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Computer Networks - Theory and Practice

Computer Networks - Theory and Practice. CSE 434 / 598 Spring 2001. Sourav Bhattacharya Computer Science & Engineering Arizona State University. Class Objectives. Technical Goals: Provide basic training in the area of “Computer and Communication Networks”

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Computer Networks - Theory and Practice

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  1. Computer Networks - Theory and Practice CSE 434 / 598 Spring 2001 Sourav Bhattacharya Computer Science & Engineering Arizona State University

  2. Class Objectives • Technical Goals: • Provide basic training in the area of “Computer and Communication Networks” • A comprehensive protocol/algorithm level understanding of the “essentials” of a network • Concept driven, not implementation/package driven • Focus on core communication aspects, and not on cosmetics • Achieve a level where you are ready to learn about specific network implementations • Other Goals: • Learn to learn, Job well done, intellectual honesty, mutual “good wish”, promote research careers, …

  3. Success Criteria • At the end of the class • Class does well in the tests, and projects • Class has learnt the subject matter from the instructor • Instructor has inspired few (at least !!) career advancements • Instructor has improved the class material … • Don’t Do List • Instructor: Demonstration of “research” • Class: Inhibitions, shy to ask questions, interrupt...

  4. Text and Syllabus • Computer Networks, by Andrew S. Tannenbaum, 3rd ed., Prentice Hall, 1996 • Flow of Discussion • Chapter 1 and 2 - background, assumed !! • You are graduate students or undergrad seniors !! • Chapter 4 - Medium Access Sublayer • Chapter 3 - Data Link Layer • Chapter 5 - Network Layer • Chapter 6 - Transport Layer • Sporadic Coverages: Security and Encryption, Network Management, Multimedia, WWW, ... (as time permits)

  5. References • High-Speed Networks: TCP/IP and ATM Design Principles, by William Stallings, Prentice Hall • Network Analysis with Applications, by William D. Stanley, Prentice Hall • Local and Metropolitan Area Networks, by William Stallings, Prentice Hall • Protocol Design for Local and Metropolitan Area Networks, by Pawel Gburzynski, Prentice Hall • Introduction to Data Communications: A Practical Approach, by Larry Hughes, Jones and Burlett Publishers. • High-Speed LANs Handbook, by Stephen Saunders, McGraw-Hill

  6. The Network Design Problem: At A Glance • Design Analogy: N persons can successfully, and efficiently communicate amongst themselves, sharing individual, group and global views • Step 1: Two remote persons can communicate • Step 2: Three or more remote persons can efficiently share a common medium to exchange distinct views (but these people have to do the entire co-ordination by themselves) • Step 3: Increasingly convenient ways of doing Step 2 • Abstraction • Quality of Service • Value added features...

  7. Layered Protocol Hierarchies • Basic data transfer occurs at the lowest layer • The rest is merely solving “human problems” • Abstraction and convenience of access • Inter-operability • Making sure that multiple users do not fight • Or, if they do, at least gracefully, and with a recourse ... ... Layer N Layer N Layer 3 Layer 3 Layer 2 Layer 2 Layer 1 Layer 1 Physical Medium

  8. Quality of Connections • Issues • Layered Protocol Interfaces • Protocol Header, and Body • Nework Architecture • Network Architecture • Connections Type • Simplex, vs. Duplex • Connection-oriented, vs. Connection-less (datagram) • Life of connection, vs. Delay of setting up a new connection • QoS of Connections

  9. OSI Model Application Application • Open System Interconnection (OSI) Model • Data header at each layer • Real data transfer at the lowest layer • Logical data flow at upper layers Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Physical Medium

  10. TCP / IP Model Telnet, Ftp, Smtp, DNS, ... Application Application • Application layer controls everything above the Transport Layer (theme: “reduce the overhead”) TCP, or UDP Transport Transport IP Network Network Data Link Data Link ArpaNet, NSFNet, various LANs, ... } Physical Physical Physical Medium

  11. Network Standardization • International Standards Organization (ISO) • Various TCs, and Working Groups • ANSI (Am. Nat’l Standards Inst.) • NIST • IEEE • Internet Engineering Task Force (IETF) • Produces stream of RFCs

  12. Medium Access Control Chapter 4 of the Text

  13. Problem Introduction • Two or more contenders for a common media • Contenders: Independent nodes or stations with its own data/information to distribute • Distribute: one-to-one, one-to-many, one-to-all (routing, multicast, broadcast) • Data/Information: anything from a bit to a long message stream • Common media • Fiber, cable, radio frequency channel, ... • Characteristics of the media -- refer Chapter 2

  14. The Most Obvious Solution • N cars to share a common road • Two approaches • Slice the road width into N parallel parts, i.e., Lanes(hopefully each part will still be wide enough for a car) • Each car drives in its own Lane • Regulate the cars to drive on a rotation basis, i.e., one after the other • Careful co-ordination is critical • No width restriction. Each car can enjoy the entire road width !! • Problems • Naive, and simplistic • Opportunity for resource wastage

  15. The Two Naive Solutions... • Frequency Division Multiplexing (FDM) • For N user stations, partition the bandwidth into N (equally sized?) frequency bands • Each user transmits onto a particular bandwidth slot • No contention. But, likely under-utilization of bandwidth. • Time Division Multiplexing (TDM) • For N user stations, create a cycle of N (equally sized ?) time slots • Each user takes its turn, and transmits only during the corresponding time slot • No contention. But, likely under-utilization of the time slots

  16. Channel Allocation on “as needed” Basis • Instead of apriori partitioning of the channel resource (bandwidth, time) - employ dynamic resource management • Advantages include: reduced channel resource wastage • Disadvantages: • Require explicit (or, implicit) co-ordination of transmission schedules • Co-ordination can be of several categories • Detection and Correction • Avoidance • Prevention (contention-free !)

  17. Model and Assumptions • User stations or Nodes • Probability of a frame being generated in an interval T is L*T, where L is a constant for a particular user. • Independent in their transmissions. Can transmit a frame any time. • Concerns: • This model is not valid for co-related transmissions (e.g., performance analysis for a set of parallel/distributed programs or threads) • Single channel Assumption • No second medium is available among the stations to communicate (data, and/or control information) • Concern: this assumption is not true for many environments, where the control information may be carried on a second channel.

  18. Model (contd...) • Carrier Sense, or No Carrier Sense • Before transmission nodes can (or, cannot) sense if the channel is currently busy due to another user’s message • Protocols can be lot more efficient if “Carrier Sense” is true • Issue: It is hardware, and analog device specific • Activation Instances • Continuous time: a message can be attempted for transmission at any time. There is no master clock. • Slotted time: a message can be delivered only at a fixed set of points in time. Time axis is discretized. Requires a master clock.

  19. ALOHA - A Simple Multiple Access Protocol • N user stations, randomly generating data frames • Anytime data is ready ---> transmit on the media(without care for collison) • Listen to the channel, and find out if there is/was collison • If collison, then wait for a random time and goto step 1 • Collision vulnerability period • If frame time = t, then vulnerability period = 2t • Reason: two frames can collide (head, tail) or (tail, head) at the extreme ends • Refer Figure 4-2

  20. insert figure 4-2 here

  21. Performance of ALOHA • A lot of nodes are suddenly jumping into the shared, common channel - What can you expect about the performance ? • G = # frame transmission attempts (including new, and re-transmission) • Thus, during a 2-frame vulnerability period (refer Fig 4-2) there will be 2G frames generated • Probability that k frames are generated during a given vulnerability period = ((2G)^k * e^(-2G)) / k! • Probability that no frame will be generated, i.e., k=0, => e^(-2G) • Successful transmissions, or throughput = rate * prob(none else transmits) = G * e^(-2G)

  22. ALOHA => Slotted ALOHA • Best case performance of ALOHA • G = 0.5, Throughput = 1/(2e), nearly 18% • What else can you expect from purely random, and no carrier sense protocols • Slotted ALOHA • Like ALOHA, in every sense, except when a transmission request can originate • Discretize the time axis into slots, 1 slot = 1 frame width • A node can only transmit a frame at a slot beginning • Requires a master clock, typically one node transmitting a special control signal at the beginning of each frame • Issue: Is clock synchronization that easy ?

  23. Performance of Slotted ALOHA • Effect of restricted transmission request time instants • Vulnerability period is reduced from 2t to t, where t is the frame width (refer Figure 4-2, and explain why ?) • Probability of no other transmission during one frame = e^(-G) • Thus, Throughput = G * e^(-G) • Best throughput is for G=1, with nearly 37% throughput • 37% utilization, 37% empty slots and 26% collisions • About twice better than pure ALOHA • Exercise • Increasing G would reduce the # of empty slots. Why that will not increase the throughput ? • Work out few examples...

  24. ALOHA ==> Slotted ALOHA Insert Fig 4-3 here

  25. Carrier Sense Protocols • Best performance of Slotted ALOHA = 1/e • Since, nodes cannot sense the carrier prior to transmission • In other words, they cannot avoid collision, can only detect • Carrier Sense Protocols • Can listen for a carrier, i.e., shared channel, to become idle and then transmit • Carrier Sense Multiple Access (CSMA) class of protocols • Persistent CSMA • Also, called as 1-persistent, since it transmits with a probability = 1 • A node with ready data • Listen for idle channel, if line is busy then WAIT Persistently • When channel is free, transmit the packet, and then listen for a collision • If collision, then sleep for a random time and goto Step 1

  26. Persistent CSMA • How does contention resolution occur ? • Depends on the “randomness” of the wait periods • If a set of random wait periods, one from each user, are in effect then eventually everyone will get through... • Role of Propagation Delay • Collision detection time depends on the propagation delay • If d is the propagation delay, then worst case collision detection time = 2d • d = 0, there may still be some collisions • Analogous to round table conference discussions among human users • Improvement over ALOHA • Nodes do not jump in at the middle of another node’s transmission

  27. Non-Persistent CSMA • Persistent CSMA • When looking for an idle channel, it keeps a continuous wait • A greedy mode for “seize asap” • Consequence: multiple contenders, each in the “seize asap” mode will lead to followup collisions • Non-Persistent CSMA • If an idle channel is not found, the node desiring to transmit does not wait in a “grab as soon as available” mode • Instead, the node attempting to transmit goes into a random wait period. It wakes up at the end of the random wait, and re-tries for an idle channel • Benefit: reduced contention (Note: it includes a 2-level randomness) • Random wait, if not found idle channel • Random wait, if found idle channel, transmitted but had collision

  28. Non-Persistent CSMA => p-Persistent CSMA • Contention reduction strategy • Involve more and more random delays in each user activities • Throughput will increase, but individual user delays will decrease • p-Persistent CSMA • Channel is time slotted, similar to Slotted ALOHA • A node with ready data • Look for an idle channel, if channel is busy then wait for the next slot • If idle channel found then transmit with probability = p (i.e., defer until the next slot with prob = 1-p) • If next slot is also idle, then transmit with prob=p, and defer for the second next slot with prob = 1-p • Continue until the data is transmitted, or some other node starts transmitting • If so, wait for a random time and goto Step 1

  29. Why p-Persistent CSMA ? • The more probabilistic events, and randomness => the less contention and increased throughput • Degrees of uncertainty • Persistent CSMA = 1, random delay when a collision occurs • Non-Persistent CSMA = 2, random delay both at the channel seek, and at the collision • p-Persistent CSMA = 2 (but different kind from Non-Persistent) • Random delay at collision (as Non-Persistent) • Deterministic seizure attitude at channel seek time (like Persistent) • Slotted time (like Slotted ALOHA) • But, non-deterministic transmission even when channel is idle • An additional level of uncertainty beyond Persistent CSMA)

  30. Performance of CSMA Class of Protocols • Throughput and individual user delays are against each other • Throughput • Non-persistent is better than Persistent • Non-Persistent VS. p-Persistent • Depends on the value of p • Both have 2 degrees of uncertainty, but different kinds • Refer Figure 4-4 for an aggregate performance depiction • In increasing throughput • Pure ALOHA • Slotted ALOHA • 1-Persistent, or Persistent CSMA • 0.5 Persistent CSMA • (Non-Persistent, 0.1 Persistent) CSMA • 0.01 Persistent CSMA

  31. include figure 4-4 here

  32. CSMA with Collision Detection • CSMA does not abort a transmission when a collision occurs • Colliding transmissions will continue (until the frame completion) • A fair (!!) amount of garbage being generated, once a collision occurs • Why not abort transmission as soon as a collision is detected • CSMA with Collision Detection • IEEE 802.3, Ethernet protocol • Quickly terminate damaged frames • Contention periods are single slot each, not a frame width (Fig 4-5) • Resource wastage = width of the slots (and not those of the frames) • Slot width = worst case signal propagation delay • Actually, twice of that • Includes the delay of the analog devices as well

  33. include fig 4-5 here

  34. Collision-Free Protocols • Channel co-ordination can be of several categories • Detection and Correction • Avoidance • Prevention (contention-free !) • Static MAC Policies • Collision-free by design, i.e., avoidance • Resource utilization may be questionable • Dynamic MAC with Collision Detection • Like CSMA/CD • Dynamic MAC with contention prevention • Protocol does few extra steps in run-time to prevent collision

  35. Reservation-Based Dynamic MAC Protocols • Protocols consist of two phases • Reservation or bidding process • Actual usage, after the bidding process • Reservation phase • All nodes with data to transmit go through the reservation phase • Result: one or more winners ==> implicit reservations • Transmission phase • The winner channel(s) transmits (one after another) • Bit-Map Protocol - One Reservation Policy • Basic idea stems from Link List approach • Refer Figure 4-6

  36. include fig 4-6 here

  37. Bit-Map Protocol • N Contention Slots for N stations • Node i transmits a “1” in Slot i, iff node i has data to send • The collection of 1’s in the Contention Slot will indicate which stations are with data (to transmit) • Followed by Transmission Phase • Allocate Frames only for those Nodes with a 1 in the Contention Slots • Performance • Low load :- • Frames’ time << Contention Slot time • Contention Slot’s delay for Low numbered station -- 1.5N (why ?) • Contention Slot’s delay for High numbered station -- 0.5N (why?) • Average wait = N slots (sloppy analysis !!) • For d-bit data frames, efficiency = d / (d +N)

  38. Performance of Bit-Map Protocol • At high load • Multiple (k) frames per each group of N Contention Slots • Efficiency = k*d / (N + k*d) • For k ==> N, efficiency = d/(d+1) • Question ? • Is this a realistic analysis ? • Can you do a queueing analysis for this protocol ? • Is there any fundamental bottleneck ?

  39. Binary Countdown Protocol • 2-phase Protocol : Reservation followed by Transmission • Reservation phase • Each station, with ready data, transmits its bit address in msb to lsb order • At each bit-position, binary OR of all the respective bits from each node. If a node with a 0-bit, observes a 1 after the OR operation - then it withdraws from the competition. The latest surviving node is the winner. • Transmission phase: Winner (single) transmits the data • Example, nodes 3, 4 and 6 have data to transmit • Node ids (0011), (0100) and (0110) get transmitted • First transmission: 0, 0, and 0 • Second transmission: 0, 1 and 1 ==> Node 3 withdraws • Third transmission: none, 0, and 1 ==> Node 4 withdraws • Node 6 is the winner. Node 6 transmits data frame.

  40. Performance of Binary Countdown Protocol • Note: only a single winner in this approach • The node with the highest bit address • This approach may starve the lower numbered users • For N nodes, ln(N) bit addresses will be transmitted • d bits frame ==> efficiency = d / (d + ln(N)) • Enhancements: • Bit ordering different from (msb --> lsb) type • Parallelized version of binary countdown, instead of serial • Efficiency can reach upto 100%

  41. Limited Contention Protocols • Design features: • Low traffic load - Collision detection approaches are better, they offer low delay, and not much collision occurs anyways • High traffic load - Collision free protocols are better, they have higher delay, but at least the channel efficiency is much better... • What if we combine the advantages of the two ? • Limited Contention Protocols • Idea: Do not let every station compete for the channel with equal probability. Allow different groups of nodes to compete at different times... • Refer Figure 4-8, for Success Probability = f(# ready stations) • Question: give an analogy of this idea using the car/road domain...

  42. include fig 4-8 here

  43. Adaptive Tree Walk - Limited Contention Protocol • Group the N nodes as a log(N) height binary tree • Tree leaves are the N nodes • Starting phase, or immediately after a successful transmit • All N nodes can compete for the channel • If one of the nodes acquire a channel, then repeat with all “N nodes” as the contenders’ list • Else, if collision then narrow the contenders’ list = left subgroup of nodes • If one of the nodes acquire a channel, then shift to the right sibling group of nodes for the next slot • Else, if there is a further collision, narrow down the contenders’ list to the leftward children subtree (Repeat...) • Refer Figure 4-9, essentially walk around with various subgroups of the tree leaves at each time as the Contenders’ list

  44. Figure 4-9

  45. Wavelength Division Multiplexed MAC Protocol • Analogous to FDM, used popularly for optical networks • Partition the wavelength spectrum into (equal ?) slices • One slice for each node / user • Can apply TDM in conjunction as well • Useful for implementation of broadcast topologies • Refer Figure 4-10, each wavelength slice has two parts - for control information, and for data values • Can also implement point-to-point network topologies (how ?) • Collectively it is called TWDM (time-wave-division multiplexed) MAC protocol • Key design issue: #transmitters, and #receivers at each node • Frequencies and Tunability of the transceivers...

  46. Figure 4-10

  47. WDMA - A Particular WDM MAC • WDMA - a broadcast based protocol • Each node is assigned two channels, for Control and for Data • The data channel is slotted • One slot for every other node • One slot for status information of the host node itself • The control channel is also slotted • Supports three classes of traffic • Constant data rate connection-oriented traffic • Variable data rate connection-oriented traffic • Datagram traffic, e.g., UDP packets • Each node has two receivers (one fixed freq, another tunable) and two transmitted (one fixed freq, another tunable)

  48. Arbitrary Topology Configurations using WDM and TDM • Consider any graph topology • Replace every bi-directional edge using two back-to-back simplex edges • Assign each simplex edge of the graph topology to one slot in the (frequency, time) • Select #time slots just adequate enough so that #freq * #time slots >= the #simplex edges • Work out an example

  49. Wireless LAN Protocols • Consider a Cellular Network, with Cell sizes anywhere between few meters to several miles • Frequency reuse is adopted, as a feature of Cellular system • What could be a typical MAC ? Can CSMA work ? • No, since there is no common broadcast channel which everyone eventually listens to • Refer Figure 4-11 • Design difficulty: how to detect interference at the Receiver ? • Hidden station problem: Two nodes transmit to a common receiver located in the middle • Competitor station is too far away • Exposed station problem: Two adjacent nodes transmitting in opposite directions. False sense of competition...

  50. figure 4-11

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