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Data and Computer
Chapter 11 – Asynchronous Transfer
streamlined packet transfer interface
 similarities to packet switching
transfers data in discrete chunks
supports multiple logical connections over a
single physical interface
uses fixed sized packets called cells
 with minimal error control and flow control
 data rates of 25.6Mbps to 622.08Mbps
Protocol Architecture
Reference Model Planes
 user
provides for user information transfer,
associated controls (flow control, error
 control
call and connection control
 management
plane management
• whole system functions
layer management
• Resources and parameters in protocol entities
ATM Logical Connections
virtual channel connections (VCC)
basic unit of switching between two end users
analogous to virtual circuit in X.25
full duplex
fixed size cells
also for
user-network exchange (control)
network-network exchange (network mgmt & routing)
ATM Virtual Path Connection
virtual path connection (VPC)
bundle of VCC with same end points
Advantages of Virtual Paths
 simplified
network architecture
 increased network performance and
 reduced processing
 short connection setup time for new
Using VPs
Virtual Channel Connection
between end users
end to end user data
VPC provides overall capacity
• VCC organization done by users within the capacity
between end user and network
control signaling
between network entities
network traffic management
VP/VC Characteristics
quality of service - cell loss ratio, cell delay
switched and semi-permanent channel
cell sequence integrity
traffic parameter negotiation and usage
monitoring - average rate, peak rate, burstiness,
and peak duration
VPC only
virtual channel identifier restriction within VPC
Fixed vs. Variable Length Cell
Efficiency Consideration:
 Efficiency N = Information Octets /
(Information Octets + Header Octets)
 Fixed Length Packets:
 L = Data Field Size (Octets) in a Packet
 H = Header Size (Octets)
 X = Total Message Size (Octets)
 # Expression for N = ? (Needs ceiling
function), Nopt = ?
Fixed vs. Variable Length Cell
Variable Length Packets:
 Needs an additional length field,
 Hv = Additional overhead octets
 H = Header Size (Octets)
 X = Message Size (Octets)
 # Expression for N = ? (Assume Single Cell)
 # Plot of N vs. Message Size (X = 48, 96,
144, 192, 240), for fixed and variable length
What size of Fixed Length
 Assume
that the cells are completely filled
(X / L = integer)
 Expression for N = ?
 Packetization Delay = buffering bits until
the entire packet is filled before
 Expression for this delay D = ? (function of
L and source data rate R)
What size of Fixed Length
 Plot
of D vs. Data Field Size (L = 16, 32,
64, 128 octets) (R = 64 kbps for voice
 Plot
of efficiency N vs. Data Field Size (16,
32, 64, 128 octets)
 =>
48 octet provides a trade-off between
efficiency and delay
ATM Cells
ATM Header Fields
 generic
flow control
 Virtual path identifier - routing
 Virtual channel identifier – end to end user
 payload type (3-bits, user information,
 cell loss priority
 header error control
Consider compressed video
transmission in ATM network, Cells must
pass through 5 switches. The data rate is
43 Mbps.
 - Transmission time of one cell through
one switch?
 - Assume other cells have lower priority
(but non-premptive) than cell A. What is
the maximum time from arrival at the first
 Switch
to the completion of transmission
by the fifth for cell A? (all other delays are
 - if the probability that a switch is busy is
0.6 and the average delay to wait for
current transmission completion is one half
a cell transmission time, what is the
average time from arrival at the first Switch
to the completion of transmission by the
fifth for cell A?
what is the maximum and average
variability in encountered delay (the jitter)?
IP datagrams are segmented into ATM
cells and sent over the ATM network. Loss
of a cell means loss of entire IP packet.
 Pc = cell loss rate
 n = number of cells for a datagram
 Expression for Pp = datagram loss rate?
 Asynchronous
Transfer Mode (ATM)
 architecture & logical connections
 ATM Cell format
 transmission of ATM cells