Transcript Presentation3

Department of Computer and IT Engineering
University of Kurdistan
Computer Networks II
Virtual Circuit Switching and ATM
By: Dr. Alireza Abdollahpouri
A Taxonomy of Communication Networks
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Broadcast vs. Switched Communication Networks
 Broadcast communication networks
 information transmitted by any node is received by every
other node in the network
 examples: usually in LANs (Ethernet, Wavelan)
 Problem: coordinate the access of all nodes to the shared
communication medium (Multiple Access Problem)
 Switched communication networks
 information is transmitted to a sub-set of designated nodes
 examples: WANs (Telephony Network, Internet)
 Problem: how to forward information to intended node(s)
 this is done by special nodes (e.g., routers, switches)
running routing protocols
A Taxonomy of Communication Networks
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Circuit Switching
 Three phases
1. circuit establishment
2. data transfer
3. circuit termination
 If circuit not available: “Busy signal”
 Examples
 Telephone networks
 ISDN (Integrated Services Digital Networks)
Timing in Circuit Switching
Host 1
Node 1
Node 2
Host 2
processing delay at Node 1
propagation delay
between Host 1
and Node 1
Circuit
Establishment
propagation delay
between Host 2
and Node 1
Data
Transmission
DATA
Circuit
Termination
A Taxonomy of Communication Networks
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Packet Switching


Data are sent as formatted bit-sequences, so-called packets.
Packets have the following structure:
Header



Data
Trailer
 Header and Trailer carry control information (e.g.,
destination address, check sum)
Each packet is passed through the network from node to node
along some path (Routing)
At each node the entire packet is received, stored briefly, and then
forwarded to the next node (Store-and-Forward Networks)
Typically no capacity is allocated for packets
A Taxonomy of Communication Networks
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Datagram Packet Switching
 Each packet is independently switched
 each packet header contains destination address
 No resources are pre-allocated (reserved) in
advance
 Example: IP networks
Timing of Datagram Packet Switching
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Host 2
Node 2
propagation
delay between
Host 1 and
Node 2
Packet 2
Packet 1
Packet 3
processing
delay of
Packet 1 at
Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 4
Node 7
Host E
A Taxonomy of Communication Networks
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
Virtual-Circuit Packet Switching
 Hybrid of circuit switching and packet
switching
 data is transmitted as packets
 all packets from one packet stream are sent along
a pre-established path (=virtual circuit)
 Guarantees in-sequence delivery of packets
 However: Packets from different virtual
circuits may be interleaved
 Example: ATM networks
Virtual-Circuit Packet Switching
 Communication with virtual circuits takes
place in three phases
1. VC establishment
2. Data transfer
3. VC disconnect
 Note: packet headers don’t need to contain
the full destination address of the packet
Timing of Virtual-Circuit Packet Switching
Host 1
Node 1
Host 2
Node 2
propagation delay
between Host 1
and Node 1
VC
establishment
Packet 1
Packet 2
Packet 1
Data
transfer
Packet 3
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
VC
termination
Virtual-Circuit Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 4
Node 7
Host E
Virtual Circuit
Connection setup establishes a path through switches
 A virtual circuit ID (VCI) identifies path
 Uses packet switching, with packets containing VCI
 VCIs are often indices into per-switch connection tables;
change at each hop
VC1
VC2
1
2
VC1
In, VC Out, VC
1, 1
4, 1
1, 2
4, 3
2, 1
4, 2
4
VC1
VC2
3
1
VC3
2
VC1
4
3
VC2
VC1
Virtual Circuits Examples
In-port
Lookup table for
Switch R1
Packet
Sender
In-VCI
Out-port
Out-VCI
1
5
3
7
4
11
0
8
5
7
1
2
2
R1
R2
4
3
1
4
1,7  4,2
3
2
1,5  3,7
2
1
R3
4
3
2,2  3,6

Subsequence packets follow the same circuit
Receiver
6
Virtual circuit wide area network
VCI
Switch and table
Source-to-destination data transfer
SVC setup request
SVC setup acknowledgment
Case Study: ATM
Multiplexing using different frame sizes
Note:
A cell network uses the cell as the
basic unit of data exchange. A cell is
defined as a small, fixed-sized
block of information.
Multiplexing using cells
ATM multiplexing
Architecture of an ATM network
UNI = User-to-Network Interface
NNI = Network-to-Network Interface
Example of VPs and VCs
VP: Virtual Path
VC: Virtual Channel
Note:
Note that a virtual connection is
defined by a pair of numbers:
the VPI and the VCI.
Connection identifiers
Virtual connection identifiers in UNIs and NNIs
An ATM cell
Why 53?
Routing with a switch
ATM layers
(SONET/SDH)
Figure 19-21
Layers
in End-Point
Devices and
and Switches
ATMATM
layers
in endpoint
devices
switches
User
information
McGraw-Hill
User
information
©The McGraw-Hill Companies, Inc., 2001
ATM layer
ATM headers
ATM Cell
 Payload Type (2 bits) identifies cell category
 00 = user information (data)
 reserved for later definition (1 bit)
8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
VCI (cont)
PT
Header Error Control
res
CLP
ATM Cell
 Cell Loss Priority Bit
 0 if sending within committed information rate
 1 if sending faster: switches will discarded if
congestion occurs
8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
VCI (cont)
PT
Header Error Control
res
CLP
ATM Cell
 Header Error Control Field
 To check the header (not the payload) for
errors
 Discard cell if an error is found
8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
VCI (cont)
PT
Header Error Control
res
CLP
ATM Adaptation Layers (AAL)

ATM Adaptation Layer (AAL)




A protocol for converting between higher layer
protocol data units (PDU’s), such as TCP
packets, IP packets, or JPEG images,
and ATM cells for actual transmission
Defines procedures for segmentation and
reassembly (SAR)
segmentation: packets to cells (done by sender)
reassembly: cells to packets (done by receiver)
AAL present only in end systems, not in switches
ATM Services
•
•
•
•
•
Constant Bit Rate (CBR)
Real-time Variable Bit Rate (rt-VBR)
Non-real-time Variable Bit Rate (nrt-VBR)
Available Bit Rate (ABR)
Unspecified Bit Rate (UBR): best effort service
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ATM Services
• CBR : User declares required rate.
Throughput, delay and delay variation guaranteed.
• VBR : User declares average and max rate.
rt-VBR : Conferencing. Max delay and delay variation
guaranteed.
nrt-VBR : Stored video.
• ABR (Available bit rate): Follows feedback instructions.
Network gives maximum throughput with minimum loss.
• UBR (Unspecified bit rate): User sends whenever it wants.
No feedback mechanism. No guarantee. Cells may be
dropped during congestion.
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AAL1
 AAL1, a connection-oriented service, is suitable for
handling constant bit rate sources (CBR), such as
uncompressed audio and videoconferencing
AAL2

Protocol standard for supporting time-dependent slow or Variable Bit
Rate (rt-VBR) connection-oriented traffic (e.g. packetized and
compressed audio and MPEG video).
AAL3/4
Protocol standard for supporting both Connectionless and
connection-oriented Variable Bit Rate (nrt-VBR); e.g., airline
reservation, banking transactions.
AAL5
AAL5 is similar to AAL3/4, but has a simplified information
header scheme that requires only one header per data unit.
Examples of this service include datagram traffic and data
network applications where no connection is set up before
data is transferred. (UBR, ABR).
IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
Questions
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