Physical and Data Link layers - NAISTData link layer Protocol Data Unit (PDU) – Defining the frame...

Physical and Data Link layers

Youki Kadobayashi Graduate School of Information Science�

Nara Institute of Science and Technology

Physical Layer�

Copyright(C)2015 Youki Kadobayashi. All rights reserved. 2 15/05/01

Types of transmission medium�! Cables�

–  Optical fiber�

–  Copper�

! Wireless�

15/05/01� Copyright(C)2015 Youki Kadobayashi. All rights reserved.� 3�

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� Cost�10Gbit/s� Optical (SMF)� 10 km� $$$�10Gbit/s� Copper� 10 m� $$�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�

Basics of protocol�


Computer and network�! Computer�! Network interface�! Protocol�


ihihhiiihhi

ihihhiiihhi hiiiiiih end bit pattern start bit pattern hiiiiiih

Fundamental aspect of network: Protocol�! 3 major elements of protocol�

–  Finite State Machine�–  Message�–  Timer�


a� b�

c� d�

α�

β�

γ�

δ�ε�

Basic constructs of protocol�! Main goalj0Transmission, recovery from errors�

! Message�–  Header, trailer�–  Error detection�–  Sequence number�–  Acknowledgement�

! State machine�–  Negotiation�–  Retransmission�–  Error recovery�

! Timer�–  Timeout�


Header�

Data�

Trailer�

Error detection�Sequence number�

Acknowledgement�

Data Link Layer�


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 ! Half-duplex and full-duplex

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

15/05/01 Copyright(C)2015 Youki Kadobayashi. All rights reserved. 12

Frame ! Data link layer Protocol Data Unit (PDU)

–  Defining the frame borders (delimiters)

! Can determine if any failures (bit errors) occured –  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 01111110 checksum

header payload

Information Network 1 / 2013

Copyright(C)2015 Youki Kadobayashi. All rights reserved. 14

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 sequencej 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 01111110 checksum

Information Network 1 / 2013 15/05/01

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

–  Protocol techniques •  Timer •  Retransmission


Basic idea of CRC�! Given:

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

! 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)

! Successul 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��26��23��22��16��12��11��10��8��7��5��4��2��

–  CRC-CCITT •  x16+x12+x5+1

! There are many other error detection codes.�



Questions?�


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

Copyright(C)2015 Youki Kadobayashi. All rights reserved. 20 Information Network 1 / 2013 15/05/01


Stop-and-wait ARQ (1)

t1 t2 t3

t4

t5 t1 Sender

Receiver

t1: Round Trip Time t2: Frame Transmission Time t3: Frame Processing Time t4: ACK Transmission Time t5: 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 6 5 3 4 5 4 3 2

1 6 5 3 4 5 4 2

Time out for Frame3�

!!



Selective-Repeat ARQ

1 8 7 3 6 5 4 3 2

1 8 7 3 6 5 4 2

Time out for Frame3�

!!


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 –  Complicated scheme that requires buffer, timer, and ACK

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



Window Flow Control for selective-repeat ARQ

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 …

sent frame� sendable framedmaximum size W)

already received ACK�

last sent frame� if sent move to right

If ACK is received move to right�

keep frame until ACK is received�

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 …

received frame� receivable framedmaximum size W)

already transmitted ACK�

last received frame�if received move to right�

if ACK is sent move to right

Sender

Receiver

15/05/01


Burden sharing among layers�! Assignment of function depends on communication

system designs ! Various solutions exist�

Data Link

Network

Transport sequence assurance flow control retransmission interconnection of network error detection and correction frame boundary�



Questions?�



Sublayers of the Data Link Layer�

Physical Layer

Data link Layer

Network Layer

CCITT X.25 (HDLC/LAPB)

Media Access Control Sublayer

8802/2 LLC

8802/3 CSMA/CD

8802/5 Token Ring

8802/4 Token Bus

Logical Link Control Sublayer

ISO/OSI Local Area Network Definitions (8802)

CCITT Data link Layer Definition


Media Access Control (MAC) (1)�


Data link layer provides packet send/receive service to network layer

Physical Layer provides binary send/receive to data link layer

Different media have different constraints about multiple nodes

accessing the medium

15/05/01

Media Access Control (MAC) (2)�


MAC layer provides medium access service to the data link layer

A separate protocol is needed to implement the service for each different transmission medium

Subsequent slides: learn about channel allocation (multiplexing)�

15/05/01

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)�Three techniques: ! Channel Partitioning

–  Divide channel into smaller “pieces” (time slots, frequency, code)

–  Allocate piece to node for exclusive use ! Random Access

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

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


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�



Questions?�

15/05/01

Evolution of data link technologies�! Wide bandwidth

!  Large scale

!  Virtualization

! Coverage expansion�

!  Switched media

!  Bridges

!  VLAN

!  Broadband wireless, residential access, etc.


LAN performance secrets: shared media → switched media ! High-bandwidth and commodity LAN ! Effectively a channel partitioning scheme


medium

access

Non-blocking crossbar switch

15/05/01

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 media Ethernet

1996 1998 2000 2002 2004 2006

Voice Video (MPEG2)

Video (D1)

Video (MotionJPEG)

Video (HD D1)

Voice Video (MPEG2)

Video (MotionJPEG)

Shared media Wireless LAN

15/05/01

Large scalej Bridges ! Compatibility between physical limitations and LAN

convenience –  Coverage, capacity


Wiring between buildingsjoptical fiber d�5kme�

Wiring in floorjcoax d�100me�

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, D Fwd to 2 E, F, G, H

15/05/01

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).



Questions?�


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 III

1.  Given: a.  Message: ‘Aloha’ (without the apostrophe)

(www.asciitable.com)

b.  Generator polynomial : 1+X+X3+X4+X8

Show how CRC-8 works on sender and receiver side

1.  Do you think the CRC method is foolproof? Please elaborate.

2.  On selecting the best generator polynomial, what is your recommendation?

Guidelines for submitting the assignments

bDeadline: 5/8 (Fri) 23:59 bFile Naming: Name_StudentID bPreferred File Format: PDF bLength: at most 2 pages bLanguage: English/Japanese bSend to: [email protected]

&/I 1.  `[ZcY: ’Aloha’(‘U,?��LO)

(www.asciitable.com) ��-�: 1+X+X3+X4+X89HAD� CRC-8=(��f��GILR;K��BT< Cg

2.  CRCM��G�.G>TH�;<g$'AJ@: 3.  ��-�U*�BTHAFf)�JQLMJK<'NJ@:g

&/��KE:F

b!P�S: 5/8 (+) 23:59 b��]VWa : _��ID b��]VWa]Xc_[\: PDF b^cY�: ��2^cY b#%: "%/��% b��: [email protected]

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Physical and Data Link layers - NAISTData link layer Protocol Data Unit (PDU) – Defining the frame...

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