Physical and Data Link layers - NAISTSublayers 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

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Physical and Data Link layers

Youki Kadobayashi Graduate School of Information Science

Nara Institute of Science and Technology

Physical Layer

Copyright(C)2014 Youki Kadobayashi. All rights reserved. 14/04/25

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Types of transmission medium ●  Cables

○  Optical fiber

○  Copper

●  Wireless

14/04/25 Copyright(C)2014 Youki Kadobayashi. All rights reserved.

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

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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

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Basics of protocol


Computer and network ●  Computer ●  Network interface ●  Protocol


１０１００１１１００１

１０１００１１１００１

０１１１１１１０

end bit pattern start bit pattern ０１１１１１１０

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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 goal： Transmission, 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

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Ethernet frame

http://www.maximumpc.com/files/u22694/ethernet-howitworks.jpg

14/04/25 Information Network 1 / 2014

Data Link Layer


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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


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 / 2014

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Copyright(C)2014 Youki Kadobayashi. All rights reserved.

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 01111110 checksum


14/04/25

Errors in Physical Layer


Noise

Attenuation

Distortion

Information Network 1 / 2014 14/04/25

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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


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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.



Questions?


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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)2014 Youki Kadobayashi. All rights reserved. Information Network 1 / 2014 14/04/25


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


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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

ACK

ACK

ACK

ACK

ACK

ACK

Time out for Frame3

!!


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Selective-Repeat ARQ

1 8 7 3 6 5 4 3 2

1 8 7 3 6 5 4 2

ACK

ACK

ACK

ACK

ACK

ACK

Time out for Frame3

!!

ACK

ACK


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.


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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 frame（maximum 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 frame（maximum size W)

already transmitted ACK

last received frame if received move to right

if ACK is sent move to right

Sender

Receiver

14/04/25


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


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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


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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

14/04/25

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)

14/04/25

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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


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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.


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MAC Throughput Performance



Questions?

14/04/25

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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

14/04/25

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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

14/04/25

Large scale： Bridges ●  Compatibility between physical limitations and LAN

convenience ○  Coverage, capacity


Wiring between buildings：optical fiber （～5km）

Wiring in floor：coax （～100m）

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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

14/04/25

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).


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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

●  How can we bind Datalink layer & network layer? ○  Address mapping – ARP and ND ○  Multiplexing & demultiplexing – protocol architecture in OS


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Hands-On Activity 1


Objectives

●  Part 1 ○  Learn basic use of VMWare and Wireshark ○  Understand the TCP/IP Model via packet inspection

using Wireshark

●  Part 2 ○  Familiarize with the protocol stack ○  Understand the data link layer thru Wireshark

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Part 1: Protocol Architecture

http://mabricksmumblings.files.wordpress.com/2014/01/osi_model_lan1.jpg

Recall: The OSI Model

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WWW - World Wide Web

DNS - Domain Name System

P2P - Peer to Peer

FTP - File Transfer Protocol

SMTP - Simple Mail Transfer Protocol

ICMP - Internet Control Message Protocol

IP - Internet Protocol

IPsec - Security Architecture for Internet Protocol

Ethernet

PPP - Point to Point Protocol

ARP - Address Resolution Protocol

RJ-45 - Registered Jack 45

TCP - Transmission Control Protocol

UDP - User Datagram Protocol

JPEG - Joint Photographic Experts Group

MP3 - Moving Picture Experts Group Layer-3 Audio

HTML - Hyper Text Markup Language

SIP - Session Initiation Protocol

RTP - Real-time Transport Protocol

Review: Glossary of Layer Protocols

Recall: OSI Model vs. TCP/IP


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Virtual Machine (VM)

•  Virtual Machine allows you to run another OS on your PC o  e.g, run a Linux OS on a Windows/Mac host OS

•  Virtual Machine players o  Oracle VirtualBox, VMware, etc.

Let’s parallely run a Linux OS in your PC!!!


Running VM and Logging in

user: information-network2014 pwd: network2014

VM player VM


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Wireshark command menus

Listing of captured packets

Details of selected packet headers

Packet contents in hexadecimal and in ASCII

Perfect Encapsulation

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Physical Layer

ethernet, IP, TCP, and http are coming!


Data Link

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Network

Transport

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Application


Part 2: Data Link

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Indications •  Turn off your wireless connection •  Wireshark: start the live capture •  Turn on your wireless connection •  Wireshark: stop the live capture

Filtering

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Filtering

Ethernet Frame: Exploring 802.3

14 bytes

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Ethernet Address: Src & Dst Hexadecimal value of the field Type

Address Resolution Protocol: ARP (RFC 826)

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ARP cache arp -a

ARP cache: Multiple Network interfaces

ARP cache of interface 1



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Wireshark ARP packets

ARP request

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ARP reply

ARP Gratuitous

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Assignment 2 (1) Using wireshark, observe traffic generated by other applications and depict protocol stack, according to wireshark output. Indicate which protocol belongs to which OSI-7 layer in an illustration.


Assignment 2 (2)


A frame: ff ff ff ff ff ff 84 2b 2b 47 94 62 08 89 00 01 08 00 06 04 00 02 84 2b 2b 47 94 62 a3 dd 34 5e 00 00 00 00 00 00 a3 dd 34 5e 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 Inspect the above frame and look for error(s). Then, explain the root cause of the error(s).

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Assignments: Miscellaneous

Deadline: 2014/05/1 at 23:59

Submit to: network1-2014 _at_ is.naist.jp

File nomenclature: Name_StudentID.pdf

Assignment 2: at most 4 pages.

Language: Japanese or English


Documents

Physical and Data Link layers - NAISTSublayers 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