Midterm Exam Review

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Midterm ExamReview
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Communication Networks

• A communication network provides a general
  solution to the problem of connecting many
  devices
• Connect each device to a network node (router)
• Network nodes exchange information and carry the
  information from a source device to a destination
  device
• Note Network nodes do not generate information
• Connect devices to a single shared medium (LAN)

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Communication Networks

• A generic communication network

Other names for Device station, host,
terminal Other names for Node switch, router,
gateway
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Classification of Communications

• Communication networks can be classified based on
  the way in which the nodes exchange information
• Communication Network
• Switched Communication Network
• Circuit-Switched Communication Network
• Packet-Switched Communication Network
• Datagram Network
• Virtual Circuit Network
• Broadcast Communication Network

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Broadcast Network Examples
Packet Radio Network
Satellite Network
Bus Local Network
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Circuit Switching

• A node in a circuit-switching network

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Circuit Switching
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Packet Switching
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Datagram Packet Switching
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Virtual-Circuit Packet Switching
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Network Technologies

• Telephone Networks
• IP Networks
• ATM Networks

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Three Network Technologies

• Telephone Network
• The largest worldwide computer network,
  specialized for voice
• Switching technique Circuit-switching
• Internet
• A newer global and public information
  infrastructure
• Switching technique Datagram packet switching
• ATM
• Was intended to replace telephone networks and
  data networks, but lost momentum due the success
  of the Internet
• Switching technique VC packet switching

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Telephone Networks
Starting in 1876, the public switched telephone
network (PSTN) has become a global infrastructure
for voice communications
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Addressing and Routing

• Each subscriber has an address (telephone number)
• Addresses are hierarchical
• The information contained in a telephone address
  is exploited when establishing a route from
  caller to callee

Country code
Number of local exchange
Subscriber number
Area code
852
2358
6984
My office number
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The Internet - A Network of Networks

• The Internet is a loose collection of networks
• Networks are organized in a (loose) multi-layer
  hierarchy

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What defines the Internet

• Use of a globally unique address space (Internet
  Addresses)
• Support of the Transmission Control
  Protocol/Internet Protocol (TCP/IP) suite for
  communications
• The physical networks widely differ (cable,
  optical, wireless, radio, etc.) - IP on top of
  ANYTHING.

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Internet Addresses

• Each network interface on the Internet has a
  unique global address, called the IP address.
• An IP address
• is 32 bits long
• encodes a network number and a host number
• IP addresses are written in a dotted decimal
  notation. 128.142.136.146 means
• 10000000 in 1 st Byte
• 10001110 in 2 nd Byte
• 10001000 in 3 rd Byte
• 10010011 in 4 th Byte

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Domain Names and IP Addresses

• Users and applications on the Internet normally
  do not use IP addresses directly. No one says
  http//128.142.136.29/
• Rather users and applications use domain names
  http//www.cs.ust.hk
• A service on the Internet, called the Domain Name
  System (DNS) performs the translation between
  domain names and IP addresses

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Traditional Network Infrastructure
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B-ISDN
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Protocol Architecture

• Layered Protocol Architectures
• OSI Reference Model
• TCP/IP Protocol Stack

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Need for Protocols

• The task of exchanging information between
  devices
• requires a high degree of cooperation between the
  involved parties
• can be quite complex
• Protocols are a set of rules and conventions. By
  enforcing that communicating parties adhere to a
  common protocol, communication is made possible.
• The complexity of the communication task is
  reduced by dividing it into subtasks
• Each subtask is implemented independently.
• Each subtask provides a service to another
  subtask.

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OSI Reference Model

• In 1977 the International Standardization
  Organization (ISO) developed a model for a
  layered network architecture
• This effort was completed in 1983 and is known as
  the Open Systems Interconnection (OSI) Reference
  Model
• The OSI model defines seven layers
• Layer 7 Application Layer
• Layer 6 Presentation Layer
• Layer 5 Session Layer
• Layer 4 Transport Layer
• Layer 3 Network Layer
• Layer 2 Data Link Layer
• Layer 1 Physical Layer
• (Layer 0 Interconnection Media)

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OSI Layers
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OSI Layers and Encapsulation
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OSI Model in a Switched Communication Network
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TCP/IP Protocol Suite

• The TCP/IP protocol suite was first defined in
  1974
• The TCP/IP protocol suite is the protocol
  architecture of the Internet
• The TCP/IP suite has four layers Application,
  Transport, Internet, and Network Interface Layer

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Encapsulation in the TCP/IP Suite

• As data is moving down the protocol stack, each
  protocol is adding layer-specific control
  information.

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Comparison of OSI Model and TCP/IP Suite
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Physical Layer

• Fundamentals
• Transmissions factors
• Transmission Media

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Physical Layer

• The physical layer deals with transporting bits
  between two machines.
• The goal is to understand what happens to a
  signal as it travels across some physical media.

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Theoretical Basis for Data Communication

• Fourier AnalysisFourier showed that a periodic
  function g(t) can be represented mathematically
  as an infinite series of sines and cosines
• f is the function's fundamental
  frequency
•  T1/f      is the function's period
•  an  and bn are the amplitudes of the nth
  harmonics

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Theoretical Basis for Data Communication

• The series representation of g(t) is called its
  Fourier series expansion.
• In communications, we can always represent a data
  signal using a Fourier series by imagining that
  the signal repeats the same pattern forever.

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Theoretical Basis for Data Communication

• We can compute the coefficients  an and bn
• Suppose we use voltages (on/off) to represent
  1''s and 0''s, and we transmit the bit string
  011000010'. The signal would look as follows

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Theoretical Basis for Data Communication
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Theoretical Basis for Data Communication

• Points to note about the Fourier expansion
• The more terms in the expansion, the more exact
  our representation becomes.
• The expression represents the
  amplitude or energy of the signal (e.g., the
  harmonics contribution to the wave).

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Theoretical Basis for Data Communication

• Conclusion it's essentially impossible to
  receive the exact signal that was sent. The key
  is to receive enough of the signal so that the
  receiver can figure out what the original signal
  was.
• Note bandwidth'' is an overloaded term.
  Engineers tend to use bandwidth to refer to the
  spectrum of signals a channel carries. In
  contrast, the term bandwidth'' is often also
  used to refer to the data rate of the channel, in
  bps.

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Nyquist Theorem

• Noise-free channel
• Limiting factor on transmission is channel BW
• If bandwidth is B, highest signal rate is 2B
• Multi-level signaling
• C 2B log2 M where
• C is the data rate
• B is the bandwidth
• M is the number of levels
• For example, a noiseless 3-kHz channel cannot
  transmit binary signals at a rate exceeding 6000
  bps.

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Shannons Theorem

• If random noise is present, the situation
  deteriorates rapidly. The amount of noise present
  is measured by the ratio of the signal power to
  the noise power, called the signal-to-noise ratio
  (S/N).
• Usually, the ratio itself is not quoted instead,
  the quantity 10 log10S/N is given. These units
  are called decibels (dB).
• Maximum number of bits/secHlog2(1S/N)
• For telephone line 3000log2(130dB)?30000bps.

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Transmission Media

• The purpose of the physical layer is to transport
  a raw bit stream from one machine to another.
• Various physical media can be used for the actual
  transmission.
• Each one has its own niche in terms of bandwidth,
  delay, cost, and ease of installation and
  maintanence.
• Media are roughly grouped into guided media, such
  as copper wire and fiber optics, and unguided
  media such as radio and lasers through air.

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Transmission Media

• Twisted Pair
• Coaxial Cable
• Fiber Optic

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Transmission MediaWireless Transmission

• Radio omnidirectional, AM, FM Radio, TV, ALOHA
  data network
• Microwave directional
• Terrestrial Microwave, long-haul common carrier,
  government communications.
• Satellite Microwave
• A communication satellite is a microwave relay
  station.

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Data Link Layer

• Framing
• Error Detection
• Flow Control
• Error Control (via Retransmission)

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Introduction

• Main Task of the data link layer
• Provide error-free transmission over a physical
  link

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Introduction

• The PDU at the Data Link Layer (DL-PDU) is
  typically called a Frame. A Frame has a header, a
  data field, and a trailer
• Example

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Framing

• Problem Identify the beginning and the end of a
  frame in a bit stream
• Solution (bit-oriented Framing) A special bit
  pattern (flag) signals the beginning and the end
  of a frame (e.g., "01111110") use bit stuffing

• Problem The sequence 01111110 must not appear
  in the data of the frame

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Error Control

• Two basic approaches to handle bit errors
• Error-correcting codes
• Too many additional bits are needed for
  correction (used only in simplex communication
  (e.g., satellite))
• Error-detecting codes plus retransmission
• Used if retransmission of corrupted data is
  feasible
• Receiver detects error and requests
  retransmission of a frame.

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Cyclic-Redundancy Codes (CRC)

• General Method
• The transmitter generates an n-bit check sequence
  number (known as Frame Checksum Sequence (FCS))
  from a given k-bit frame such that the resulting
  (kn)-bit frame is divisible by some number
• The receiver divides the incoming frame by the
  same number
• If the result of the division does not leave a
  remainder, the receiver assumes that there was no
  error

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Step 2 CRC Encoding Method

• Define
• M(x) Data block is a polynomial ( Message,
  Frame)
• P(x) "Generator Polynomial" which is known to
  both sender and receiver (degree of P(x) is n)

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Step 2 CRC Encoding Method

• (I) Append n zeros to M(x), i.e., M(x)xn
• (II) Divide M(x)xn by P(x) and obtain
• M(x)xn Q(x)P(x) R(x)
• (III) Set T(x) M(x)xn R(x). T(x) is the
  encoded message
• Note T(x) is divisible by P(x). Therefore, if
  the received message does not contain an error
  then it can be divided by P(x).

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Flow Control

• Flow Control is a technique for speed-matching of
  transmitter and receiver. Flow control ensures
  that a transmitting station does not overflow a
  receiving station with data
• We will discuss two protocols for flow control
• Stop-and-Wait Protocol
• Sliding Window Protocol

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Stop-and-Wait Flow Control

• Simplest form of flow control
• In Stop-and-Wait flow control, the receiver
  indicates its readiness to receive data for each
  frame
• Operations
• 1. Sender Transmit a single frame
• 2. Receiver Transmit acknowledgment (ACK)
• 3. goto 1.

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Analysis of Stop-and-Wait
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Analysis of Stop-and-Wait

• Notation
• C Channel capacity in bps
• I Propagation delay
• H Number of bits in a frame header
• D Number of data bits in a frame
• F Total length of a frame (F DH)
• A Total length of an ACK frame
• F/C Transmission delay for a frame

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Analysis of Stop-and-Wait
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Analysis of Stop-and-Wait

• Transmission of a frame (in Stop-and-Wait)

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Analysis of Stop-and-Wait

• Efficiency of a protocol is the maximum fraction
  of time when the protocol is transmitting data
• Efficiency of Stop-and-Wait Flow Control (1)

• Assuming that H and A are negligible we obtain (2)

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Sliding Window Flow Control

• Major Drawback of Stop-and-Wait Flow Control
• Only one frame can be in transmission at a time
• This leads to inefficiency if agt1
• Sliding Window Flow Control
• Allows transmission of multiple frames
• Assigns each frame a k-bit sequence number
• Range of sequence number is 0...2k-1, i.e.,
  frames are counted modulo 2k

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Operation of Sliding Window

• Sending Window
• At any instant, the sender is permitted to send
  frames with sequence numbers in a certain range
• The range of sequence numbers is called the
  sending window

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Operation of Sliding Window

• Receiving Window
• The receiver maintains a receiving window
  corresponding to the sequence numbers of frames
  that are accepted

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Analysis of Sliding Windows

• Define
• We use the same parameters for as in
  Stop-and-Wait
• To simplify notation we set
• F/C 1
• I a (Normalization)
• W Maximum window size (identical for sender and
  receiver)

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Analysis of Sliding Windows
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Analysis of Sliding Windows

• If the window size is sufficiently large the
  sender can continuously transmit packets
• W gt 2a1 Sender can transmit continuously
• normalized efficiency 1
• W lt 2a1Sender can transmit W frames every 2a1
  time units
• normalized efficiency W/(12a)

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ARQ Error Control

• Two types of errors
• Lost frames
• Damaged Frames
• Most Error Control techniques are based on
• (1) Error Detection Scheme (e.g., Parity checks,
  CRC)
• (2) Retransmission Scheme
• Error control schemes that involve error
  detection and retransmission of lost or corrupted
  frames are referred to as Automatic Repeat
  Request (ARQ) error control

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ARQ Error Control

• All retransmission schemes use all or a subset of
  the following procedures
• Receiver sends an acknowledgment (ACK) if a frame
  is correctly received
• Receiver sends a negative acknowledgment (NAK) if
  a frame is not correctly received
• The sender retransmits a packet if an ACK is not
  received within a timeout interval
• All retransmission schemes (using ACK, NAK or
  both) rely on the use of timers

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ARQ Schemes

• The most common ARQ retransmission schemes
• Stop-and-Wait ARQ
• Go-Back-N ARQ
• Selective Repeat ARQ

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Go-Back-N ARQ

• Go-Back-N uses the sliding window flow control
  protocol. If no errors occur the operations are
  identical to Sliding Window
• Operations
• A station may send multiple frames as allowed by
  the window size
• Receiver sends a NAK i if frame i is in error.
  After that, the receiver discards all incoming
  frames until the frame in error was correctly
  retransmitted
• If sender receives a NAK i it will retransmit
  frame i and all packets i1, i2,... which have
  been sent, but not been acknowledged

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Selective-Repeat ARQ

• Similar to Go-Back-N ARQ. However, the sender
  only retransmits frames for which a NAK is
  received
• Advantage over Go-Back-N
• Fewer Retransmissions.
• Disadvantages
• More complexity at sender and receiver
• Each frame must be acknowledged individually (no
  cumulative acknowledgements)
• Receiver may receive frames out of sequence

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Analysis of ARQ Protocols

• What is the efficiency of the discussed ARQ
  protocols?
• A number of assumptions
• ACKs and NAKs are never lost, and frames are not
  dropped.
• Sizes of ACKs, NAKs, and frame headers are
  negligible.

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Analysis of Stop-and-Wait ARQ

• Parameters
• Uefficiency
• TtF/C (transmission delay of a frame)
• Ipropagation delay
• aI/Tt
• Pprobability that a frame is in error
• Without Errors (P0)
• UTt/(Tt2I)

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Stop-and-Wait ARQ With Errors

• Probability that k transmission attempts are
  needed to successfully transmit a frame

• Expected number of attempts (EA)

• Expected efficiency with errors

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Analysis of Selective Reject ARQ
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Local Area Networks (LANs)

• Broadcast Networks
• Multiple Access Protocols
• Ethernet (IEEE 802.3)
• Token Ring (IEEE 802.5, FDDI)

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Examples of Broadcast Network

• If more than one station transmits at a time on
  the broadcast channel, a collision occurs
• Multi-access problem How to determine which
  station can transmit?

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Multi-access Protocols

• Protocols that solve the resolution problem
  dynamically are called Multiple Access
  (Multi-access) Protocols
• Different types of multi-access protocols
• Contention protocols resolve a collision after it
  occurs. These protocols execute a collision
  resolution protocol after each collision
• Collision-free protocols ensure that a collision
  can never occur

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Contention Protocols

• ALOHA Protocols
• (Pure) Aloha
• Slotted Aloha
• CSMA (Carrier Sense Multiple Access)
• persistent CSMA
• non-persistent CSMA
• CSMA/CD - Carrier Sense Multiple Access with
  Collision Detection ( Ethernet)
• There are many more

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Collisions in (Pure)ALOHA
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Slotted ALOHA (S-ALOHA)

• The Slotted Aloha Protocol
• Slotted Aloha - Aloha with an additional
  constraint
• Time is divided into discrete time intervals
  (slot)
• A station can transmit only at the beginning of a
  frame
• As a consequence
• Frames either collide completely or do not
  collide at all
• Vulnerable period 1

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Collisions in S-ALOHA
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Performance of (Pure)ALOHA

• Question What is the maximum throughput of the
  ALOHA protocol?
• Notation
• S Throughput Expected number of successful
  transmissions per time unit. Normalization Frame
  transmission time is 1, maximum throughput is 1
• G Offered Load Expected number of transmission
  and retransmission attempts (from all users) per
  time unit

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Modeling Assumptions

• All frames have a fixed length of one time unit
  (normalized)
• Infinite user population
• Offered load is modeled as a Poisson process with
  rate G, that is,
• Probk packets are generated in t frame times

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Throughput of Aloha

• Fundamental relation between throughput and
  offered load
• S G x Prob frame suffers no collision

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Performance of (pure)ALOHA

• Prob frame suffers no collision
• Prob no other frame is generated during the
  vulnerable period for this frame
• Prob no frame is generated during a 2-frame
  period

Throughput in ALOHA
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Results

• Maximum achievable throughput
• Take the derivative and set
• Maximum is attained at G 0.5
• We obtain
• That is about 18 of the capacity!!!

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Performance of S-ALOHA

• Derivation is analogous to Aloha
• S G x Probframe suffers no collision
• Prob frame suffers no collision
• Prob no other frame is generated during a
  vulnerable period
• Prob no frame is generated during 1 frame
  period

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Performance of S-ALOHA

• Total Throughput in ALOHA
• Maximum achievable throughput

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Comparison of ALOHA and S-ALOHA
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CSMA - Carrier Sense Multiple Access

• Improvement to ALOHA protocol
• If stations have carrier sense capability
  (stations can test the broadcast medium for
  ongoing transmission), and
• if stations only transmit if the channel is idle,
• then many collisions can be avoided.
• Caveat This improves ALOHA only if the ratio a
  is small.

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CSMA - Carrier Sense Multiple Access

• CSMA protocol
• A station that wishes to transmit listens to the
  medium for an ongoing transmission
• Is the medium in use?
• Yes Station back of for a specified period
• No Station transmits
• If a sender does not receive an acknowledgment
  after some period, it assumes that a collision
  has occurred
• After a collision a station backs off for a
  certain (random) time and retransmits

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Variations of CSMA Protocols

• There are a number of variations of CSMA
  protocols
• Each variant specifies what to do if the medium
  is found busy
• Non-Persistent CSMA
• 1-Persistent CSMA
• p-Persistent CSMA

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Comparison of ALOHA and CSMA
Load vs. Throughput Assumption propagation
delay ltlt transmission delay
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CSMA / CD

• Improvement to CSMA protocol
• Carrier Sense Multiple Access with Collision
  Detection
• Widely used for bus topology LANs (IEEE 802.3,
  Ethernet)
• Only works if propagation delay is small relative
  to transmission delay (in other words, a must be
  small)

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CSMA/CD

• CSMA has an inefficiency
• If a collision has occurred, the channel is
  unstable until colliding packets have been fully
  transmitted
• CSMA/CD overcomes this as follows
• While transmitting, the sender is listening to
  medium for collisions. Sender stops if collision
  has occurred
• Note
• CSMA Listen Before Talking
• CSMA/CD Listen While Talking

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CSMA/CD

• Question How long does it take to detect a
  collision?
• Answer In the worst case, twice the maximum
  propagation delay of the medium

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Collision Detection in CSMA/CD
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CSMA/CD

• Restrictions of CSMA / CD
• Packet should be twice as long as the time to
  detect a collision (2 maximum propagation
  delay)
• Otherwise, CSMA/CD does not have an ad-vantage
  over CSMA
• Example Ethernet
• Ethernet requires a minimum packet size and
  restricts the maximum length of the medium

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Exponential Backoff Algorithm

• Ethernet uses the exponential backoff algorithms
  to determine when a station can retransmit after
  a collision
• Algorithm
• Set "slot time" equal to 2a
• After first collision wait 0 or 1 slot times
• After i-th collision, wait a random number
  between 0 and 2i -1 time slots
• Do not increase random number range, if i10
• Give up after 16 collisions

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Performance of CSMA/CD

• Parameters and assumptions
• End-to-end propagation delay a
• Packet transmission time (normalized) 1
• Number of stations N
• Time can be thought of as being divided in
  contention intervals and transmission intervals.
• Contention intervals can be thought of as being
  slotted with slot length of 2a (roundtrip
  propagation delay).

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Performance of CSMA/CD

• Contention slots end in a collision
• Contention interval is a sequence of contention
  slots
• Length of a slot in contention interval is 2a
• We assume that the probability that a station
  attempts to transmit in a slot is P

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Performance of CSMA/CD
Derivation of maximum throughput of CSMA/CD

• Let A be the probability that some station can
  successfully transmit in a slot. We get
• In the above formula, A is maximized when P1/ N.
  Thus

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Performance of CSMA/CD

• Prob contention interval has a length of j
  slots
• Prob 1 successful attempt x Prob j-1
  unsuccessful attempts

The expected number of slots in a contention
interval is then calculated as
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Performance of CSMA/CD

• Now we can calculate the maximum efficiency of
  CSMA/CD with our usual formula

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IEEE 802 LAN Standard
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IEEE 802 LAN standard
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IEEE 802 LAN Architecture

• Functions of the LLC
• Similar to HDLC (sliding window protocol)
• Provides SAPs to higher layers
• Provides different services
• acknowledged connectionless service
• unacknowledged connectionless service
• connection-oriented service
• Framing
• Error control
• Addressing

106
IEEE802.3 (CSMA/CD)

• Generally referred to as Ethernet
• Based on CSMA/CD
• Applies exponential back-off after collisions
• Data Rate 2 - 1,000 Mbps
• Maximum cable length is dependent on the data
  rate
• Uses Manchester encoding
• Bus topology

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Ethernet

• There are many different physical layer
  configurations for 802.3 LANs
• The following notation is used to denote the
  configuration

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Ring Local Area Network
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States of the Ring Interface

• Listen State Incoming bits are copied to output
  with 1-bit delay
• Transmit State Write data to the ring
• Bypass State Idle station does not incur
  bit-delay

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Ring LANs

• If a frame has traveled once around the ring it
  is removed by the sender
• Ring LANs have a simple acknowledgment scheme
• Each frame has one bit for acknowledgment.
• If the destination receives the frame it sets the
  bit to 1.
• Since the sender will see the returning frame, it
  can tell if the frame was received correctly.

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What is the "Length" of a Ring?

• The length of a ring LAN, measured in bits, gives
  the total number of bits which are can be in
  transmission on the ring at a time
• Note Frame size is not limited to the length of
  the ring since entire frame may not appear on the
  ring at one time.
• Bit length propagation speed length of ring
  data rate No. of stations bit delay at
  repeater

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Ring LAN

• Advantages
• Can achieve 100 utilization
• No collisions
• Can achieve deterministic delay bounds
• Can be made efficient at high speeds
• Disadvantages
• Long delays due to bit-delays
• Solution Bypass state eliminates bit-delay at
  idle station
• Reliability Problems
• Solution 1 Use a wire center
• Solution 2 Use a second ring (opposite flow)

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Token Ring LANS

• Token is a small packet that rotates around the
  ring
• When all stations are idle, the token is free and
  circulates around the ring
• Possible Problem All stations are idle and in
  the Bypass state. What is the problem?

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802.5(Token Ring) MAC Protocol

• In order to transmit a station must catch a free
  token
• The station changes the token from free to busy
• The station transmits its frame immediately
  following the busy token
• IF station has completed transmission of the
  frame AND the busy token has returned to the
  station THEN station inserts a new free token
  into the ring

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Properties of the 802.5 Token Ring

• No collisions of frames
• Full utilization of bandwidth is feasible
• Transmission can be regulated by controlling
  access to token
• Recovery protocols is needed if token is not
  handled properly, e.g., token is corrupted,
  station does not change to "free" etc

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Priority of Transmission in 802.5

• Eight levels of priorities
• Priorities handled by 3-bit priority field and
  3-bit reservation field
• Define
• Pm priority of the message to be transmitted
• Pr token priority of received token
• Rr reservation priority of received token

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Token Ring Priority Scheme Example
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(No Transcript)
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Effect of propagation delay

• Effect of propagation delay on throughput
• Case 1 a lt 1 (Packet longer than ring)
• T2 time to pass token to the next station a/N
• Case 2 a gt 1 (Packet shorter than ring)
• Note Sender finishes transmission after T1 1,
  but cannot release the token until the token
  returns
• T1T2 max(1, a) a/N

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FDDI

• FDDI distinguishes 4 Service Classes
• Asynchronous
• Synchronous
• Immediate (for monitor and control)

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Station Types - Class A Station

• Two PHY (and one or two MAC) entities
• Connects to another Class A station or to a
  concentrator

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Station Types - Class B Station

• Class B station has one PHY (and one MAC) entity
• Connects to a concentrator

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dual attach node
c o n c e n t r a t o r
dual attach node
single attach nodes
FDDI Dual Ring Structure
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FDDI Media Access Control

• FDDI uses a Token Ring Protocol, similar to 802.5
• Differences of FDDI and 802.5
• To release a token, a station does not need to
  wait until the token comes back after a
  transmission. The token is released right after
  the end of transmission
• In FDDI, multiple frames can be attached to the
  token
• FDDI has a different priority scheme

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Timed Token Protocol

• FDDI has a timed token protocol which determines
  how long a station can transmit
• Each station has timers to measure the time
  elapsed since a token was last received
• TTRT Target Token Rotation Time
• Value of TTRT is negotiated during initialization
  (default is 8 ms)
• Set to the maximum desired rotation time

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Parameters of Timed Token Protocol

• Station Parameters
• TRT Token Rotation Time
• Time of the last rotation of the token.
• If TRT lt TTRT, then token is early,
  asynchronous traffic can be transmitted
• If TRT gt TTRT then token is late, asynchronous
  traffic cannot be transmitted.
• THT Token Holding Time
• Controls the time that a station may transmit
  asynchronous traffic.
• fi Percentage of the TTRT that is allocated for
  synchronous traffic at station i.

127
Timed Token Protocol

• If a station receives the token it sets
• THT TRT
• TRT TTRT
• Enable TRT (i.e., start the timer)
• If the station has synchronous frames are waiting
  the transmit synchronous traffic for up to time
  TTRTfi (with sum(fi) lt1)
• If the station has asynchronous traffic
• enable THT
• while THT gt 0 transmit asynchronous traffic.

128
FDDI MAC Operation

• When a token arrives each station follows this
  procedure
• THT TTRT TRT
• TRT 0
• Send Synchronous Data
• IF THT gt 0, enable THT and start sending
  Asynchronous data as long as THT gt 0

129
FDDI MAC Example
130
FDDI MAC Example
131
FDDI MAC Example
132
FDDI MAC Example
133
FDDI MAC Example-Sync. (TTRT 80)
Maximum Throughput 80 / 84 95.23
134
FDDI MAC Example-Asyn. (TTRT100)
135
FDDI MAC Example(Asyn. TTRT100)
Time 404, station 1 gets the token
136
Analysis of FDDI

• Analysis of
• Synchronous traffic
• Asynchronous traffic
• Synchronous Traffic
• Recall that each station can transmit synchronous
  traffic for up to time TTRTfi (with sum(fi)lt1)
• If sum(fi)1, the maximum throughput of
  synchronous traffic is 100.
• One can show that the maximum delay until a frame
  is completely transmitted is
• Maximum Access Delay lt 2TTRT

137
Analysis of FDDI

• Asynchronous Traffic
• Parameters
• D Ring latency
• n Number of active sessions (all heavily loaded)
• T Value of TTRT
• Assumption
• No synchronous traffic

138
Analysis of FDDI

• From the Example we see
• Cycle in a system has a length of nT D
• Time in a cycle used for transmission n(T - D)
• We obtain for the maximum throughput for
  asynchronous traffic is

• ... and the maximum access delay for asynchronous
  traffic
• Max. Access DelayT(n-1)2D

139
IEEE 802.4 (Token Bus)

• Problems with 802.3
• Collisions of frames can lead to unpredictable
  delays
• In some real-time scenarios, collisions and
  unpredictable delays can be catastrophic
• Solution via Token Bus
• A control packet (Token) regulates access to the
  bus
• A station must have the token in order to
  transmit
• A station can hold the token only for a limited
  time
• The token is passed among the stations in a
  cyclic order
• This structures the bus as a logical ring

140
IEEE 802.4 (Token Bus)

• Stations form a logical ring
• Each station knows its successor and predecessor
  in the ring

141
Feature of Token Bus

• Bandwidth is 1, 5, or 10 Mbps
• The token bus MAC protocol is very complex
• Typically, token bus is free of collisions
• Defines priority transmissions and can offer
  bounded transmission delays

142
IEEE 802.4 (Token Bus)

• 802.4 requires each station to implement the
  following management functions
• Ring Initialization
• Addition to ring
• Deletion from ring
• Fault management

143
Adding a Station to the Token Bus

• Each node periodically sends a solicit successor
  packet which invites nodes with an address
  between itself and the next node to join the ring
• Sending node waits for response for one round
  trip
• One of the following three cases apply
• (1) No Response
• Pass token
• (2) Response from one node
• Reset successor node
• Pass token to new successor node
• (3) Response from more than one node
• Collision has occurred
• Node tries to resolve contention

144
Add a station to the Token Bus

• Assume Response from more than one node has
  resulted in a collision.
• Station sends a resolve contention packet and
  waits for four windows
• (window 1 round trip time) for a response
• In window 1, stations with address prefix 00 can
  reply
• In window 2, stations with address prefix 01 can
  reply
• In window 3, stations with address prefix 10 can
  reply
• In window 4, stations with address prefix 11 can
  reply
• If there is a another collision, procedure is
  repeated for the second pair of bits. Only the
  nodes which replied earlier can join the next
  round
• First successful reply joins the ring

145
IEEE 802.4 (Token Bus)

• Four priority levels
• Levels 6, 4, 2, 0
• Priority 6 is the highest level
• Token Holding Time (THT)
• Maximum time a node can hold a token
• Token Rotation Time for class i (TRTi)
• Maximum time of a full token circulation at which
  priority i transmissions are still permitted

146
Token Bus Transmission Rules

• Each station can transmit class 6 data for a time
  THT
• For i 4, 2, 0
• Transmit class i traffic if all traffic from
  class i2 or higher is transmitted
• and the time of the last token circulation
  (including the transmission time of higher
  priority packets during the current holding of
  the token) is less than TRT i .