Transcript Part 2.1 Network Properties (Ownership, Service Paradigm, Measures of Performance)

Part 2.1
Network Properties
(Ownership, Service Paradigm,
Measures of Performance)
Robert Probert, SITE, University of Ottawa
SEPT, 2005
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Network Ownership
And Service Type
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Private
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Owned by individual or corporation
Restricted to owner’s use
Typically used by large corporations
Public
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Owned by a common carrier
Individuals or corporations can subscribe
“Public” refers to availability, not data
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Advantages and
Disadvantages
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Private
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Complete control
Installation and operation costs
Public
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No need for staff to install/operate network
Dependency on carrier
Subscription fee
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Public Network Connections
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One connection per subscriber
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Typical for small corporation or individual
Communicate with another subscriber
Multiple connections per subscriber
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Typical for large, multi-site corporation
Communicate among multiple sites as well as
with another subscriber
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Virtual Private Network
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A service
Provided over public network
Interconnects sites of single corporation
Acts like private network
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No packets sent to other subscribers
No packets received from other subscribers
Data encrypted
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Network Service Paradigm
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Fundamental characteristic of network
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Understood by hardware
Visible to applications
Two basic types of networks
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Connectionless
Connection-oriented
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Connectionless ( CL )
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Sender
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Forms packet to be sent
Places address of intended recipient in packet
Transfers packet to network for delivery
Network
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Uses destination address to forward packet
Delivers
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Characteristics of
Connectionless Networks
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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)
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Sender
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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)
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Network
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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
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In conventional telephone system
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Circuit
In CO data network
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Virtual Circuit
Virtual Channel
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Comparison of CO and CL
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CO
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More intelligence in network
Can reserve bandwidth
Connection setup overhead
State in packet switches
Well-suited to real-time applications
CL
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Less overhead
Permits asynchronous use
Allows broadcast / multicast
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Two Connection Types
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Permanent Virtual Circuit (PVC)
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Entered manually
Survives reboot
Usually persists for months
Switched Virtual Circuit (SVC)
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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
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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
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Integer value
One per active VC
Not an address
Allows multiplexing
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Computer supplies when sending data
Network supplies when delivering data
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Two Primary
Performance Measures
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Delay
Throughput
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Delay
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Time required for one bit to travel through the
network
Three types (causes)
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Propagation delay
Switching delay
Queuing Delay
Intuition: “length” of the pipe
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Latency
Latency  Propagatio n  Transmit  Queue
Propagatio n  Distance / Speed of light
Transmit  Size / Bandwidth
 2.0  108 m / s in a fiber
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Speed of light   2.3  108 m / s in a cable
3.0  108 m / s in a vaccum
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Throughput
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Number of bits per second that can be
transmitted
Capacity
Intuition: “width” of the pipe
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Throughput
Throughput  Transfer size / Transfer t ime
(effective end-to-end throughput)
Transfer t ime  RTT  1/Bandwidt h  Transfer size
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Components of Delay
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Fixed (nearly constant)
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Propagation delay
Switching delays constant
Variable
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Queuing delay
Depends on throughput
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Relationship Between
Delay and Throughput
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When network idle
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As load on network increases
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Queuing delay is zero
Queuing delay rises
Load defined as ration of throughput to
capacity
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Called utilization
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Relationship Between
Delay and Utilization
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Define
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D0 to be the propagation and switching delay
U to be the utilization delay
D to be the total delay
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Then
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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
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Delay
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Throughput
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Time to cross network
Measured in seconds
Capacity
Measured in bits per second
Delay * Throughput
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Measured in bits
Gives quantity of data “in transit”
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Delay x Bandwidth
Delay
Bandw idth
This product is analogous to the volume of a pipe or the
number of bits it holds. It corresponds to how many bits the
sender must transmit before the first bit arrives at the
receiver.
Delay may be thought of as one-way latency or round-trip
time (RTT) depending on the context.
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Jitter
Interpacket gap
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Packet
source
3
2
1
4
3
2
Network
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Packet
sink
Jitter is a variation (somewhat random) of the latency from
packet to packet. Jitter is most often observed when packets
traverse multiple hops from source to destination.
Question: What is the cause of jitter?
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Summary
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Network can be
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Public
Private
Virtual Private Network
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Uses public network
Connects set of private sites
Addressing and routing guarantee isloation
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Summary (continued)
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Networks are
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Connection types
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Connectionless
Connection-oriented
Permanent Virtual Circuit
Switched Virtual Circuit
Two performance measures
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Delay
Throughput
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Summary (continued)
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Delay and throughput interact
Queuing delay increases as utilization
increases
Delay x Throughput
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Measured in bits
Gives total data “in transit”
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