Computer Networks Theory and Practice

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
• Dont 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
TCP, or UDP
Transport
Transport
IP
Network
Network
Data Link
Data Link
ArpaNet, NSFNet, various LANs, ...

Physical
Physical
Physical Medium

• Application layer controls everything above the
  Transport Layer (theme reduce the overhead)

11
Network Standardization

• International Standards Organization (ISO)
• Various TCs, and Working Groups
• ANSI (Am. Natl 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 LT, 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
  users 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 ---gt 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., k0, gt e(-2G)
• Successful transmissions, or throughput rate
  prob(none else transmits) G e(-2G)

22
ALOHA gt 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 G1, 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 gt 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
  nodes 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 gt 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
  probp, 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 gt
  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 gt 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 1s 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 ltlt Contention Slot time
• Contention Slots delay for Low numbered station
  -- 1.5N (why ?)
• Contention Slots 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 kd / (N kd)
• For k gt N, efficiency d/(d1)
• 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 gt Node 3
  withdraws
• Third transmission none, 0, and 1 gt 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 gt efficiency d / (d ln(N))
• Enhancements
• Bit ordering different from (msb --gt 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 gt 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

51
MACA - Multiple Access with Collision Avoidance

• Idea have both the sender and receiver ackn each
  other stating the length of upcoming transmission
• Consequently, neighbors both around the sender
  and receiver will be aware of the transmission
  activity and its duration (from the bits in the
  transmission)
• Figure 4-12
• Protocol
• Sender send a request-to-send (RTS) signal to
  receiver with bits in the upcoming data frame
• Receiver ackn to sender using a clear-to-send
  (CTS), if no collision.
• Sender start transmitting upon receiving the CTS
• Where is the catch ? Both the senders and
  receivers neighbor can hear the message
  initiation along with size !!

52

• figure 4-12

53
MACA and MACAW

• Collisions in MACA
• Still possible, but chances are much reduced
• If two nodes initiate an RTS simultaneously
• Collision gt backoff and re-try later (like
  CSMA)
• Backoff approach is based upon a binary
  exponential scheme
• MACAW - an enhanced MACA Protocol
• ACK signal at the MAC layer, after each data
  frame
• Include carrier sensing to further reduce
  collision(Although, carrier could only be sensed
  locally.)
• Random wait and re-try transmission at every
  message level, instead of at every node level
• Congestion information exchange between pairwise
  stations, leading to better congestion control
  and backoff approaches

54
Protocols for Digital Cellular Radio

• Significant usage for mobile telephony
• Each connection lasts longer than few msec
• Hence, channel allocation per Call is better than
  per Frame (why ?)
• Preferably use digital coding, instead of analog
• Allows compression of data/speech
• Allows to integrate voice, data, fax, ...
• Can include error-correcting codes (for
  reliability) and encryption (for security)
• GSM - Global System for Mobile Communication
• Allocated in the 900 MHz band, later re-shuffled
  to the 1800 MHz range as well (called DCS 1800)
• Employs 124 bi-directional freuqncy channels
  within each cell
• Refer Figure 4-13

55

• figure 4-13

56
GSM - Details

• Each cell has 124 (base station --gt user nodes)
  frequency channels 124 (user nodes --gt base
  station) freq. channels
• These are used for Data In/Out and Control
  signals
• Each freq. channel is 200KHz wide, allowing a
  fair bit rate !!
• Each freq. channel is 8-way TDM slotted
• Thus, a total of 992 (124 8) logical
  connections are possible
• Not all of the 992 connections are implemented
• for avoiding frequency conflicts with neighboring
  cells
• Also, for enhancing the bps within each logical
  connection
• Format of the TDM slots
• 148 bit in each slot, 8 slots per frame for time
  division multiplexing, and 26 frames to create a
  multiframe

57
Data Format of GSM Frames

• Refer Fig. 4-14
• Each TDM slot, of 148 bits, consist of
• 3 start bits
• 57 bit Information
• 1 bit Voice/Data toggle
• 26 bit synchronization information
• 1 bit Voice/Data toggle
• 57 bit Information
• 3 stop bits
• 8 TDM slots create a TDM frame
• Slots are separated by 30 microsec guard time
  (worth 8.25 bit)
• Guard times accommodate lack of sync, and data
  overflow

58

• figure 4-14

59
GSM (contd...)

• 26 TDM Frames constitute a TDM multi-frame
• 24 frames are data use, 1 frame for control, 1
  left for future use
• Time spent for a TDM multiframe is 120 milisec
• Effective data rate in each logical connection is
  9600 bps
• Other GSM channels
• Apart from the GSM framing structure, it also
  supports other specific purpose channels
• Broadcast Control Channel
• Continuous stream of outputs from the Base
  Station to all the nodes describing the Base
  Station id
• Mobile nodes check the strength of this signal to
  detect the cellular parenthood

60
Other GSM Channel (contd...)

• Dedicated Control Channel
• for location updating, registration and call
  setup
• each base station maaintains a data structure
  with all intra-cell mobile nodes the control
  channel exchanges information to keep this data
  structure updated
• Common Control Channel
• Paging Channel
• Base station uses this for announcing Incoming
  Calls
• Mobile nodes listen to this for answering
  Incoming calls
• Random Access Channel
• Slotted ALOHA to setup a call in the Dedicated
  Control Channel
• A node can setup a Call using this Channel
• Access Grant Channel
• response of Random Access Channel

61
GSM vs. CDPD Cellular Digital Packet Data

• GSM
• Circuit Switched, not packet switched
• Not friendly to cellular handoffs, each handoff
  can miss some data
• Increased error rate
• CDPD
• A packet switched, digital datagram service
• Using 30 KHz channels, it can offer 19.2 Kbps
  links (excluding protocol overhead gt 9600 bps
  data channels)
• CDPD System Architecture
• Three kinds of nodes mobile end system, base
  stations and base interface stations (which
  connect between base stations and to the
  Internet)
• Refer Figure 4-15

62

• figure 4-15

63
CDPD Details

• Uses three types of interfaces
• E-Interface Connects a CDPD Network to the
  outside world networks, e.g., the Internet
• I-Interface Connects between multiple CDPD areas
  (basically, between multiple cells)
• A-Interface Between base station and mobile
  nodes
• One Downlink part, from Base Station to Mobile
  Noeds
• Not difficult to manage, since it has only one
  user (the Base Station)
• One Uplink channel, shared by all the mobile end
  users
• Digital Sense Multiple Access protocol adopted by
  the mobile end nodes
• Similar to Slotted, p-Persistent CSMA
• Data is packetized, time axis is slotted, and
  re-entry attempts are spread out to
  non-consecutive time slots
• Combines the benefits of Slotted ALOHA,
  p-Persistent CSMA

64
Collision in CDPD

• Possible, when two or more mobile end nodes start
  on a time slot together
• Mobile hosts may not immediately detect a
  collision (sensing delay due to RF propagation)
• Microblock transmission is faster than the rate
  of detection of a failure
• Correct/Incorrect reception of microblock n is
  delayed until microblock n2
• In between, the mobile node just goes ahead and
  continues transmission
• If a failure is detected (later), it stops -
  otherwise transmission continues
• Voice data has higher priority, data transmission
  is next

65
Code Division Multiple Access

• CDMA - a completely new line of MAC approach
• MAC approaches so far TDM, FDM, WDMA, slotted
  ALOHA, ...
• CDMA - each user transmits across the entire
  spectrum
• However, nobody collides with each other
• Each node has a unique code, called Chip, using
  which it transmits
• The uniquness of the Chips ensure no eventual
  collision
• Analogy - Multiple people speaking in a room
• TDM everyone takes turn in speaking
• FDM Separate clusters of people, each speaking
  within its cluster, yet not being overheard at
  other clusters
• CDMA Everybody speaks loud and clear to
  everybody else, but using different languages

66
CDMA - Summary

• Each node has a unique sequence, called Chip
• Usually its a 64 or 128 bit pattern, but we
  demonstrate using a 8-bit Chip
• Example As chip (0, 0, 0, 1, 1, 0, 1, 1)
• If A wants to transmit a 1, it will send the
  above chip
• If A wants to transmit a 0, then it will send
  1s complement of the Chip
• Another node, B, will have a different Chip
  sequence
• Orthogonal from every other nodes Chip
• Normalized inner product of any pair of Chip
  sequences 0
• Thus, As Chip ltnormalized inner productgt Bs
  Chip 0
• By definition, As Chip ltnorm. inner prod.gt
  Complement(Bs Chip) 0
• Bit sequence within the Chips are transmitted
  across the entire spread spectrum

67
CDMA - Bandwidth Usage

• Consider 100 nodes, and 1 MHz spectrum with 1
  Mbps
• FDM allocates 10 KHz per station
• Each station has a 10 Kbps data rate
• CDMA, with m bit Chips
• Allocates the entire 1 MHz to each station
• Thus, each stations data rate 1000/m Kbps
• When m is smaller than 100, CDMA is a better
  bandwidth utilization
• Where is the catch ?
• CDMA will expect to treat the RF media in an
  analog fashion
• Voltages (RF transmission powers) will be
  expected to be additive in value
• It can get more noisy, likely to be more erroneous

68
CDMA - Example (refer Figure 4-16)

• Four nodes, A, B, C, and D each with unique 8-bit
  Chip
• 0-bits in the Chip sequence can be treated as -1
  for voltage or transmission power point of view
• Two or more nodes transmitting together simply
  adds their voltages (addition of negative values
  indicate voltage or RF power reduction --gt this
  is a major source of error, in analog handling)
• The design of the Chip sequences ensure that
• A ltnorm. inner prod.gt B 0
• A ltnorm. inner prod.gt (complement of B) 0
• Suppose, A and C transmit a 1, while B transmit a
  0
• T (A not(B) C) is transmitted. Everyone
  receives this.
• Receiver node D, trying to listen to C, computes
  C ltnorm. inner prodgt T
• C.A C.(not(B)) C.C 1, where 1 is what C
  transmitted

69

• figure 4-16

70
CDMA Example (contd...)

• Suppose, C transmitted a 0 in the previous
  example
• T (A not(B) not(C))
• The receiving node D will compute
• C . T C.A C.(not(B)) C.(not(C))
• 0 0 (-1) -1
• 0-bit is assumed to have a value -1
• Efficiency of CDMA
• Theoretically, can be arbitrarily large
• In practice, the noise level, analog value
  handling and bits/Chip pose limitations
• Design rule if you want to enhance b/w, and can
  live with some noise - go for CDMA (Korean
  Telecom)
• Question Why the name Chip

71
Theory to Practice

• CSMA/CD MAC Protocol with various degrees of
  Persistency
• IEEE 802.3 is a specific implementation
• Random delay, if collision occurs, is based on a
  Binary Exponential Backoff algorithm
• Average case performance Moderate
• However, no worst case delay guarantee for
  individual stations
• Token Bus and Token Ring Protocols
• Worst case bounded delay, may be useful for
  Real-Time application
• IEEE 802.4 and 802.5 LAN standards
• LAN to MAN and fairness issues
• Distributed Queue Dual Bus (DQDB), IEEE 802.6
• IEEE 802.2 Logical Link Control

72
Ethernet ??802.3

• Essentially, it is a 1-persistent CSMA/CD
  Protocol
• Looking for an idle channel
• If not found, i.e., Channelbusy, station waits
  in a greedy mode
• If Channel idle, station immediately attempts
  to transmit data
• If no collision, then successful transmission
• If collision, stop transmission immediately and
  go into a random delay wait more
• Requires broadcast mode cable topology
• Linear, Backbone, Tree, Segments with Repeaters
• Figure 4-19
• Worst case delay in broadcast transmission
  affects performance (Efficiency, for example)

73

• figure 4-19

74
Binary Exponential Backoff Algorithm for Random
Delay Wait

• Motivation
• Random delay to ensure that collissions will
  eventually be resolved
• Minimize the probability that two (or more)
  colliding stations will keep colliding again and
  again
• Once done so, then minimize the absolute ranges
  of delay periods during these random wait cycles
• If few stations compete, the range of random
  delays should be smaller
• Chances of consecutive collisions is less, hence
  minimize the random delay period
• If collisions occur in consecutive attempts, then
  the range of random delays should be increased
  (perhaps, rapidly) to quickly resolve the
  colliding stations
• Here, two or more stations are repeatedly
  colliding. Hence, most immediate priority is to
  resolve the conflict between them.

75
Binary Exponential Backoff (contd...)

• After the first collision
• random wait period is either 0 (i.e., re-try next
  slot) or 1
• After the second consecutive collision
• random wait period is in the range 0, 1, 2, and
  3
• After the i-th consecutive collision, ilt 10
• random wait period is in the range 0, 1, 2, ...,
  2i -1
• For, 11 lt i lt15, the random wait period range
  is fixed 0, 1023
• For i16, an abnormal transmission event
  interrupt is sent to the message source
• Features
• For fewer stations, and fewer collisions gt
  average randon wait is small
• For many stations, and lot of collisions gt
  collision gets resolve quickly

76
Ethernet Addressing

• Frame Format
• Transmission _at_ frame quantums (viz. collision
  detection advt.)
• Preamble 7 Bytes
• Each byte 10101010 gt 10 MHz square wave for
  5.6 microsec
• Used for clock synchronization
• Start Delimitter 1 Byte (10101011)
• Destination (and, Source) Address 2 or 6 Bytes
• Data Length 2 Bytes Actual Data 0 to 1500
  Bytes
• Pad 0 to 46 Bytes (used for ensuring gt 64 bytes
  after dest.addr)
• Checksum 4 Bytes (32-bit CRC 8 bit
  end-delimiter)

7 1 2 or 6 2 or 6
2 0-1500 0-46 4
Preamble Start Dest.Addr. Source Addr
Length Data Pad Checksum
77
802.3 Frame Format
Insert Figure 4-21 here
78
Ethernet Addressing (contd.)

• Data length 0 to 1500 Bytes
• Effects of short data frames
• Too small data length can confuse the receiver
• Is it a collided frame, or real (short) data ?
• Also, two frames may start at distant ends of the
  cable
• Answer Each frame must be at least 64 bytes
  after the destination address
• If actual data size is small, then create a Pad
  (upto 46 bytes)
• Why 64 Bytes ?
• 10-Mbps LAN, 2.5 km cable (specs), and 2t
  collision window
• Minimum frame width 51.2 microsec gt 64 Bytes
  length

79
Broadcast and Multicast Addresses

• Destination Address
• Msb 1 for group (multicast or broadcast), 0
  for unicast
• Address all 1s indication of Broadcast
• How does multicast work ?
• Group addr. id programmed to listen at
  individual nodes
• 2nd Msb Local vs. Global addresses
• Useful for address filtering, and flooding
  control
• Uniqueness of Node Addresses
• Total 46 bit addressing (6 bytes - 2 msb)
• Approx. 7 1013 addresses
• Can provide unique address to every node !!
• Manufacturers procure a bulk of address ranges

80
Broadcast, Multicast, and Unicast

• Each transmitted frame is listened to by every
  adapter
• Adaptors act as filters
• Frames that are ok-ed by the filter are sent to
  the backend host computer
• Filter Modes
• Listen to self-address only Unicast
• Promiscuous Listen to all addresses (useful for
  gateway design)
• Listen to addresses with all 1s Broadcast
• Listen to specific group-ID Multicast

81
ARP vs. RARP

• Issue Upper Layer Address vs. Ethernet Address
• Forward and Reverse Mapping
• Address Resolution Protocol
• 32-bit IP address gt 48 bit Ethernet address
• Naive Approach Configuration Files (IP address
  vs. Ethernet address)
• ARP Algorithm Broadcast IP address and seek a
  response
• ARP records can be cached, optimized for locality
• Reverse Address Resolution Protocol
• Host machine (at boot time) transmits ethernet
  address and seeks IP address (from RARP server)

82
Ethernet Connectors

• 10Base 5 (Thick Ethernet)
• Vampire Tap
• 10Base 2 (Thin Ethernet)
• Flexible Connector
• 10Base T (Central Hub)
• Nodes connect twisted pair cable to a switch
• 10Base F
• Version for optical fiber

83
Worst Case Collision Detection
Insert Fig 4-22 here
84
Performance of 802.3

• Simplistic analysis
• Assume a fixed number of, k, stations always with
  data to transmit
• p probability with which each station transmits
  during a contention slot
• Then, the probability that one of those k
  stations will successfully acquire the channel is
  A k p (1-p)k-1
• k times, one for each station being the channel
  winner
• (k-1) stations did not transmit, while the winner
  stations did transmit gt Probability p
  (1-p)k-1
• Probability that the contention interval is
  exactly j slots, will be A (1-A)j-1
• Contention interval is not in the (j-1) slots gt
  (1-A)j-1
• It is at the j-th slot gt A (1-A)j-1

85
Performance of 802.3 (contd...)

• Mean number of slots per contention
• sum (from j0, to jinfinity) j A
  (1-A)j-1
• 1/A
• Each slot is a duration 2????where ??is the
  worst case broadcast delay
• Hence, mean contention interval 2?? 1/A
• If the average frame takes P time units to
  transmit, then the total time taken to transmit
  P mean contention interval P 2??
• Hence, Channel efficiency P / ( P 2??/A )
• Refer Figure 4-23, for channel efficiency as a
  function of the stations trying to send data
• Large P gt higher efficiency, but increased
  frame fragmentation

86

• figure 4-23

87
Switched Ethernet

• Switched Ethernet
• Intelligent processing allows packet filtering
• Useful for traffic reduction, containment
• Example multicast filtering, broadcast
  filtering,
• Other usage security, workgroup establishment
• Design Paradox
• Ethernet had not been initially meant to be
  point-to-point
• However, design needs led it to becoming
  point-to-point
• Its still called Ethernet, and behaves like
  Ethernet - for compliance, and ability to (still
  !!) use existing ethernet adapter cards
• Sometimes, it is an expensive mistake to carry
  one !!

88
Full Duplex Ethernet

• Design Rationale
• Ethernet does not scale well
• Connect Points, also bandwidth...
• Solution Several 802.3 LAN connected via a
  faster switch
• Each 802.3 LAN is in reality a plug-in card at
  the switch
• Full Duplex Switched Ethernet
• FDSE Architecture
• Not a shared bus LAN
• Instead, a point-to-point protocol around a fast
  switch
• Switch has several (lt32) Plug-in Cards
• Each Plug-in Card has few (lt8) Connectors
• Each connector is a 10Base T link to a host
  computer

89
FDSE Block Diagram
Insert Fig 4-24 here
90
FDSE Structure
host
to other FDSE
host
FDSE
802.3 LAN
Hub
hosts
91
FDSE Design

• Idenaitcal frame format, addressing, ....
• On-Card LAN
• If a frame is addressed to another node on the
  same card, then the frame is locally copied
• Else, it is transmitted over the high-speed
  backbone bus to another on-card LAN
• Input Buffering
• Collision resolution with on-card LAN(Btw,
  collision never occurs across multiple cards)
• Approach 1 adopt CSMA/CD within each card
• Approach 2 Input packet buffering scheduling
• Whao !! Feasibility for Packet Prioritization,
  Periodic traffic support...

92
Packet Priority in FDSE LAN

• 802.3 has no support for priority
• 802.4 and 5 evolved precisely for these reasons
• However, FDSE is a much digressed version of the
  initial ethernet
• It is point-to-point, instead of shared media
• It is input buffered, and scheduled, instead of
  collision and re-try
• Hence, packet priority establishment is feasible
  in FDSE
• Priority implementation in the scheduling of
  input buffer
• Still, ethernet frame format does not accommodate
  priority values
• One way to accommodate priority is as part of the
  data field
• Priority support from upper OSI layers (e.g.,
  TCP) is always feasible

93
Periodic Traffic Support in FDSE LAN

• Not directly supported
• But, can always be implemented from TCP or IPX
  layer
• Admission Control Stage
• At TCP or upper application layer
• Dynamic Scheduling Stage
• At FDSE input buffer scheduling algorithm
• Upper OSI Layer Connection-Oriented Virtual
  Circuit can solve this problem
• Aperiodic RT Traffic Support
• Use placeholder (i.e., stub) periodic traffic

94
FDSE LAN of LANs

• FDSE as a switch easily lends itself to
  hierarchical construction as LAN or MAN / WAN (as
  LAN os LANs)

95
FDSE Flow Control

• Prevent over-bandwidth situations, and recover
  from congestions and hot spots
• Objective Forward packets from ingtout ports
  without any loss of packets and minimum (0 ?)
  latency
• TCP or Window Based Protocol
• Several packets transmitted before a destination
  port overloaded message can be reverse ackn-ed
• Solution modest sized buffer, time to fill up
  the buffer (due to destnation port jamming) is
  adequate to inform the sender node
• Disadvantage Large buffer gt large (individual)
  packet latency
• Another solution reduce the window size of the
  upper layer protocol (e.g., TCP or IPX)

96
Learn Table (Address Mapping)

• Learn table a table of information associating
  48-bit Ethernet addresses with ports
• New frame arrival
• Look up the port address, from (destinations)
  ethernet address
• If port address unavailable, then broadcast
  (unfortunate situation, cannot be helped) - are
  you out there, please respond type
• Learn Table updated bu current lookup
  information
• Recent failures in lookup, and eventual
  resolution (by broadcast)
• Old entries are flushed in a cache-page update
  manner
• LRU, or FIFO

97
FDSE and Fast Ethernet Connectors

• 100 Base - Fx
• Specs for 100 Mbps Fast Ethernet over fiber
• Similar to FDDI specs
• Signals are unscrambled, 4B5B encoded
• 100 Base - T4
• same as above, except for category 3 or better
  twisted pair cabling
• Full duplex not supported under T4
• 100 Base - TX
• same as above, except for category 5 twisted pair
  cabling
• Similar to CDDI specs, signals are scrambled,
  4B5B encoded

98
Limitations of 802.3

• No worst case delay bound for any given stations
• No notion of priorities to any of the
  nodes/stations
• Focusses on the overall channel efficiency, not
  on the individual user station needs
• Certainly not good for time-critical traffic
• IEEE 802.4 evolves from 802.3
• Token Bus structure, logically
• Each one of the N nodes takes turn in sending
  their respective frames
• If each node takes T time units, then no node
  will have to wait more then NT time units
• Figure 4-25 as an example Token Bus

99

• figure 4-25

100
IEEE 802.4 Token Bus

• Logical linear connection
• Each node has a predecessor and a successor node
• The Token arrives from the predecessor node, and
  is destined to the successor node after usage by
  the current node
• The highest numbered station sends the first
  frame
• If a node has no data to send, it passes the
  Token immediately
• Logically the nodes are organized as a Ring (Fig.
  4-25)
• Collison avoidance by mutual exclusion in Token
  ownership
• Physically, the nodes may be in any connection
  pattern
• Tree, Bus, ...
• Essentially, a broadcast transmission medium is
  needed
• Logical ordering of the stations is independent
  of the physical locations

101
Priority in Token Bus

• Worst case response time for each node lt NT time
  units, for N nodes and T time units per node
  (i.e., per Token)
• This prevents from unbounded response delay
  situations
• Yet, may not follow hard real-time guarantees
• How to assign priorities to the traffic within
  each node ?
• Token Bus defines four priority classes, 0, 2, 4
  and 6
• Priority 6 is the highest, Priority 0 is the
  least
• When a node acquires the Token, say for T time
  units
• First, it allocates transmission from Priority 6
  messages
• After all the data from Priority 6 set is
  exhausted, if any more time is still left gt
  allocate traffic from Priority 4 messages
• After all Priority 4 messages are over, if still
  some time is left, then use for Priority 2
  messages, and so on

102
Synchronous Traffic in Token Bus

• The bandwidth for at least one of the Priority 6
  messages is guaranteed
• ltT (as much as desired) time units of
  transmission per every NT time units
• Synchronous traffic, e.g., live video,
  multimedia, automated factories and production
  environments, are supported
• Limitations
• Ranges of deadline that can be honored
• No notion of periodic traffic support
• Fault-Tolerance
• What if a node/station goes down while holding a
  Token ?
• A max-time parameter for claiming tokens

103
Token Ring IEEE 802.5

• Token Bus
• Requires a broadcast channel
• Large delay
• Analog characteristics
• Enjoys the freedom of logical predecessor/successo
  r assignment
• Token Ring