Physical and Data Link layers. Youki Kadobayashi Graduate School of Information Science Nara Institute of Science and Technology

Size: px

Start display at page:

Download "Physical and Data Link layers. Youki Kadobayashi Graduate School of Information Science Nara Institute of Science and Technology"

Dorthy Arnold
5 years ago
Views:

1 Physical and Data Link layers Youki Kadobayashi Graduate School of Information Science Nara Institute of Science and Technology

Types of transmission medium = Cables Optical fiber Copper Source: siemon.

Cables and connectors = Copper UTP STP =

4 Cables and connectors = Copper UTP STP = Connectors RJ45 RJ11 Source: aisan = Optical fibers Single mode fiber Multimode fiber = Connectors LC, SC, FC, MT-RJ Source: aisan RJ45 connector. Source: flukenetworks.com Source: aisan.co.jp Copyright(C)2016 Youki Kadobayashi. All rights reserved. 4

5 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 5

6 Physical characteristics: a crude comparison 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 = 54Mbit/s in wireless cannot be delivered as advertised, whereas 1Gbit/s in optical fiber can be delivered as advertised. Copyright(C)2016 Youki Kadobayashi. All rights reserved. 6

7 Hands-on: create and test your own UTP cables o o o Let s form 3 groups Each group will collaborate using Google Docs on a tutorial on how to make an UTP cable. o The tutorial should be in all the mother languages of the group members. (e.g. if the group members are from France, Malaysia and Thailand, the tutorial should be in French, Malay and Thai) After putting together the tutorial, the students should follow it. The end goal is to have a working UTP cable. *Note: We will provide all the necessary materials to make and test the cables. Copyright(C)2016 Youki Kadobayashi. All rights reserved. 7

Computer and network = Computer = Network interface = Protocol 10100111001 start bit pattern

11 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 Sequence number Heade r Acknowledgemen t Data Error detection Trailer Copyright(C)2016 Youki Kadobayashi. All rights reserved. 11

13 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 13

14 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 address control data checksum head er payloa d Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 14

15 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: if sender detects in data, it stuffs a 0 right after. if receiver detects in data, it deletes the following stuffed address control data checksum Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 15

17 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 Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 17

18 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 x r M(x): frame with r zeros Compute modulo of x r M(x) divided by G(x): R(x) Frame for transmission: F(x) F(x) = x r M(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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 18

19 Standardized CRC polynomials = Commonly known standards CRC-12 x 12 +x 11 +x 3 +x 2 +x+1 CRC-16 x 16 +x 15 +x 2 +1 CRC-32 x 32 +x 26 +x 23 +x 22 +x 16 +x 12 + x 11 +x 10 +x 8 +x 7 + x 5 +x 4 +x 2 +x+1 CRC-CCITT x 16 +x 12 +x 5 +1 = There are many other error detection codes. Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 19

21 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)2016 Youki Kadobayashi. All rights reserved. 21 Information Network 1 / 2016

22 Stop-and-wait ARQ (1) Send er t 1 t 5 t 4 t 1 t 2 t 3 t 1 : Round Trip Time t 2 : Frame Transmission Time t 3 : Frame Processing Time t 4 : ACK Transmission Receiv er Time Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 22

23 Stop-and-wait ARQ (2) = Procedure Waiting to receive ACK on each frame transmission Setting a sender timer greater than 2t 1 +t 2 +t 3 +t 4 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 Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 23

26 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. Copyright(C)2016 Youki Kadobayashi. All rights reserved. 26 Information Network 1 / 2016

27 Window Flow Control for selective-repeat ARQ Send er sent frame keep frame until ACK is received sendable frame(maximum size W) Receiv er already received ACK last sent frame received frame if sent move to right If ACK is received move to right receivable frame(maximum size W) already transmitted ACK if received if ACK is sent last received Copyright(C)2016 frame move Youki to Kadobayashi. right All rights move reserved. to right 27

28 Burden sharing among layers = Assignment of function depends on communication system designs = Various solutions exist Transport Network Data Link sequence assurance flow control retransmission interconnection of network error detection and correction frame boundary Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 28

30 Sublayers of the Data Link Layer Network Layer Data link Layer Logical Link Control Sublayer Media Access Control Sublayer ISO/OSI Local Area Network Definitions (8802) 8802/2 LLC 8802/3 CSMA/C D 8802/4 Token Bus 8802/5 Token Ring CCITT Data link Layer Definition CCITT X.25 (HDLC/LAPB) Physical Layer Copyright(C)2016 Information Youki Kadobayashi. Network 1 / 2016 All rights reserved. 30

Media Access Control (MAC) (1) Data link layer provides packet send/receive service to network layer Physical Layer provides binary send/receive to data

31 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 31

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

32 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) Copyright(C)2016 Youki Kadobayashi. All rights reserved. 32

33 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 wireless LAN Copyright(C)2016 Youki Kadobayashi. All rights reserved. 33 Information Network 1 / 2016

34 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 34 Information Network 1 / 2016

35 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 35 Information Network 1 / 2016

36 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. Copyright(C)2016 Youki Kadobayashi. All rights reserved. 36 Information Network 1 / 2016

39 Evolution of data link technologies = Wide bandwidth = Large scale = Virtualization = Coverage expansion = Switched media = Bridges = VLAN = Broadband wireless, residential access, etc. Copyright(C)2016 Youki Kadobayashi. All rights reserved. 39

40 LAN performance secrets: shared media switched media = High-bandwidth and commodity LAN = Effectively a channel partitioning scheme medium access Non-blocking crossbar switch Copyright(C)2016 Youki Kadobayashi. All rights reserved. 40

41 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 Shared media Wireless LAN Voice Video (MPEG2) Video (MotionJPEG) Switched media Ethernet Voice Video (MPEG2) Video (MotionJPEG) Video (D1) Video (HD D1) Copyright(C)2016 Youki Kadobayashi. All rights reserved. 41

42 Large scale: Bridges = Compatibility between physical limitations and LAN convenience Coverage, capacity Wiring in floor:coax ( 100m) Wiring between buildings:optical fiber ( 5km) Copyright(C)2016 Youki Kadobayashi. All rights reserved. 42

43 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 C Transparent bridge E G B D 1 2 F H Fwd to 1 Fwd to 2 A, B, C, D E, F, G, H Copyright(C)2016 Youki Kadobayashi. All rights reserved. 43

44 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 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). Copyright(C)2016 Youki Kadobayashi. All rights reserved. 44

46 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 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 46 Information Network 1 / 2016

47 Homework = Imagine you are a Network Architect in a start-up network operator (no more than 1000 subscribers) and you have been tasked with building the Physical and Data link infrastructure = Please build an action plan for building the L1/L2 infrastructure Build a diagram of the L1/L2 infrastructure Explain why you chose one technology over another by highlighting the pros/cons = Format: [Name]-[StudentID].pdf = Submit it to: network1-2016@is.naist.jp = Deadline: April 20 (Wed) by 23:59 Copyright(C)2016 Youki Kadobayashi. All rights reserved. 47

Physical and Data Link layers

Physical and Data Link layers Youki Kadobayashi Graduate School of Science Nara Institute of Science and Technology Physical Layer All rights reserved. 2 Types of transmission medium! Cables Optical fiber