CSCD 330 Network Programming

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1 CSCD 330 Network Programming Spring 2018 Lecture 17 Link Layer Hardware and Protocols Who is this? Reading: Chapter 5 in text Some slides provided courtesy of J.F Kurose and K.W. Ross, All Rights Reserved, copyright

2 Robert (Bob) Metcalf Inventor of Ethernet Bob Metcalfe Helped build early Internet while still an undergraduate Invented Ethernet while at Xerox Palo Alto Research Center Founded 3Com Corp. Served as publisher at IDG's Infoworld Publishing Co. Wrote three books Since January 2001 has been a venture capitalist with Polaris Ventures Nice article on him if you want to read it 2

3 Introduction to Link Layer So far, Application Layer - applications interface to people Messages get delivered Transport Layer application to application Segments get delivered Network Layer destination to destination Datagrams get delivered Link Layer node to node delivery Frames are delivered 3

4 Link Layer Overview Main question at this layer How do I deliver packets or frames between two nodes in the network? Historical reasons look at other link layer schemes But, Ethernet is currently dominant wired technology Wireless another protocol Actually a variation of Ethernet Covered in a different chapter, in a different course and at a future time in a galaxy far away... 4

5 Link Layer a Introduction and Services NIC Cards Error detection and correction Multiple access protocols Link-layer Addressing Ethernet 5

6 Link Layer - Introduction Link Layer Terms Both Hosts and Routers Called nodes Communication channels connect adjacent nodes along communication path, links Wired links Wireless links Layer-2 packet is a Frame encapsulates datagram Data-link layer responsible for transferring a frame from one node to adjacent node over a link 6

7 Link Layer Services The services offered by Link Layer are... Framing Link Access Reliable Delivery Flow Control Error Detection and Correction Half duplex and full duplex 7

8 Link Layer Services Framing Encapsulate datagram into frame, add header, trailer MAC (Media Access Control) addresses used in frame headers to identify source, destination Example MAC address: 00:13:02:BA:43:56 Reliable Delivery Between Adjacent Nodes Wireless links: higher error rates, Does more for reliability than wired Does compute errors Wired relies on upper layers to handle it Channel Access Share channel if shared medium Point to point if not shared 8

9 Reliable Delivery Question on Reliability So, why would you need both link-level and transport layer reliability? Because dealing with reliability at two levels! Transport layer reliability For end-to-end delivery between two processes Link layer reliability For delivery between two nodes connected by a single link 9

10 Link Layer Services Continued Flow control Pacing between adjacent sending and receiving nodes Error detection Errors caused by signal attenuation, noise Receiver detects presence of errors: Signals sender for retransmission or drops frame 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 10

11 Link Layer Implementation Where is the link layer implemented? Hardware or software? Separate card or on chip? Mostly Implemented Hardware Network Interface Card (NIC) 11

12 Link Layer Implementation Alternative to actual NIC card... Built in to the motherboard

13 Link Layer Implemented In each host Link layer implemented in Network adaptor Network Interface Card (NIC) Ethernet card, PCMCI card, card Implements link and physical layers Attaches to host s system buses Combination of hardware, software, firmware host schematic application transport network link cpu memory host bus (e.g., PCI) controller link physical physical transmission network adapter card 13

14 Network Interface Cards (NIC) Basics A Network Interface Card (NIC) Establishes link between computer and network, and manages link A NIC performs two crucial tasks: 1. Establishes and manages computer s network connection 2. Encodes and translates digital data into signals for outgoing messages, and Translates signals into digital computer data for incoming messages DataLink Layer 14

15 Parallel vs. Serial Transmission NICs also manage transformation of network data s form Computer bus has series of parallel data lines (Parallel transmission) Signals traversing network media consist of a linear sequence of bits of data (serial transmission) NIC takes outgoing transmission in parallel form and recasts them into their serial equivalents For incoming messages, process reverses Converting between serial and parallel transmission, NIC has memory, to hold data temporarily DataLink Layer 15

16 Parallel vs. Serial Transmission Bus width Bus width - number of parallel lines in a computer bus Industry Standard Architecture (ISA) bus 16-bit Now obsolete!!! Peripheral Component Interconnect (PCI) bus 32-bit, 64-bit, of course faster DataLink Layer 16

17 Network Interface Card Heart of card is a special purpose processor chip Does framing, link access, flow control and error detection Encryption too, for wireless Examples Intel 8254x controller implements Ethernet, Atheros AR5006 controller handles protocol Trend... Integrate adapters into motherboard, not separate cards anymore Particularly true for laptops and phones devices 17

18 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 18

19 Error Detection and Correction Causes of Transmission Errors Signal noise, attenuation or distortion Both on wired and wireless mediums See next slide...

20 Definitions What is Signal Attenuation? Loss of signal strength measured in decibels (db) dec i bel - Defined a unit used to express differences in power, in acoustics or electronics: equal to ten times the common logarithm of the ratio of two signals. Occurs for several reasons Range - both wireless and wired transmissions gradually dissipate in strength over longer distance Interference - on wireless networks, radio interference or physical obstructions like walls dampen communication signals Wire size - on wired networks, thinner wires suffer from higher (more) attenuation than thicker wires

21 Errors at Link Layer Two Ideas for Errors at Link Layer Error detection Parity checks, cyclic redundancy codes, checksum Error correction Send redundant information with frame When receiving frame incorrectly, receiver makes educated guess about the original frame Often done for missing frames in real-time applications

22 Error Detection Error Detection Lets Receiver Determine whether received message has been corrupted Checksum simplest method Checksum, function of message, gets appended to message Receiver uses same function to calculate checksum of received message and compare it with appended checksum

23 Example of Checksum Internet Checksum Goal: Detect errors (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Sender: Treat segment contents as sequence of 16-bit integers Checksum: Addition (1 s complement sum) of segment contents Only done on IP packet header Receiver: Compute checksum of received segment Check if computed checksum equals checksum field value: NO - error detected YES - no error detected DataLink Layer 23

24 Error Detection Parity Checking Even Parity If you get odd number of bits including parity bit > A bit changed in transmission Single parity bit is an error detecting code, Not an error correcting code No way to determine which bit is corrupted Two-dimensional parity checks parity of both column and row, pinpoints error and allows for correction 24

25 Parity Checking Simplest Technique Two Dimensional Bit Parity Single Bit Parity Detect and correct single bit errors Detect single bit errors s Add a 1 Simple detection For Even Parity Number of 1 s in data + parity bit must be an Even number Can locate and correct errors 25

26 Parity Checking Comment Is a limit to parity schemes A parity bit only guaranteed to detect an odd number of bit errors One, three, five, and so forth If even number of bits (two, four, six and so on) are flipped, the parity bit appears to be correct, even though the data is corrupt This is for even parity Odd parity will have the reverse problem 26

27 Checksumming Cyclic Redundancy Check (CRC) Want algorithm detects some number of changed bits Ideally... implemented in hardware CRC Scheme CRC algorithms treat message as an enormous binary number, Divide it by another fixed binary number, Make remainder from this division the checksum Widely used in practice (Ethernet, WiFi, ATM) It has better powers of detection than bit parity or checksums uses entire number, handle more bit errors nice example below 27

28 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3 Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 28

29 Access Links and Protocols 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) Old-fashioned Ethernet wireless LAN Satellite shared wire (e.g., shared RF cabled non-switch Ethernet)(e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) 29

30 Communication Between Nodes Point to Point Links These are dedicated channels between a node and switch or modem No need to share the link Broadcast Links Multiple sending and receiving nodes all connected to single broadcast channel Central problem at link layer How to share the link between nodes Must coordinate access between competing nodes Known as Multiple Access Problem

31 Multiple Access Problem Question is How do you fairly share the medium at the Link level? Everyone is connected to same network All want to send their data all the time. How do we fairly share the medium?

32 Multiple Access Protocols Idea is... can send and receive in both directions single shared broadcast channel Two or more simultaneous transmissions by nodes means interference Collision if node receives two or more signals at the same time gibberish, must retransmit, BAD! Multiple Access Protocol Distributed algorithm, determines how nodes share channel, i.e., determine when node can transmit Communication about channel sharing must use channel itself! No out-of-band channel for coordination 32

33 Examine Following Strategies Look at each of the strategies and see how close each meets the IDEAL Multiple Access Channel properties...

34 Multiple Access Protocols: A Taxonomy Three broad classes 1. Channel Partitioning Divide channel into smaller pieces Time slots, frequency, code division Allocate piece to node for exclusive use 2. Random Access Channel not divided, allow collisions Recover from collisions 3. Taking turns Nodes take turns, but nodes with more to send can take longer turns 34

35 Channel Partitioning

36 Channel Partitioning Protocols: TDMA TDMA: Time Division Multiple Access Access to channel in "rounds" Each station gets fixed length slot Length = packet transit time, in each round Whats the problem? Unused slots wasted Example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame

37 TDMA: Time Division Multiple Access Analogy Take turns talking Fix each person a time slot All get a turn even when they have nothing to say Our class 5 slot limit Eric Talks Kevin Silent Ian Talks Lana Silent Mark Talks Wasted two slots where others could have had useful things to say 37

38 Channel Partitioning Protocols: FDMA FDMA: frequency division multiple access FDM cable frequency bands Channel spectrum divided into frequency bands Each station assigned fixed frequency band Whats the problem? Some frequency bands are idle, you are limited to your band Example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle time 38

39 Random Access Protocols

40 Random Access Protocols When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes Two or more transmitting nodes - collision Random access Medium Access Control protocol specifies Key Problems How to detect collisions How to recover from collisions Examples Slotted ALOHA, ALOHA CSMA, CSMA/CD, CSMA/CA 40

41 Aloha Protocol Simple protocol developed by Norman Abramson and colleagues at University of Hawaii, in 1968 Used low-cost amateur radio-like systems to create a computer network linking distributed campuses of the University Protocol allows every system to send a frame if its ready to send Based on a shared medium network This was basis for Ethernet!!!!

42 Slotted ALOHA Assumptions All frames same size Time divided into equal size slots Time to transmit 1 frame Nodes transmit only at slot beginning Nodes are now synchronized If 2 or more nodes transmit in slot, all nodes detect collision 42

43 Slotted ALOHA Operation When node obtains fresh frame, transmits in next slot If no collision, node can send new frame in next slot If collision, node retransmits frame in each subsequent slot with probability p until success Probability is key and is between 0 and 1

44 Slotted ALOHA Probability is key Probability p means each node independently tosses a coin, heads or tails P says how likely it is node gets a head Node transmits under this event (1-p) is the probability of getting tail on a coin Means node skips a slot and tosses again in next slot 44

45 Slotted Aloha C collision E empty S - Success

46 Slotted ALOHA Pros Single active node can continuously transmit at full rate of channel Highly decentralized: Slots in nodes need to be in sync Simple C collision E empty S - Success Takes 9 slots Cons Collisions, wasting slots Idle slots Nodes may be able to detect collision in less than time to transmit packet Clock synchronization 46

47 Slotted Aloha efficiency Whats the Efficiency of Aloha? Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send) Suppose: N nodes with many frames to send, each transmits in slot with probability p Probability that given node has success in a slot = p(1-p)n-1 Prob that any node has a success = Np(1-p)N-1 Max efficiency: find p* that maximizes, successful nodes Np(1-p)N-1 For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives: Max efficiency = 1/e =.37 At best: channel used for useful transmissions 37% of time!! 47

48 Slotted ALOHA Throughput versus offered traffic for ALOHA systems.

49 CSMA (Carrier Sense Multiple Access) Max throughput achievable by slotted ALOHA is CSMA improved throughput compared to Aloha protocols. Big Improvement... Listen before you speak! Listens to the channel before transmitting a packet (avoid avoidable collisions) 49

50 Assumptions with CSMA Networks 1. Constant length packets 2. No errors, except those caused by collisions 3. Each host can sense the transmissions of all other hosts 4. The propagation delay is small compared to the transmission time 50

51 CSMA (Carrier Sense Multiple Access) CSMA: Listen before transmit: If channel sensed idle Transmit entire frame If channel sensed busy Defer transmission Human analogy: Don t interrupt others! 51

52 CSMA collisions spatial layout of nodes Collisions can still occur: propagation delay means two nodes may not hear each other s transmission B sends at t0 D sends at t1 Collision: entire packet transmission time wasted 52

53 CSMA/CD (Collision Detection) CSMA/CD: Carrier sensing as in CSMA Collisions detected within short time Colliding transmissions aborted, reduces channel wasting time Collision Detection Easy in wired LANs Measure signal strengths, compare transmitted and received signals Difficult in wireless LANs: Received signal has problems with signal strength due to attenuation and interference 53

54 CSMA/CD Collision Detection 54

55 CSMA/CD Sense the channel If idle, transmit immediately If busy, wait until the channel becomes idle Collision detection Abort a transmission immediately if a collision is detected Try again later after waiting a random amount of time Will revisit this later when we describe Ethernet... 55

56 Taking Turns 56

57 Taking Turns MAC protocols Polling: Master node invites slave nodes to transmit in turn Typically used with dumb slave devices Bluetooth uses polling Concerns: Polling overhead Latency Single point of failure (master) data poll master data slaves 57

58 Token Passing Protocols Token Ring protocol was developed by IBM mid1980s Access involves token-passing Token Ring, computers are connected so that signal travels around the network from one computer to another in a logical ring A single electronic token moves around ring from one computer to the next If computer does not have information to transmit, it simply passes the token on to the next workstation If computer wishes to transmit and receives an empty token, it attaches data to the token

59 Token Passing Protocols Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable It operates, transmission speeds of 4 Mbps or 16 Mbps Due to the increasing popularity of Ethernet, Use of Token Ring decreased dramatically

60 Taking Turns MAC protocols Token passing: Control token passed from one node to next sequentially. Token message Concerns: Token overhead Latency Single point of failure (token) T (nothing to send) T data 60

61 Summary of Protocols Channel partitioning, by time, frequency or code Time Division, Frequency Division Random access (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD Carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in Taking turns Polling from central site, token passing Bluetooth, FDDI, IBM Token Ring 61

62 End 62

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