Transcript Chapter11 - William Stallings, Data and Computer

Data and Computer
Communications
Chapter 11 – Asynchronous Transfer
Mode
ATM
a
streamlined packet transfer interface
 similarities to packet switching


transfers data in discrete chunks
supports multiple logical connections over a
single physical interface
 ATM
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

plane
provides for user information transfer,
associated controls (flow control, error
control)
 control

plane
call and connection control
 management

plane
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
reliability
 reduced processing
 short connection setup time for new
channel
Call
Establishment
Using VPs
Virtual Channel Connection
Uses

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
routing
VP/VC Characteristics





quality of service - cell loss ratio, cell delay
variation
switched and semi-permanent channel
connections
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
cells
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
retransmission
 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
coding)
 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,
congestion)
 cell loss priority
 header error control
Examples
#
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
Examples
 Switch
to the completion of transmission
by the fifth for cell A? (all other delays are
negligible)
 - 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?
Examples
-
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?
Summary
 Asynchronous
Transfer Mode (ATM)
 architecture & logical connections
 ATM Cell format
 transmission of ATM cells