Transcript Chapter 10 - William Stallings, Data and Computer Communications

Data and Computer
Communications
Tenth Edition
by William Stallings
Data and Computer Communications, Tenth
Edition by William Stallings, (c) Pearson
Education - 2013
CHAPTER 9
WAN Technology and Protocols
Switched Communications
Networks
Switching nodes - provide a switching facility that move data between
nodes
Stations – devices attached to the network
Nodes – switching devices that provide communication
Communications network – collection of nodes
Switched communication network – data entering the network from a
station are routed to the destination by being switched from node to node
Mainframe
Personal
computer
C
2
1
Server
3
B
D
5
Personal
computer
4
A
E
Switching
Node
6
7
Personal
computer
Figure 9.1 Simple Switching Network
F
Personal
computer
Circuit Switching

Uses a dedicated
path between two
stations
 Can be inefficient


Channel capacity
dedicated for duration
of connection
If no data, capacity is
wasted

Has three phases
Establish
Transfer

Set up (connection)
takes time
 Once connected,
transfer is transparent
Disconnect
Long-distance
office
Long-distance
office
End Office
End Office
Digital PBX
Figure 9.2 Example Connection Over a Public Circuit-Switching Network
a
b
End
office
c
Trunk
Intermediate
exchange
Trunk
d
End
office
Figure 9.3 Circuit Establishment
Circuit-Switching Technology

Driven by applications that handle voice traffic


Key requirement is no transmission delay and no
variation in delay
Efficient for analog transmission of voice signals
 Inefficient for digital transmission
 Transparent
 Once a circuit is established it appears as a
direct connection; no special logic is needed
Full-duplex lines
to attached devices
Control Unit
Digital Switch
Network
Interface
Figure 9.4 Elements of a Circuit-Switch Node
Circuit-Switching Concepts
Blocking or Non-blocking
Blocking network


May be unable to connect
stations because all paths
are in use
Used on voice systems
because it is expected for
phone calls to be of short
duration and that only a
fraction of the phones will
be engaged at any one
time
Non-blocking network



Permits all stations to
connect at once
Grants all possible
connection requests as
long as the called party is
free
When using data
connections terminals can
be continuously connected
for long periods of time so
nonblocking configurations
are required
Space Division Switching
 Originally
developed for analog, space
division switching has been carried over
into the digital realm
 Signal paths are physically separate from
one another
 Path is dedicated solely to transfer signals
 Basic building block of switch is a metallic
crosspoint or semiconductor gate
Input Lines
Output Lines
Figure 9.5 Space-Division Switch
FIRST STAGE
1
SECOND STAGE
5 2
switch
THIRD STAGE
2 5
switch
2
1
2
2 2 switch
3
3
4
4
5
5
2 2 switch
6
5 2
switch
2 5
switch
6
7
7
8
8
9
9
10
10
Figure 9.6
Three-Stage Space-Division Switch
Time Division Switching
 Modern
digital systems use intelligent
control of space & time division elements
 Use digital time division techniques to set
up and maintain virtual circuits
 Partition low speed bit stream into pieces
that share higher speed stream
 Individual pieces manipulated by control
logic to flow from input to output
1
I
a
I
b
J
b
I
a
J
1
I
TSI
J
J
N
N
(a) TSI Operation
Data in
Data out
Write address
I
J
a
b
Data store
Read address
Time slot
counter
Read
address
J
I
I
J
Address
store
(b) TSI Mechanism
Figure 9.7 Time-Slot Interchange
SEL
SEL
SEL
SEL
TSI
TSI
Figure 9.8 A Time-Multiplexed Switch
SWITCH
Call
processing
SS7
Network
Request to generate progress
tones (e.g., ringback, engaged).
Instructions to establish switch
fabric connections.
Supervisory events
(e.g., off-hook, on-hook)
Circuitswitching
fabric
Circuitswitched
trunks
(a) Traditional circuit switching
Media
gateway
controller
Request to generate progress
tones (e.g., ringback, engaged) .
Instructions to establish switch
fabric connections.
Supervisory events
(e.g., off-hook, on-hook)
Circuit- or packetswitched
access
SS7
Network
Media
gateway
Circuit- or packetswitched
trunks
(b) Softswitch architecture
Figure 9.9 Comparison between Traditional Circuit Switching and Softswitch
Packet Switching

Circuit switching was designed for voice
 Packet switching was designed for data
 Transmitted in small packets
 Packets contain user data and control info



User data may be part of a larger message
Control information includes routing (addressing)
Packets are received, stored briefly (buffered)
and passed on to the next node
Application data
control information
(packet header)
Packet-Switching
Network
packet
Figure 9.10 The Use of Packets
Advantages

Line efficiency



Data rate conversion



Single link shared by many packets over time
Packets queued and transmitted as fast as possible
Stations connect to local node at own speed
Nodes buffer data if required to equalize rates
Packets accepted even when network is busy
 Priorities can be used
Switching Techniques
 Station
breaks long message into packets
 Packets sent one at a time to the network
 Packets can be handled in two ways:
Datagram
• Each packet is treated
independently with no
reference to previous
packets
Virtual circuit
• A preplanned route is
established before
any packets are sent
3 2
1
(a)
3
2
1
(b)
3
2
1
(c)
3
2
1
(d)
3
(e)
Figure 9.11 Packet Switching: Datagram Approach
2
1
3 2
1
(a)
3
2
1
(b)
3
2
1
(c)
3
2
1
(d)
3
2
1
(e)
Figure 9.12 Packet Switching: Virtual-Circuit Approach
Virtual Circuits vs. Datagram
 Virtual



circuits
Network can provide sequencing and error
control
Packets are forwarded more quickly
Less reliable
 Datagram



No call setup phase
More flexible
More reliable
(a) 1-packet message
(b) 2-packet message
(c) 5-packet message
(d) 10-packet message
1
1
Header
Data
1
Data
Data Data
2
1
2
1
3
2
4
3
2
5
4
3
Data Data
2
1
Data
1
5
4
2
1
3
2
1
4
3
2
5
4
3
6
5
4
7
6
5
8
7
6
9
8
7
10
9
8
10
9
5
Data
2
X
a
b
10
Y
X
X
a
b
a
Y
Data
X
a
b
Y
Figure 9.13 Effect of Packet Size on Transmission Time
b
Y
External Network Interface
 ITU-T
standard for interface between host
and packet switched network
 Almost universal on packet switched
networks and packet switching in ISDN
 Defines three layers
Physical
Link
Packet
Mainframe
C
Personal
computer
Server
D
Personal
computer
B
Personal
computer
Packet-Switching
Network
E
Personal
computer
A
Solid line = physical link
Dashed line = virtual circuit
Figure 9.14 The Use of Virtual Circuits
F
(a) Circuit switching
propagation
delay
Call
request
signal
(b) Virtual circuit packet switching
processing
delay
(c) Datagram packet switching
Call
request
packet
Pkt1
Call
accept
packet
Call
accept
signal
Pkt2
Pkt1
Pkt3
Pkt2
Pkt1
Pkt3
Pkt2
Pkt1
User
data
Pkt3
Pkt2
Pkt1
Acknowledgement signal
Pkt3
Pkt2
Pkt1
Pkt3
Pkt2
Pkt3
link
Nodes:
1
link
2
Acknowledgement packet
link
3
4
1
2
3
4
1
2
3
Figure 9.15 Event Timing for Circuit Switching and Packet Switching
4
Circuit Switching
Datagram Packet Switching
Virtual Circuit Packet
Switching
Dedicated transmission path
No dedicated path
No dedicated path
Continuous transmission of
data
Transmission of packets
Transmission of packets
Fast enough for interactive
Fast enough for interactive
Fast enough for interactive
Messages are not stored
Packets may be stored until
delivered
Packets stored until delivered
The path is established for
entire conversation
Route established for each
packet
Route established for entire
conversation
Call setup delay; negligible
transmission delay
Packet transmission delay
Call setup delay; packet
transmission delay
Busy signal if called party
busy
Sender may be notified if
packet not delivered
Sender notified of connection
denial
Overload may block call
setup; no delay for established
calls
Overload increases packet
delay
Overload may block call
setup; increases packet delay
Electromechanical or
computerized switching nodes
Small switching nodes
Small switching nodes
User responsible for message
loss protection
Network may be responsible
for individual packets
Network may be responsible
for packet sequences
Usually no speed or code
conversion
Speed and code conversion
Speed and code conversion
Fixed bandwidth
Dynamic use of bandwidth
Dynamic use of bandwidth
No overhead bits after call
setup
Overhead bits in each packet
Overhead bits in each packet
Table 9.1
Comparison
of
Communication
Switching
Techniques
(Table can be found
on page 315 in
textbook)
Asynchronous Transfer Mode
(ATM)





A switching and multiplexing technology that
employs small, fixed-length packets called cells
A fixed-size packet ensures function could be
carried out efficiently, with little delay variation
Small cell size supports delay-intolerant interactive
voice service with a small packetization delay
Designed to provide the performance of a circuitswitching network and the flexibility and efficiency
of a packet-switching network
Standardization effort was to provide a powerful
set of tools for supporting a rich QoS capability
and a powerful traffic management capability
ATM
 Commonly
used by telecommunications
providers to implement wide area
networks
 Used by many DSL implementations
 Used as a backbone network technology
in numerous IP networks
 Multiprotocol Label Switching (MPLS) has
reduced the role for ATM
Virtual Channel Connection
(VCC)
 Logical
connection in ATM
 Analogous to a virtual circuit
 Basic unit of switching in an ATM network
 Set up between two end users through the
network, and a variable-rate, full duplex
flow of fixed-size cells is exchanged over
the connection
 Also used for user-network exchange and
network-network exchange
Virtual Channels
Virtual Path
Physical
Transmission Path
Figure 9.16 ATM Connection Relationships
Virtual Path Advantages
Virtual Channel Characteristics

ITU-T Recommendation I.150 lists the
following characteristics of VCCs:
Quality of service (QoS)
•Specified by parameters such as cell loss ratio and cell delay variation
Switched and semipermanent VCCs
•A switched VCC is an on-demand connection which requires a call control signaling for setup and tearing down
•A semipermanent VCC is one that is of long duration and is set up by configuration or network management
action
Cell sequence integrity
•The sequence of transmitted cells within a VCC is preserved
Traffic parameter negotiation and usage monitoring
•Traffic parameters can be negotiated between a user and the network for each VCC
•The network monitors the input of cells to ensure that the negotiated parameters are not violated
ITU-T Recommendation I.150 lists the following characteristics of VPCs:
Quality of service (QoS)
Switched and semipermanent VPCs
A switched VPC is an on-demand connection which
requires a call control signaling for setup and tearing
down
A semipermanent VPC is one that is of long duration
and is set up by configuration or network management
action
Cell sequence integrity
The sequence of transmitted cells within a VPC is preserved
Traffic parameter negotiation and usage monitoring
Traffic parameters can be negotiated between a user
and the network for each VPC
The network monitors the input of cells to ensure that
the negotiated parameters are not violated
Virtual channel identifier restriction within a VPC
One or more virtual channel identifiers, or numbers, may not be available to the user of the VPC but may be
reserved for network use
Virtual Path
Characteristics
Specified by parameters such as cell loss ratio and cell delay variation
Control Signaling
VCCs

SemipermanentVCCs may be used
for user-to-user exchange




User-to-network signaling virtual
channel
The meta-signaling channel can
also be used to set up a user-touser signaling virtual channel

Such a channel must be set up within a
preestablished VPC
Can be established on a
semipermanent basis by prior
agreement


Meta-signaling channel
The meta-signaling channel can be
used to set up a VCC between the
user and the network for call control
signaling


No control signaling is required
If there is no preestablished call
control signaling channel, then one
must be set up

VPCs
VPC establishment/release may
be customer controlled


No control signaling is required
The customer uses a signaling
VCC to request the VPC from the
network
VPC establishment/release may
be network controlled


The network establishes a VPC
for its own convenience
The path may be network-tonetwork, user-to-network, or userto-user
8
7
6
5
4
3
2
1
8
Generic flow control Virtual path identifier
7
6
5
4
3
2
1
Virtual path identifier
Virtual path identifier
5-octet
header
Virtual channel identifier
Payload type
Virtual channel identifier
Payload type
CLP
Header error control
Header error control
53-octet
cell
Information field
(48 octets)
(a) User-Network Interface
Information field
(48 octets)
(b) Network-Network Interface
Figure 9.17 ATM Cell Format
CLP
Table 9.2
Payload Type (PT) Field Coding
PT Coding
Interpretation
000
User data cell, congestion not experienced,
SDU-type = 0
001
User data cell, congestion not experienced,
SDU-type = 1
010
User data cell, congestion experienced,
SDU-type = 0
011
User data cell, congestion experienced,
SDU-type = 1
100
OAM segment associated cell
101
OAM end-to-end associated cell
110
Resource management cell
111
Reserved for future function
SDU = Service Data Unit
OAM = Operations, Administration, and Maintenance
Summary



Switched
communications
networks
Circuit-switching
networks
Circuit-switching
concepts



Space division switching
Time-division switching
Softswitch architecture

Packet-switching
principles





Switching technique
Packet size
External network
interface
Comparison of circuit
switching and packet
switching
Asynchronous transfer
mode


ATM logical connections
ATM cells