunk (“frame”) over a single communication media.

Fundamental Transmission procedures for “bits” over communication media.

Physical Layer


Types of transmission medium=Cables

– Optical fiber

– Copper

=Wireless


Source: siemon.com

Source: blackbox.com

Cables and connectors=Copper

– UTP– STP

=Connectors– RJ45– RJ11


=Optical fibers– Single mode fiber– Multimode fiber

=Connectors– LC, SC, FC, MT-RJ…

RJ45 connector.Source: flukenetworks.com

Source: aisan.co.jp

Source: aisan

Source: aisan

Cable speed, distance and cost


Speed Medium Distance Cost10Gbit/s Optical (SMF) 10 km $$$10Gbit/s Copper 100 m $$1Gbit/s Optical (MMF) 550 m $$1Gbit/s Copper 100 m $1Mbit/s Copper 4 km $

Source: cable360.net

Physical characteristics: a crude comparison

= 54Mbit/s in wireless cannot be delivered as advertised,whereas 1Gbit/s in optical fiber can be delivered as advertised.


Copper Fiber Wireless

Attenuation XX XXXX

Attenuation distortion X X XX

Noise XX X XXXX

Bend XX

Chromatic dispersion X XXXX

Crosstalk XX XXXX

EM interference XX XXXX

Echo XX XXXX

Role of physical layer=Transmit every “bit” by electrical signal, radio wave,

or something that could carries 0/1 values.


��

��end bit patternstart bit pattern��

Physical layer

Example of encoding bits

=Manchester code (phase encoding)

– IEEE 802.3 (10base-T / 10Mbps�– A transition of electric voltage indicates 0 or 1


low → high

= 1high → low

= 0

More detail=take the lectures

– Principles of Signal Processing (3003)• https://syllabus.naist.jp/subjects/preview_detail/28• Prof. Kato

– Wireless Communication Systems (4015)• https://syllabus.naist.jp/subjects/preview_detail/70• Prof. Okada

– Signal Detection Theory (4016) • https://syllabus.naist.jp/subjects/preview_detail/71• Prof. Okada


Data Link Layer


Coverage of datalink


ApplicationPresentation

SessionTransportNetwork

Data LinkPhysical

ES (End System)

IS (Intermediate System)

Physical connection

Router handling network layer

datalink

Data Link Layer Services overview= Framing, link access:

– Encapsulate datagram into frame, adding header, trailer– Channel access if shared medium– “MAC” addresses used in frame headers to identify source &

destination= Flow Control:

– Pacing between adjacent sending and receiving nodes= Error Detection:

– Errors caused by signal attenuation and noise– Receiver detects presence of errors

= Error Correction: – Receiver identifies and corrects bit error(s) without resorting to

retransmission= Transmission mode (Half-duplex and full-duplex)

– With half duplex, nodes at both ends of link can transmit, but not at same time


Frame=Data link layer Protocol Data Unit (PDU)

– Defining the frame borders (delimiters)=Can determine if any failures (bit errors) occurred

– Adding error-detection / error-correction code to bit sequences in order to delimit the appropriate frame length

=Frame header– error detection and flow control– control information


01111110 address control data 01111110checksum

header payload

Inform ation N etw ork 1 / 2016


Frame Synchronization

=Bit-sequence-based frame synchronization

– A special bit sequence is inserted to the data header and footer.

• synchronization

– Insertion of a bit sequence composed of the “same” bit

• bit stuffing

– special bit sequence only appears at the frame header and footer

– e.g.

• special bit sequence� 01111110

• if sender detects “11111” in data, it “stuffs” a “0” right after.

• if receiver detects “11111” in data, it deletes the following

stuffed “0”.

01111110 address control data 01111110checksum


Ethernet frame=A frame captured by wireshark (introduced in today’s

hands-on)


Error detection & correlation


Errors in Physical Layer


Noise

Attenuation

Distortion



Error Control=Goal

– Detecting and correcting transmission error in channel• Was the frame correctly sent?• Was the frame sequence order correct?

=Techniques– Introducing the concept of frame (failure localization)– Coding techniques

• Error Detection Code – Parity, CRC (Cyclic redundancy check)• Error Correction Code – FEC

– Protocol techniques for error correction• Timer• Retransmission


Basic idea of CRC=Given:

– Polynomial expression of m bit frame M(x) (degree m-1)– Generator polynomial G(x), of degree r (r < m)

=Compute:– prepare xrM(x): frame with r zeros– Compute modulo of xrM(x) divided by G(x): R(x)– Frame for transmission: F(x)

• F(x) = xrM(x) + R(x)=Successful transmission: F(x) / G(x) = 0

– Nonzero otherwise. i.e., error detection.– Consecutive errors less than r bits can be detected



Standardized CRC polynomials=Commonly known standards

– CRC-12• x12+x11+x3+x2+x+1

– CRC-16• x16+x15+x2+1

– CRC-32• x32+x26+x23+x22+x16+x12+ x11+x10+x8+x7+ x5+x4+x2+x+1

– CRC-CCITT• x16+x12+x5+1

=There are many other error detection codes.


Flow control


Flow Control=Flow Control Protocols deal with how to send

sequences of frames=They have two goals:

– Recover from lost frames– Prevent buffer overflows

=Network Layer may want to receive same set of frames in the same order they were sent

=Automatic Repeat Request (ARQ)– Stop-and-wait– Go-back-N– Selective-repeat

Internet Engineering / 2018 23Inform ation N etw ork 1 / 2016

In a case without flow control= Sender sends frames at an arbitrary timing

Frames may be dropped on an intermediate medium

= Receiver takes time to process a frame


sender receiver

3 frames arrives while processing 1 frame→ buffer overflow

sender receiver

X

X


Stop-and-wait ARQ (1)

t1 t2 t3

t4

t5t1Sender

Receiver

t1: Round Trip Timet2: Frame Transmission Timet3: Frame Processing Timet4: ACK Transmission Timet5: ACK Processing Time



Stop-and-wait ARQ (2)

=Procedure– Waiting to receive ACK on each frame

transmission

– Setting a sender timer greater than 2t1+t2+t3+t4

– Retransmission when sender timer times out.

=Characteristics– Simple

– The buffer never contains more than one frame for the receiver and the sender

– Very low utilization of channel capacity



Go-back-N ARQ

1 653 45432

1 653 4542

ACK

ACK

ACK

ACK

ACK

ACK

Time out for Frame3

!!



Selective-Repeat ARQ

1 873 65432

1 873 6542

AC

K

AC

K

AC

K

AC

K

AC

K

AC

K

Time out for Frame3

!!

AC

K

AC

K


ARQ: simplicity vs efficiency, adaptability=Stop-and-Wait

– Simple– No large buffer required in both ends

=Go-back-N– Still simple, but buffer management has to be done at

SENDER.– N means the buffer size– There is no large buffer required at RECEIVER side.

=Selected Repeat– Efficient but complicated scheme that requires buffer, timer,

and ACK managements.– Buffers are required in both ends.– Window Flow Control is needed for buffer management.



Burden sharing among layers=Assignment of function depends on communication

system designs=Various solutions exist

Data Link

Network

Transport sequence assuranceflow controlretransmissioninterconnection of networkerror detection and correctionframe boundary


Sublayers of the Data Link Layer



Sublayers of the Data Link Layer

Physical Layer

Data link Layer

Network Layer

Media Access Control Sublayer

802.2 LLC

802.3CSMA/CD

802.5Token Ring

802.4Token Bus

Logical Link Control Sublayer

IEEE standard 802


802.11Wireless LAN

CCITT X.25(HDLC/LAPB)

CCITT Data linkLayer Definition

Access Channel=Two types of “links”:

– Point-to-point• PPP for dial-up access• Point-to-point link between Ethernet switch and host

– Broadcast (shared wire or medium)• Traditional Ethernet• 802.11 wireless LAN


MAC Protocols (1)=Single shared broadcast channel

– Two or more simultaneous transmissions can interfere with each other

– Collision will be observed whenever node receives two or more signals at the same time

=Ideal Media Access Protocol– When one node wants to transmit, it can send at rate R– When M nodes want to transmit, each can send at average

rate R/M– Fully decentralized:

• No special node to coordinate transmissions• No synchronization of clocks, slots

– Simple


MAC Protocols (2)=Control-based MAC

– Channel Partitioning• Divide channel into smaller “pieces” (time slots, frequency,

code)• Allocate piece to node for exclusive use

– Taking turns• Nodes take turns• Nodes with more to send can take longer turns

=Contention-based MAC– Random Access

• Channel not divided, allow collisions• “Recover” from collisions


Channel Partitioning (1)=TDMA (Time division multiple access)

– Each node occupy channel at a specific time slot


Time

Node A Node B Node C

Channel Partitioning (2)=FDMA (Frequency division multiple access)

– Allocate frequency to a node


Time

Frequency

Node ANode B

Node C

Contention-based MAC (1)=Pure ALOHA (1970s) at Hawaii Univ.

– Wireless communication=How to communicate

– Outlying station transmits a packet on the inbound frequency– The central transmitter repeats the transmission on the outbound

frequency (which all stations can receive) – If a copy of its packet arrives, the sending station moves to the

next packet; if no copy arrives, the sending station waits a random period and tries again.

Internet Engineering / 2018 39Computer Networks & Internets: Douglas E. Comer

CSMA=Carrier Sense Multiple Access

– Sensing carriers to determine whether the medium is busy (or free) before sending a frame

=Variants– Non-persistent CSMA

• Send immediately if free. Otherwise, exponential backoff (wait for randomly determined period)

– 1-persistent CSMA• Send immediately if free. Otherwise, send immediately after

channel becomes free• Used in old Ethernet

– p-persistent CSMA• Send in probability p if free. Otherwise, send in same

probability after channel becomes free


Collision Detection/Avoidance=CSMA/CD

– For ethernet (but going to be not used)– terminate transmission as soon as a collision is detected

even while transmitting

=CSMA/CA– For wireless (still important mechanism)– Deferring transmission for a random interval if the channel is

busy

=CSMA/CA with RTS/CTS– Channel reservation mechanism by interaction of request-to-

send frame and clear-to-send (approved) frame


Controlled or Contention?

=Controlled assignment of partitioned channel is for higher efficient channel occupying (high throughput)

– TDMA (time), FDMA (frequency), WDM (wave length)

– Code Divided Multiple Access (CDMA)

=Contention type (random access) has its long history,

but CSMA/CD with binary back-off is the final answer.

– Pure ALOHA, Slotted ALOHA

• classic & primitive form of random access

– CSMA, CSMA/CD, CSMA/CD with binary back-off (Ethernet)

• More complicated form for avoiding unnecessary collisions.

• Carrier Sense is pre-action, Collision Detection is post-action.

– CSMA/CA (Collision Avoidance)

• More aggressive way to manage channels, WiFi.


MAC Throughput Performance


Evolution of data link technologies


Conventional Ethernet (10base5/2)=Electrical signal will be

attenuated as going far=Repeater amplifies

electrical signal


Physical

Repeater

Share a same medium (same datalink)

Bus

https://www.sqa.org.uk/e-learning/NetTechDC01CCD/page_12.htm

Bridge basics: Transparent bridge=Host is not aware of the bridge=Transparent bridge

– No modification of MAC frame– Promiscuous: capture all flowing packets– Administrator builds the bridge forwarding table


A

B

C

D

E

F

G

H

Transparent bridge

1 2

Fwd to 1 A, B, C, DFwd to 2 E, F, G, H

not share a medium but same datalink

Learning Bridge=Dynamic adaptation for topology changes & traffic

loop avoidance.– “Frame forwarding tables” in bridges are maintained for

optimizing the flow:• Any frame to unknown MAC addresses is forwarded, and the

table is updated for “unknown” MAC.• Any frame to known MAC addresses is forwarded if necessary.

• Spanning Tree Protocol (STP) is now very common for 802.3 families to avoid traffic loop.

– Exchanging data between bridges to form a singe spanning tree as their forwarding route.

– Today: improved protocol called RSTP (Rapid STP).


Switched media / full-duplex=Fully occupy a medium

– No competition

=Full-duplex


Non-blocking switch

Occupied medium but same datalink

Send (100Mbit/s)

Receive (100Mbit/s)

unused

http://www.aim-ele.co.jp/tech/metal-tech6/

Wireless LAN performance secrets=Wireless LAN performance will lag behind forever=Wireless LAN remains to be shared media

– Significantly slower, error prone– “crowded cocktail party” -- Don’t expect same performance


Switched mediaEthernet

1996 1998 2000 2002 2004 2006

Voice Video(MPEG2)

Video(D1)

Video(MotionJPEG)

Video(HD D1)

Voice Video(MPEG2)

Video(MotionJPEG)

Shared mediaWireless LAN

MAC in Wi-Fi=The MAC layer of Wi-Fi defines rate adaptation=Transmission rate is inversely proportional to the

distance from base station to the terminal node=You are effectively

sharing airtimewith others


Summary=Basic ideas of Data Link Layer

=Many simple but effective scheme to obtain good performance– Error control, flow control– Media access control (MAC)– Switched media & learning bridge– Ethernet families


Assignment 2: Explore datalinks= Use the frame generator tool provided in the hands-on

https://iplab.naist.jp/lectures/internet-engineering/2018/materials/= Collaborative work in your group

– Capture frames sent/received to/from someone in your group1. Explain frames you captured (in two cases that you sent and you received)2. Draw (or explain) the hierarchical protocol architecture the tool used

– Explore a datalink3. Identify the range of a same datalink in ethernet (LAN) of your labs, wireless

(mm2010), or wherever you choose, and then explain what you did and what you found

– You can share this in your group but you must clear which part you did

= Optional assignment1. You must have the tool written in C and Python. Explain the both codes

and their (little) difference. Then, you’ll get an extra scores2. IEEE802.11 frame format is not the exact same with ethernet frame.

Investigate the reason why we can use ethernet frame (by the tool) on WLAN.

= Report: [name]-[studentID].pdf submit via email– Both English and Japanese are acceptable.

= Deadline: July 10, 2018 17:00 JSTInternet Engineering / 2018 52

� Even if you do not finish, you should submit.

physical and data link layersphysical and data link layers yuzo taenaka graduate school of science...

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