Computer Networks: Introduction

1
Computer NetworksIntroduction

• Ivan Marsic
• Rutgers University

Chapter 1 Introduction
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TopicIntroduction to Data Networking

• ? Goals
• ? Communication Media
• ? Protocols
• ? Reliable Transmission

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User Goals and Tunable Knobs
Visible network properties
Correctness
Fault tolerance
Timeliness
Cost
Delivery
Customer
Tunable network parameters
Network topology
Communication protocols
Network architecture
Components
Physical medium
Network Engineer
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Topology vs. Robustness
Paul Baran, 1964
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Internet Map Major ISPs
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Fully Interconnected Network
New York City, 1888
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Early Telephone Switching Offices
8
1924 First Mobile Telephone
The first version of a mobile radio telephone
being used in 1924.
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Exploiting Locality
Saul Steinberg, A View of the World from Ninth
Avenue, cover of The New Yorker March 29, 1976
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Distortion of Signals
threshold "0"/"1"
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Packet Error Rate Approximation
PER packet error rate BER bit error rate n
packet length in bits
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Packet Transmission (1)
Example Sender sends a 6-bit packet 101101
to the receiver
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Transmission Link Capacity
Effect of link speed ? Link 2 can transmit 10
times more bits per unit of time ? or, Link 2
can transmit the same message in a 10 times
shorter period
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Wireless Communication
Point source
Interfering sources
Interference pattern
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Radio Signal Propagation
Ray tracing simulation in a closed office
environment. Signal intensity map for a room
with a doorway and a metal desk
door
desk
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Transmission / Interference Range
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Protocols
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Statistical Multiplexing
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3-Layer Protocol Stack
Protocol at layer i depends only on the protocols
at i?1 (not at i?1!)
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Layer 1 / 3-Layer Protocol Stack
Link Layer Protocol modules at layer 1 (bottom
layer) exchange packets over the link
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Layer 2 / 3-Layer Protocol Stack
Network Layer Protocol modules at layer 2
(middle layer) route packets from source to
destination (possibly over many links)
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Layer 3 / 3-Layer Protocol Stack
Applications ? Network games ? Internet
telephony ? Email
End-to-End Layer Protocol modules at layer 3
(top layer) create illusion of different link
types (tailored to application-specific needs)
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Protocol Layers at Hosts/Switches
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Bit Stuffing for Transparency
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ISO OSI Protocol Stack

• Application services (SIP, FTP, HTTP,
  Telnet, )

• Data translation (MIME)
• Encryption (SSL)
• Compression

• Dialog control
• Synchronization

• Reliable (TCP)
• Real-time (RTP)

• Source-to-destination (IP)
• Routing
• Address resolution

• Wireless link (WiFi)
• Wired link (Ethernet)

• Radio spectrum
• Infrared
• Fiber
• Copper

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Packet Nesting Across Layers
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How Headers Guide Packets
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Error Detection and Correction
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Interleaving
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Packet Transmission (2)
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Transmission and Propagation Delays
Transmission delay
Propagation delay
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Fluid Flow Analogy
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What Contributes to RTT
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TopicReliable Transmission via Retransmission

• ? Stop-and-Wait
• ? Go-Back-N
• ? Selective Repeat

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Automatic Repeat reQuest (ARQ)

• Stop-and-wait ARQ
• Transmit a frame and wait for acknowledgement
  (ACK)
• If positive ACK from receiver, send next frame
• If ACK does not arrive after a certain period of
  time (Timeout), retransmits the frame
• Simple, low efficiency
• Go-back-N ARQ
• Transmit frames continuously, no waiting
• The receiver only ACKs the highest-numbered
  frames received in sequence
• ACK comes back after a round-trip delay
• If timeout, the sender retransmits the frames
  that are not ACKed and N1 succeeding frames that
  were transmitted during the round-trip delay (N
  frames transmitted during a round-trip delay)
• Need buffer at sender, does not have to buffer
  the frames at the receiver,
• Moderate efficiency and complexity. Less
  efficient when the round-trip delay is large and
  data transmission rate is high
• Selective-repeat ARQ
• Transmit continuously, no waiting
• The receiver ACKs all successfully received
  frames
• The sender only retransmits (repeats) the unACKed
  frames when their timers expire
• Most efficient, but most complex, buffer needed
  at both sender receiver, needs per-frame timer

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Stop-and-Wait with Errors
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Stop Wait Sender Utilization
Stop Wait sender utilization, under error-free
transmission
Probability of successful transmission, with
error rate pe
Expected sender utilization for Stop Wait,
under errors
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Sliding Window Keeping the Pipe Full

• Goal Sender should be busy sending packets (as
  long as it has packets ready to send)
• Sender utilization as a metric of protocol
  performance
• Keeping the pipe full

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Sliding Window ARQ
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Go-back-N ARQ
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Selective Repeat ARQ
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Acknowledgements GBN vs. SR
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TopicBroadcast and Wireless Links

• ? ALOHA
• ? Hidden and Exposed Stations
• ? Carrier Sensing Multiple Access
• ? CSMA/CD, CSMA/CD

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Transmission Cone
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Transmission Cone, Collision Vulnerable Period
(b)
Collision occurs if two (or more) transmission
cones overlap.
(a)
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Parameter ?
Ratio of propagation delay vs. packet
transmission time
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Parameter ?
Ratio of propagation delay vs. packet
transmission time
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ALOHA and Slotted ALOHASenders State Diagram
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ALOHA Packet Transmission
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When transmission cones of ALOHA stations overlap?
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ALOHA Scenario
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Backlogged Stations

• Fresh stations transmit new packets
• Backlogged stations re-transmit collided packets

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Analysis of Slotted ALOHA (1)

• ASSUMPTIONS FOR ANALYSIS
• All packets require 1 slot for x-mit
• Poisson arrivals, arrival rate ?
• Collision or perfect reception (no errors)
• Immediate feedback (0, 1, e)
• Retransmission of collisions (backlogged
  stations)
• No buffering or infinite set of stations(m ?)

Time Slots
i ? 1
i
i ? 1
i ? 2
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ALOHA Model
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Analysis of Slotted ALOHA (2)

• 0 lt ? lt 1, since at most 1 packet / slot
• Equilibrium departure rate arrival rate
• Backlogged stations transmit randomly
• Retransmissions new transmissionsPoisson
  process with parameter G gt ?

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Analysis of Slotted ALOHA (2)

• Throughput arrival rate ?
  probability of no collision
• Slotted ALOHA throughput
• Pure ALOHA throughput

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Efficiency of ALOHAs
S-ALOHA In equilibrium, arrival rate departure
rate ? Ge?G Max departure rate
(throughput) 1/e ? 0.368 _at_ G 1
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Unslotted (Pure) ALOHA

• Assume all packets same size, but no fixed slots
• The packet suffers no collision if no other
  packet is sent within 2 packets long SGP0Ge?2G
• Max throughput 1/2e ? 0.184 _at_ G 0.5
• Less efficient than S-ALOHA, but simpler, no
  global time synchronization

i
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Hidden Stations
? A is transmitting to B. ? C wants to transmit
and listens before talk but cannot hear A
because A is too far away (As radio signal
is too weak for C to hear, so A and C are
hidden stations to each other). ? C concludes
that the medium is idle and transmits, thus
interfering with Bs reception.
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Exposed Stations
? B is transmitting to A. ? C wants to transmit
to D, it listens before talk and hears B so
it refrains from transmitting although its
transmission would not interfere with As
reception. ? Therefore, C is an exposed station
to B. Note that for C to be allowed to transmit,
C must know that A is not located in its
transmission range and that D is not in Bs
transmission range. ?? Hidden station problem is
much simpler to solve!
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Exponential Backoff
Key idea increasing number of choices reduces
the probability of repeated collisions
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Wired Broadcast Media CSMA
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CSMA / CD Senders State Diagm
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Analysis of CSMA/CD
Probability of a successful transmission
Channel efficiency for CSMA/CD
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CSMA/CD Collision Detection
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CSMA/CD Backoff Example
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CSMA / CA Senders State Diagm
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Efficiency of CSMA protocols
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Delay vs. Arrival Rate
TDMA
Maximum channel transmission rate
CSMA/CA
Average packet delay
ALOHA
CSMA/CD
Arrival rate ? per station
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TopicInternetworking

• ? Routing Forwarding
• ? Internet Protocol (IPv4) Datagram
  Fragmentation
• ? Link State Routing Distance Vector Routing
• ? Addressing CIDR

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Packet Switching / Routing
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Example Internetwork
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Protocol Stack atEnd-points vs. Routers
End-point protocol stack
Router protocol stack
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Routing Problem
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Network-layer ProtocolInternet Protocol (IP)
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IPv4 Header
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Dotted Decimal Notation for IPv4
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Domain Name System (DNS)
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IP Datagram Fragmentation (1)
(a)
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IP Datagram Fragmentation (2)
NO FRAGMENTATION
FRAGMENTATION OCCURS HERE
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Datagram Reassembly at Host D
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Network Routing Link State
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Network Example for Routing Protocols
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Example of Link State Routing
Step Confirmed set N ? Tentative set Comments
0 (A, 0, ?) ? Initially, A is the only member of Confirmed(A), so examine As LSA.
1 (A, 0, ?) (B, 10, B),(C, 1, C) As LSA says that B and C are reachable at costs 10 and 1, respectively. Since these are currently the lowest known costs, put on Tentative(A) list.
2 (A, 0, ?), (C, 1, C) (B, 10, B) Move lowest-cost member (C) of Tentative(A) into Confirmed set. Next, examine LSA of newly confirmed member C.
3 (A, 0, ?), (C, 1, C) (B, 2, C),(D, 8, C) Cost to reach B through C is 1?12, so replace (B, 10, B). Cs LSA also says that D is reachable at cost 7?18.
4 (A, 0, ?), (C, 1, C), (B, 2, C) (D, 8, C) Move lowest-cost member (B) of Tentative(A) into Confirmed, then look at Bs LSA.
5 (A, 0, ?), (C, 1, C), (B, 2, C) (D, 3, C) Because D is reachable via B at cost 1?1?13, replace the Tentative(A) entry for D.
6 (A, 0, ?), (C, 1, C), (B, 2, C), (D, 3, C) ? Move lowest-cost member (D) of Tentative(A) into Confirmed. END.
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Example Link State (1)
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Example Link State (2)
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Network Routing Distance Vector
shortest path (src?? dest) Min 7 ? 19, 4 ?
29, 25 ? 8 7 ? 19 26
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Distance Vector Calculation
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Example - Distance Vector
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Example - Distance Vector
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(No Transcript)
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Example DV Routing Loops
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TopicIP Addressing and CIDR

• ? Hierarchical Structure of IP Addresses
• ? CIDR (Classless Interdomain Routing)

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Example Road Map
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Forwarding Table Scalability
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OLD IPv4 Address Structure
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Address Class Sizes
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Special IPv4 Addresses
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CIDR Example (1)
(a)
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CIDR Example (2)
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CIDR Example (3)
Subnet Network prefix Binary representation Interface addresses
1 204.6.94.160/30 11001100 00000110 01011110 101000-- R2-1 204.6.94.160
1 204.6.94.160/30 11001100 00000110 01011110 101000-- R1-2 204.6.94.161
1 204.6.94.160/30 11001100 00000110 01011110 101000-- (unused)
1 204.6.94.160/30 11001100 00000110 01011110 101000-- b-cast 204.6.94.163
2 204.6.94.164/30 11001100 00000110 01011110 101001-- C 204.6.94.164
2 204.6.94.164/30 11001100 00000110 01011110 101001-- R1-3 204.6.94.165
2 204.6.94.164/30 11001100 00000110 01011110 101001-- D 204.6.94.166
2 204.6.94.164/30 11001100 00000110 01011110 101001-- b-cast 204.6.94.167
3 204.6.94.168/30 11001100 00000110 01011110 101010-- A 204.6.94.168
3 204.6.94.168/30 11001100 00000110 01011110 101010-- R1-1 204.6.94.169
3 204.6.94.168/30 11001100 00000110 01011110 101010-- B-1 204.6.94.170
3 204.6.94.168/30 11001100 00000110 01011110 101010-- b-cast 204.6.94.171
4 204.6.94.172/30 11001100 00000110 01011110 101011-- R2-2 204.6.94.172
4 204.6.94.172/30 11001100 00000110 01011110 101011-- B-2 204.6.94.173
4 204.6.94.172/30 11001100 00000110 01011110 101011-- (unused)
4 204.6.94.172/30 11001100 00000110 01011110 101011-- b-cast 204.6.94.175
5 204.6.94.176/30 11001100 00000110 01011110 101100-- R2-3 204.6.94.176
5 204.6.94.176/30 11001100 00000110 01011110 101100-- E 204.6.94.177
5 204.6.94.176/30 11001100 00000110 01011110 101100-- F 204.6.94.178
5 204.6.94.176/30 11001100 00000110 01011110 101100-- b-cast 204.6.94.179
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CIDR-based Forwarding Tables
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TopicAutonomous Systems

• ? Commercial Internet
• ? Peering and Transit Relationships
• ? Path Vector Routing

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Autonomous Systems (ASs)
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ISP Business Relationships
Figure (a) shows the pay-for-transit
relationship. In principle, a customer pays for
incoming and outgoing traffic, and expects to be
able to reach all other customers or content
providers on the global Internet. Permissible
amounts of traffic in both directions are
regulated by the service level agreement (SLA)
between the provider and the customer. ISP ? has
the same relationship with ISPs ? and ? as they
have to their customers. In other words, ? and ?
are ?s paying customers. In (b), ISP ? (not
shown) could peer with another same-tier ISP ?
(not shown) and their peering relationship would
work on the same principle as for ISPs ? and ?.
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ISP Business Relationship Example
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Providing Selective Transit (1)
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Providing Selective Transit (1)
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Providing Selective Transit (2)
AS? and its customers are customers of AS?and
AS? and its customers are customers of AS?
equivalent
customers of AS?
customers of AS?
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Routing in Global Internet (1)
router in AS? sends an update message advertising
the destination prefix 128.34.10.0/24
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Routing in Global Internet (2)
AS? advertises only its customers to its peers,
so AS? never learns that AS? has links to AS? and
AS?
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Example - Path Vector
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Example - Path Vector
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Integrating IGP EGP Tables
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Example - Path Vector
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TopicLink-Layer Technologies

• ? Point-to-Point Protocol (PPP)
• ? IEEE 802.3 a.k.a. Ethernet

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Link Layer Services

• Data-link layer transfer datagram from one node
  to adjacent node over a communication link
• Framing encapsulate datagram into a frame,
  adding header, trailer.
• Identify what set of bits constitute a frame,
  that is, determining the beginning and the end of
  a frame
• Channel access if shared medium
• MAC addresses used in frame headers to identify
  source destination
• different from IP addresses!
• Reliable delivery between adjacent nodes
• Error detection
• Error recovery forward error correction code,
  retransmission (ARQ)
• Flow control pacing between adjacent sending and
  receiving nodes
• Half-duplex and full-duplex
• with half duplex, either transmit or receive on a
  link,but not both nodes at same time

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Link Layer Sublayering
LLC Packet Data Unit
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Point-to-Point Protocol (PPP)
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PPP Functions

• Framing encapsulation of network-layer datagram
  in data-link frame
• Identify what set of bits constitute a frame,
  i.e., determine the start end of a frame
• Carry data of any network layer protocol (not
  just IP) at same time
• ability to demultiplex upwards
• Bit transparency must carry any bit pattern in
  the data field
• Error detection (no correction)
• Connection liveness detect, signal link failure
  to network layer
• Network-layer address negotiation endpoints can
  learn/configure each others network addresses
• Other characteristics of PPP
• no error correction/recovery
• no flow control
• out-of-order delivery acceptable
• no need to support multipoint links (e.g.,
  polling)

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Point-to-point (PPP) Frame Format
LCP or NCP Control Packets
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Point-to-point (PPP)State Diagram
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TopicIEEE 802.3 a.k.a. Ethernet

• ? Ethernet Medium Access Control (MAC) Protocol
• ? Ethernet Evolution
• ? Switched Ethernet

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802.3 Link-Layer Frame Format
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Ethernet Version Notation
Data rate(e.g., 10 Mbps, 10 Gbps) Baseband/Broadband transmission Wiring type (e.g., coaxial, twisted pair or fiber optic)
MAC address Network port Time last frame received
00-01-03-1D-CC-F7 1 1039
01-23-45-67-89-AB 1 1052
A3-B0-21-A1-60-35 2 1017
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Thin-Cable Ethernet
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Switched/Bridged Ethernet
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Legacy Ethernet vs. Eth. Hub
Thin-Cable Ethernet
Ethernet Hub
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Hub vs. Switch
OSI Layer-1 switching
Ethernet Hub
OSI Layer-2 switching
Ethernet Switch
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Ethernet MAC Link Duplexity
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Ethernet Switch
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Learning Switches
See the switching table in the next slide
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Switching Tablefor the Previous Example
MAC address Network port Time last frame received
00-01-03-1D-CC-F7 1 1039
01-23-45-67-89-AB 1 1052
A3-B0-21-A1-60-35 2 1017
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Loops in Switched LANs (1)
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Loops in Switched LANs (2)
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802.1D Configuration BPDU parameters and format
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TopicIEEE 802.11 a.k.a. Wi-Fi

• ? 802.11 Architecture
• ? 802.11 Medium Access Control
• ? RTS/CTS Protocol for Hidden Stations

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Components of 802.11 LANs
Ad hoc network does not have distribution system
nor access point
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IBSS and Infrastructure BSS
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Extended Service Set (ESS)
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802.11 Link Layer ProtocolArchitecture
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802.11 Link or MAC-LayerFrame Format
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802.11 PHY Frame(Long PPDU format)
PPDU PLCP protocol data unit PLCP physical
(PHY) layer convergence procedure SFD start
frame delimiter
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802.11 Address Fields
Address-1 RA Immediate recipient of the
current frame (C) Address-2
TA Transmitter which transmitted the current
frame (B) Address-3 SA Original source
(A) Address-4 DA
Original destination (D)
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802.11 Protocol Architecture
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802.11 Interframe Spaces (1)
CSMA / CA
Collision Avoidance
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802.11 Interframe Spaces (2)
EIFS definition A station ready to transmit
enters EIFS after detecting a corrupted frame
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IEEE 802.11b System Parameters
Parameter Value for 1 Mbps channel bit rate
Slot time 20 ?sec
SIFS 10 ?sec
DIFS 50 ?sec (DIFS SIFS 2 Slot time)
EIFS SIFS PHY-preamble PHY-header ACK DIFS 364 ?sec
CWmin 32 (minimum contention window size)
CWmax 1024 (maximum contention window size)
PHY-preamble 144 bits (144 ?sec)
PHY-header 48 bits (48 ?sec)
MAC data header 28 bytes 224 bits
ACK 14 bytes PHY-preamble PHY-header 304 bits (304 ?sec)
RTS 20 bytes PHY-preamble PHY-header 352 bits (352 ?sec)
CTS 14 bytes PHY-preamble PHY-header 304 bits (304 ?sec)
MTU Adjustable, up to 2304 bytes for frame body before encryption
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802.11 Basic Transmission Mode
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802.11 Protocol State Diagram Sender
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802.11 Protocol State Diagram Receiver
(b)
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Examples of Timing Diagrams for IEEE 802.11

1. A single station has two frames ready for
   transmission on an idle channel.
2. A single station has one frame ready for
   transmission on a busy channel. The
   acknowledgement for the frame is corrupted during
   the first transmission.
3. A single station has one frame ready for
   transmission on a busy channel. The data frame is
   corrupted during the first transmission.

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802.11 Timing Diagrams
(a) Timing of successful frame transmissions
under the DCF
(b) Frame retransmission due to ACK failure
(c) Frame retransmission due to an erroneous data
frame reception
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RTS/CTS for Hidden Stations
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RTS/CTS Transmission Mode
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TopicQuality of Service (QoS)

• ? Introduction Prospects
• ? Network Neutrality Debate

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Network Conceptual Model
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Players and Parameters
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Network Conceptual Model (2)
We dont know when sources will start/end their
sessions also for some types of data (video),
datarate is variable