Download Latency = Propagation + Transmit + Queue

Performance and Internet
Architecture
Networking
CS 3470, Section 1
Sarah Diesburg
Announcements
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Project 1
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Due in two weeks
Start now, if you haven’t already
Will go over missing pieces and answer questions
on Friday
Assignment 1
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Due in one week
Hand in during class (instead of eLearning)
Performance
Bandwidth
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Bandwidth
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Width of the frequency band
Number of bits per second that can be transmitted
over a communication link
1 Mbps: 1 x 106 bits/second
1 x 10-6 seconds to transmit each bit
Can imagine this on a timeline
Bandwidth
Bits transmitted at a particular bandwidth can be regarded as having
some width:
(a) bits transmitted at 1Mbps (each bit 1 μs wide);
(b) bits transmitted at 2Mbps (each bit 0.5 μs wide).
The length of bits
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How wide/long is a bit in the network?
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Propagation speed?
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Electrons in copper: 2.3x108m/s
Light pulses in fiber: 2.0x108m/s
Transmission rates
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10Mbps
100Mbps
1Gbps
10Gbps
The length of bits
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How wide is a bit in the network?
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If your transmission rate is 10Mbps, how long
does it take to put one bit on the line?
10Mbps = 10x106 bits in one second
= 1 bit in 1/ 10x106 seconds
= 1 bit in 1x10-7 seconds
= 1 bit in 0.1 μs
The length of bits
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How long is a bit in the network?
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10Mbps = 1 bit every 1x10-7 seconds
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In copper, electrons travel at 2.3x108 m/s
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2.3x108 m/s x 1x10-7 seconds ≈ 23 meters
The length of bits
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How long is a bit in the network?
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What about wireless?
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2.4GHz spectrum (802.11b)
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Transmission rate 11Mbps
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Transmission medium is taken to be the speed of light
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Unless there are physical considerations:
 Wood,Glass, Plastic (low)
 Water, Bricks, Living Animals (medium)
 Ceramic, Paper, Bullet-proof glass, Concrete (high)
 Metal (very high)
Latency
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Latency
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How long it takes for a message to travel from the
source to the destination
Always measured in time
Lots of factors can affect this – any ideas?
Latency = Propagation + Transmit + Queue
Latency
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1.
Three main factors affect latency
Propagation delay deals with the speed of
light over the medium
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Electrons in copper: 2.3x108m/s
Light pulses in fiber: 2.0x108m/s
Propagation = Distance/SpeedOfLight
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Latency
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2.
Three main factors affect latency
Transmit time
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Amount of time it takes to transmit a unit of data
Transmit = Size/Bandwidth
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Latency
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3.
Three main factors affect latency
Queue delay deals with delays in the
network
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E.g., switches that store and forward
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All Together…
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Latency = Propagation + Transmit + Queue
 Propagation = Distance/SpeedOfLight
 Transmit = Size/Bandwidth
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Sometimes, we are concerned with round-trip
time (RTT)
 Time it takes to send a message from source
to destination and back to source
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One-way latency time X 2
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The Delay x Bandwidth “Pipe”
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Okay, so it takes “latency” seconds for a bit to
go from one end to another (plus a fraction
for the transmission of the bit!).
While that one bit is “on its way,” you can still
send more bits.
How many bits can you stuff in the pipe?
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That is, how many bits can be “in transit” that the
sender knows have been sent, but the receiver
has not yet been made aware of?
The Delay x Bandwidth “Pipe”
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Think of the link as a pipe.
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The “length” of the pipe as the latency
The cross-sectional area as the transmission rate
Then, the Delay x Bandwidth product is the
volume (in bits) of the pipe.
And yet another time
Transmission
Time
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The transfer time refers to the amount of time
sending the data plus the overhead in
setup/teardown of the transfer.
We'll see a lot of these when we talk about TCP,
but for now, look at it like this:
RTT
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Jitter
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Packets that go through several congested
routers must contend for transmission slots.
The result is that an application sending
packets at a constant interval would be
perceived by the receiver to have variations
in the interpacket gap, or the time between
successive packets.
This is observable by variations in
latency, referred to as “jitter.”
Internet Architecture
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Layered Architecture
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Layering simplifies the
architecture of complex system
Layer N relies on services from
layer N-1 to provide a service
to layer N+1
Interfaces define the services
offered
Service required from a lower
layer is independent of it’s
implementation
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Layer N change doesn’t affect
other layers
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Protocols
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Protocols are rules by which network
elements communicate
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The format and the meaning of messages
exchanged
Protocols in everyday life
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Examples: traffic control, open round-table
discussion etc
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Protocol Stacks and Layering
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Layering leads to separation of tasks, which makes
it easier for programmers and hardware vendors to
implement the interface to the neighboring layers.
Protocols lead to standardization and well-defined
behaviors and expectations.
Encapsulation
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Encapsulation refers to the embedding of a
data representation at one protocol layer into
the data representation of another layer.
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Fragmentation
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Packets at one layer might be too large.
In this case, the packet might be fragmented
into smaller pieces, encapsulated into the
data representation of the underlying
protocol, and then defragmented
(reassembled) at the destination, or at a node
later on in the link.
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Common Standards
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ISO:
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International Standards Organization
Defined reference model known as OSI (Open
Systems Interconnection)
IETF
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Internet Engineering Task Force
Defined the Internet Model
The OSI Model
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Also known as the seven-layer salad.
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Application
Presentation
Session
Transport
Network
Data Link
Physical
(All pizzas sent through Nick digest promptly)
The Internet Model
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Commonly four layers—with the physical
layer implied.
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Application
Transport
Network
Link
(Physical)
ISO/OSI and Internet Reference
Models
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ISO/OSI Reference Model
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Application layer
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Examples: http, ftp, smtp etc
Process-to-process communication
All layers exist to support this layer
Presentation layer (OSI only)
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Conversion of data to common format
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Example: Little endian vs big endian byte orders
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ISO/OSI Reference Model (cont’d)
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Session layer (OSI only)
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Session setup (authentication)
Recovery from failure (broken session)
Transport layer
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Examples: TCP, UDP
End-to-end delivery
(Some typical) functions include reliable in-order
delivery and flow/error control
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ISO/OSI Reference Model (cont’d)
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Network layer
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Examples: IP
Used to determine how packets are routed from
source to destination
Congestion control
Accounting
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ISO/OSI Reference Model (cont’d)
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Data link layer
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Examples: Ethernet, PPP
Responsible for taking a raw transmission facility and
transforming it into a line that appears free of
undetected transmission errors.
Accomplished by sending data in frames, and
transmitting frames in sequence.
Acknowledgment frames.
Special delineation bit patters used to distinguish
frames.
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ISO/OSI Reference Model (cont’d)
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Physical layer
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Transmitting raw bits (0/1) over wire
Examples: 802.11 (2.4GHz wireless), Copper,
Fiber
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More on Layers
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The lower three layers are implemented on
all network nodes
The transport layer and the higher layers
typically run only on end-hosts and not on the
intermediate switches and routers
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Protocol Stacks and Layering
The OSI 7-layer Model
OSI – Open Systems Interconnection