1. Advanced topics in Computer Networks
Lecture 2: ATM: an overview
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
Adv. topics in Computer Network

2. Adv. topics in Computer Network
Outline
Why ATM
ATM overview
ATM standard
Univ. of Tehran
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3. Adv. topics in Computer Network
Why ATM?
Problems with IP?
Best effort, lack of reliability
TCP on top of IP
Being stateless
Hop to hop or point to point.
Lack of QoS.
How important is QoS? It is essential in some applications including
VoIP
Video on demand.
Teleconferencing.
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4. Adv. topics in Computer Network
Supporting QoS in IP
QoS parameter
Bounded delay and jitter.
Min rate
IP should identify flow and keep state in nodes.
Contradictory to original design goal, flexibility and scalability.
Can we have both? We try.
This is the main challenge of Internet.
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5. Adv. topics in Computer Network
ATM overview
ATM = Asynchronous Transfer Mode.
ATM = Data Network + Phone Network
Combination of Internet method of communication (packet switching) and phone companies’ method (circuit switching)
Asynchronous- The way bandwidth is allocated. The time slot is not allocated by temporal position like Synchronous transfer mode, STM, (circuit switch).
Transfer mode= Multiplexing and switching.
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6. Adv. topics in Computer Network
ATM overview (cont)
Connection oriented- Need signaling like circuit switch.
Establish connection.
Tear down connection.
Create a virtual connection or circuit, then order is preserved in carrying data.
Problems?
Overhead for connection and tearing down.
Loosing connectivity in case of failure in nodes.
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ATM vs. STM
STM or TDM
Peak rate allocation
Fast and simple logic.
Not good for bursty traffic like data.
ATM- uses statistical multiplexing.
Giving bandwidth for label.
Has to process cells .
Congestion is possible.
Variable cell delay (Queuing).
But, network controls routing, errors, flow, copying and priorities.
Overcome to congestion and delay is fundamental to ATM.
Univ. of Tehran
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8. Adv. topics in Computer Network
ATM vs. Phone Network
Current phone networks are synchronous (periodic). STM
ATM = Asynchronous Transfer Mode
Phone networks use circuit switching.
ATM networks use “Packet” Switching
In phone networks, all rates are multiple of 8 kbps.
With ATM service, you can get any rate. You can vary your rate with time.
With current phone networks, all high speed circuits are manually setup. ATM allows dialing any speed.
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9. Adv. topics in Computer Network
Interfaces
User to Network Interface (UNI):
Public UNI, Private UNI
Network to Node Interface (NNI):
Private NNI (P-NNI)
Public NNI = Inter-Switching System Interface (ISSI)
Intra-LATA ISSI (Regional Bell Operating Co)
Inter-LATA ISSI (Inter-exchange Carriers)
Broadband Inter-Carrier Interface (B-ICI)
Data Exchange Interface (DXI)
Between packet routers and ATM Digital Service Units (DSU)
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10. Adv. topics in Computer Network
ATM standard
Standard is different from concept. It deals with implementation.
ATM Forum is a standard body of developer, service provider and other interested bodies.
What standard does is to decide:
Size of cell
Header format
Signaling
Interfaces
Traffic control
Network management
Testing
Internetworking
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11. ATM Cell format GFC=Generic Flow Control
(Was used in UNI but not in NNI)
The field is used for VPI in NNI
VPI/VCI=0/0 Þ Idle cell; 0/n Þ Signaling
HEC: 1 + x + x 2 + x 8 4 7
GFC/VPI
VPI
VCI
PT CLP
Header Error Checksum(HEC)
Data
U.S ANSI proposed 64 bytes data with 5
byte header
Euro. ETSI proposed 32 bytes with 4
bytes header
Finally, 48 bytes with 5 bytes header.
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12. Connection Identifiers
VPI/VCI are local and swapped in each switch.
VCI is allocated in connection setup time.
VPI is usually used by management.
By VPI, a set of VCI are treated as the same.
VPI/VCI=0/0 Þ Idle cell; 0/n Þ Signaling
Each cell contains a 24/28-bit connection identifier
First 8/12 bits: Virtual Path, Last 16 bits: Virtual Channel
VP service allows new VC's w/o orders to carriers
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13. Header Error Check (HEC)
Protects header only
Optional Correction mode: Correct one bit errors if no earlier errors
Discard cells with bad HEC
Used for cell delineation in SONET
Recalculated on each hop
CRC generator is 1 + x + x 2 + x 8
Single bit error (correction)
Multibit error (discard)
Correction mode
Correction mode
Detection mode
No Error
Error
No error detected
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14. ATM protocol stack AAL = ATM Adaptation Layer Peer to peer connection
PHY
PHY
PHY
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15. Adv. topics in Computer Network
Protocol layers
The ATM Adaptation Layer
How to break application messages to cells
The ATM Layer
Transmission/Switching/Reception
Congestion Control/Buffer management
Cell header generation/removal at source/destination
Reset connection identifiers for the next hop (at switch)
Cell address translation
Sequential delivery
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16. Adv. topics in Computer Network
ATM network
ATM layer: Transporting cells
Cell encapsulation and decapsulation.
Insertion and extraction of idle cells
Cell header processing
Cell queuing
Cell coping
GFC management in UNI
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17. Adv. topics in Computer Network
ATM network (cont)
Connection Admission Ctrl
Connection admission or rejection
VPI/VCI assignment and removal
Exchanging and processing signaling cells
Traffic Ctrl: Maintain agreed QoS
Usage parameter ctrl
Selective cell discarding
Implicit service priorities
Explicitly forward
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18. Adv. topics in Computer Network
ATM network (cont)
ATM layer management
Configuring the network and switch fabric
Monitoring the network operation
Detecting troubles
ATM switch
Doing actual cell switching.
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19. Adv. topics in Computer Network
History of ATM
1980: Narrowband ISDN adopted
Early 80's: Research on Fast Packets
Mid 80's: B-ISDN Study Group formed
1986 ATM approach chosen for B-ISDN
June 1989: 48+5 chosen (64+5 vs 32+4).
October 1991: ATM Forum founded
July 1992: UNI V2 released by ATM Forum
1993: UNI V3 and DXI V1
1994: B-ICI V1
Univ. of Tehran
Adv. topics in Computer Network