1. Department of Computer and IT Engineering University of Kurdistan
Computer Networks II
Virtual Circuit Switching and ATM
By: Dr. Alireza Abdollahpouri

2. A Taxonomy of Communication Networks
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

3. 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

4. A Taxonomy of Communication Networks
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

5. Circuit Switching Three phases If circuit not available: “Busy signal”
circuit establishment
data transfer
circuit termination
If circuit not available: “Busy signal”
Examples
Telephone networks
ISDN (Integrated Services Digital Networks)

6. Timing in Circuit Switching
Host 1
Host 2
Node 1
Node 2
DATA
processing delay at Node 1
propagation delay
between Host 1
and Node 1
Circuit Establishment
Data Transmission
Circuit Termination
propagation delay
between Host 2
and Node 1

7. A Taxonomy of Communication Networks
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

8. Packet Switching
Data are sent as formatted bit-sequences, so-called packets.
Packets have the following structure:
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
Header
Data
Trailer

9. A Taxonomy of Communication Networks
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

10. 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

11. Timing of Datagram Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation
delay between
Host 1 and
Node 2
transmission
time of Packet 1
at Host 1
Packet 1
Packet 2
Packet 3
processing delay of Packet 1 at Node 2
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3

12. Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

13. A Taxonomy of Communication Networks
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

14. 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

15. Virtual-Circuit Packet Switching
Communication with virtual circuits takes place in three phases
VC establishment
Data transfer
VC disconnect
Note: packet headers don’t need to contain the full destination address of the packet

16. Timing of Virtual-Circuit Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation delay
between Host 1
and Node 1 VC
establishment
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Data
transfer
Packet 1
Packet 2
Packet 3 VC
termination

17. Virtual-Circuit Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

18. 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 1 4 1
VC2 4
VC1
VC2
VC3
VC1 2 3
In, VC Out, VC
1, , 1
1, , 3
2, , 2 3 2
VC1
VC2
VC1

19. Virtual Circuits Examples
In-port
In-VCI
Out-port
Out-VCI 1 5 3 7 4 11 8
Lookup table for
Switch R1
Packet 5 7 2 2
Sender R1 R2
1,7  4,2 1 3 1 3 4 4
1,5  3,7 2 2 1 R3
Receiver 3 4 6
2,2  3,6
Subsequence packets follow the same circuit

20. Virtual circuit wide area network

21. VCI

22. Switch and table

23. Source-to-destination data transfer

24. SVC setup request

25. SVC setup acknowledgment

26. Case Study: ATM

27. Multiplexing using different frame sizes

28. 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.

29. Multiplexing using cells

30. ATM multiplexing

31. Architecture of an ATM network
UNI = User-to-Network Interface
NNI = Network-to-Network Interface

32. Example of VPs and VCs
VP: Virtual Path
VC: Virtual Channel

33. Note:
Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

34. Connection identifiers

35. Virtual connection identifiers in UNIs and NNIs

36. An ATM cell
Why 53?

37. Routing with a switch

38. ATM layers
(SONET/SDH)

39. ATM layers in endpoint devices and switches
User information
User information

40. ATM layer

41. ATM headers

42. Virtual Path Identifier Virtual Channel Identifier
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
res
CLP
Header Error Control

43. Virtual Path Identifier Virtual Channel Identifier
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
res
CLP
Header Error Control

44. Virtual Path Identifier Virtual Channel Identifier
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
res
CLP
Header Error Control

45. AAL present only in end systems, not in switches
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

46. 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

47. 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.

48. AAL1
AAL1, a connection-oriented service, is suitable for handling constant bit rate sources (CBR), such as uncompressed audio and videoconferencing

49. 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).

50. AAL3/4
Protocol standard for supporting both Connectionless and connection-oriented Variable Bit Rate (nrt-VBR); e.g., airline reservation, banking transactions.

51. 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).

52. IP-Over-ATM
AAL
ATM
phy
Eth IP
app
transport

53. Questions