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(Ownership, Service Paradigm, Measures of Performance)
Chapter 15 Network Properties (Ownership, Service Paradigm, Measures of Performance)
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Comer, 4e, Ch 15 and Ch 16 Comer 5e, Ch 26
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Network Ownership And Service Type
Private Owned by individual or corporation Restricted to owner’s use Typically used by large corporations Public Owned by a common carrier Individuals or corporations can subscribe “Public” refers to availability, not data
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Advantages and Disadvantages
Private Complete control Installation and operation costs Public No need for staff to install/operate network Dependency on carrier Subscription fee
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Public Network Connections
One connection per subscriber Typical for small corporation or individual Communicate with another subscriber Multiple connections per subscriber Typical for large, multi-site corporation Communicate among multiple sites as well as with another subscriber
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Virtual Private Network
A service Provided over public network Interconnects sites of single corporation Acts like private network No packets sent to other subscribers No packets received from other subscribers Data encrypted
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Virtual Private Network
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Frame Relay Pricing Permanent Virtual Circuits (PVCs)
Leased access line must be fast enough to handle all of the PVCs it is multiplexing Example: if it multiplexes kbps PVCs, the access line must be 840 kbps (T1 line needed) PVC Leased Access Line PVC
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Network Service Paradigm
Fundamental characteristic of network Understood by hardware Visible to applications Two basic types of networks Connectionless Connection-oriented
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Connectionless ( CL ) Sender Network Forms packet to be sent
Places address of intended recipient in packet Transfers packet to network for delivery Network Uses destination address to forward packet Delivers
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Characteristics of Connectionless Networks
Packet contains identification of destination Each packet handled independently No setup required before transmitting data No cleanup required after sending data Think of postcards
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Connection-Oriented (CO)
Sender Requests “connection” to receiver Waits for network to form connection Leaves connection in place while sending data Terminates connection when no longer needed
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Connection-Oriented (CO) (continued)
Network Receives connection request Forms path to specified destination and informs sender Transfers data across connection Removes connection when sender requests Think of telephone calls
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Terminology In conventional telephone system In CO data network
Circuit In CO data network Virtual Circuit Virtual Channel
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Comparison of CO and CL CO CL More intelligence in network
Can reserve bandwidth Connection setup overhead State in packet switches Well-suited to real-time applications CL Less overhead Permits asynchronous use Allows broadcast / multicast
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Two Connection Types Permanent Virtual Circuit (PVC)
Entered manually Survives reboot Usually persists for months Switched Virtual Circuit (SVC) Requested dynamically Initiated by application Terminated when application exits
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Examples of Service Paradigm Various Technologies Use
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Connection Multiplexing
Typical computer has one physical connection to network All logical connections multiplexed over physical interconnection Data transferred must include connection identifier
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Connection Identifier
Integer value One per active VC Not an address Allows multiplexing Computer supplies when sending data Network supplies when delivering data
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Example Connection Identifier (ATM)
24 bits long (The full address is 160 bits) Divided into two parts Virtual Path Identifier Virtual Channel Identifier Known as (VPI/VCI) Different at each end of connection Mapped by switches
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Illustration of ATM VC Switch maps VPI/VCIs 17 to 12 96 to 8
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Two Primary Performance Measures
Delay Throughput
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Delay Time required for one bit to travel through the network
Three types (causes) Propagation delay Switching delay Queuing Delay Intuition: “length” of the pipe
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Throughput Number of bits per second that can be transmitted Capacity
Intuition: “width” of the pipe
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Components of Delay Fixed (nearly constant) Variable
Propagation delay Switching delay Variable Queuing delay Depends on throughput If delay is changing rapidly, we refer to it as Jitter
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Relationship Between Delay and Throughput
When network idle Queuing delay is zero As load on network increases Queuing delay rises Load defined as ration of throughput to capacity Called utilization
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Relationship Between Delay and Utilization
Define D0 to be the propagation and switching delay U to be the utilization (0 U 1) D to be the total delay Then High utilization known as congestion
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Practical Consequence
Any network that operates with a utilization approaching 100% of capacity is doomed
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Delay-Throughput Product
Time to cross network Measured in seconds Throughput Capacity Measured in bits per second Delay * Throughput Measured in bits Gives quantity of data “in transit”
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Summary Network can be Virtual Private Network Public Private
Uses public network Connects set of private sites Addressing and routing guarantee isolation
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Summary (continued) Networks are Connection types
Connectionless Connection-oriented Connection types Permanent Virtual Circuit Switched Virtual Circuit Two performance measures Delay Throughput
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Summary (continued) Delay and throughput interact
Queuing delay increases as utilization increases Delay * Throughput Measured in bits Gives total data “in transit”
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Protocols and Protocol Layering
Chapter 16 Protocols and Protocol Layering
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Protocol Agreement about communication Specifies
Format of messages (syntax) Meaning of messages (semantics) Rules for exchange Procedure for handling problems
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Need for Protocols Hardware is low level Many problems can occur
Bits corrupted or destroyed Entire packet lost Packet duplicated Packets delivered out of order
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Need for Protocols (continued)
Need mechanisms to distinguish among Multiple computers on a network Multiple applications on a computer Multiple copies of a single application on a computer
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Set of Protocols Work together
Each protocol solves part of communication problem Known as Protocol suite Protocol family Designed in layers
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Plan for Protocol Design
Intended for protocol designers Divides protocols into layers Each layer devoted to one subproblem Example: ISO 7-layer reference model
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Illustration of the 7-Layer Model
All People Seems To Need Data Processing Defined early Now somewhat dated Does not include internet layer!
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ISO Layers Layer 1: Physical Layer 2: Data Link (media access)
Underlying hardware (Example: RS-232) Layer 2: Data Link (media access) Hardware frame definitions Layer 3: Network Packet forwarding Layer 4: Transport Reliability
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ISO Layers (continued)
Layer 5: Session Login and passwords Layer 6: Presentation Data representation Layer 7: Application Services for common applications
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TCP/IP protocol suite
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Layers and Protocol Software
Protocol software follows layering model One software module per layer Modules cooperate Incoming or outgoing data passes from one module to another Entire set of modules known as stack
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Illustration of Stacks
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Layers and Packet Headers
Each layer Prepends header to outgoing packet Removes header from incoming packet
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Example of encapsulation
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Scientific Layering Principle
Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source
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Illustration of Layering Principle
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Protocol Techniques For bit corruption For out-of-order delivery
Parity Checksum CRC For out-of-order delivery Sequence numbers Duplication
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Protocol Techniques (continued)
For lost packets Positive acknowledgement and retransmission For replay (excessive delay) Unique message ID For data overrun Flow control
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Flow Control Needed because Related to buffering Two forms
Sending computer system faster than receiving computer Sending application faster than receiving application Related to buffering Two forms Stop-and-go Sliding window
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Stop-And-Go Flow Control
Sending Side Transmits one packet Waits for signal from receiver Receiving side Receives and consumes packets Transmits signal to sender Inefficient
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Acknowledgements & Timeouts
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Stop & wait sequence numbers
Sender Receiver Sender Receiver Sender Receiver Frame 0 Frame 0 Frame 0 imeout imeout ACK 0 T T ACK 0 ACK 0 Frame 0 Frame 1 Frame 0 imeout imeout T ACK 0 ACK 1 ACK 0 T Frame 0 (c) (d) ACK 0 (e) Simple sequence numbers enable the client to discard duplicate copies of the same frame Stop & wait allows one outstanding frame, requires two distinct sequence numbers CS 640
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Sliding Window Flow Control
Receiving side Establishes multiple buffers and informs sender Sending side Transmits packets for all available buffers Only waits if no signal arrives before transmission Sends signals as packets arrive
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Illustration of Sliding Window on Sending Side
Window tells how many packets can be sent Window moves as acknowledgements arrive
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Sliding Window Example
Sender Receiver 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 3 4 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 6 A4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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Performance Stop-and-go Sliding window Slow
Useful only in special cases Sliding window Fast Needed in high-speed network
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Comparison of Flow Control
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Why Sliding Window? Simultaneously Speedup Tw = min(B, TG * W) where
Increase throughput Control flow Speedup Tw = min(B, TG * W) where B is underlying hardware bandwidth TW is sliding window throughput TG is stop-and-go throughput W is the window size
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Congestion Fundamental problem in networks
Caused by traffic, not hardware failure Analogous to congestion on a highway Principle cause of delay
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Illustration of Architecture That Can Experience Congestion
Multiple sources Bottleneck
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Dealing with Congestion
Congestion results in filled buffers in packet switches => packets will be discarded => retransmission => more packets….. Solutions Increasing buffer space Reducing the amount of packets in the network Packet switches inform senders, or Senders use packet loss as an indication
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Congestion and Loss Modern network hardware works well; most packet loss results from congestion, not from hardware failure
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