1. Asynchronous Transfer Mode (ATM)
EE290F
22 April 2004
Shanna Crankshaw

2. ATM – definition
"A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".

3. What is it really?
Low-level network layer—above physical layer, below AAL (ATM adaptation layer)
Single transport mechanism for different types of traffic (voice, data, video, etc.)
Streamlined protocol, minimal error and flow control capabilities
Fixed packet size = ATM cell
Simplified processing, management
Fast packet switching, high data rates

4. Synchronous Transfer Mode
Pre-assigned “slots,” frame boundaries, global timing
Slots identified by position from the start of the frame
BW allocated in units of slots
Idle slots wasted
Efficient for Constant Bit Rate traffic

5. Contrast with STM
“Bandwidth on demand”—Slots assigned on demand, users take any empty slot
Nothing pre-assigned, no global timing
Slot  Cell, fixed size of 53 bytes
Arbitrary bit rates: can support T-1 using CBR, voice/video using real-time VBR, IP-based traffic using ABR and UBR, etc.
Each cell must be self-identifying (overhead)

6. ATM cell contents Header – 5 bytes
General Flow Control (GFC) – traffic control for different QoS, alleviates short-term overloads
VPI – routing field for network
VCI – routing to/from user
Payload type
Cell loss priority (CLP)
Header error control (HEC) – can correct single bit errors in header
Information – 48 bytes

7. ______________Interpretation____________
Stallings, Data & Computer Communications, 6th ed., Table 11.2
PT coding
000
001
010
011
100
101
110
111
______________Interpretation____________
User data cell, congestion not experienced, SDU type=0
User data cell, congestion not experienced, SDU type=1
User data cell, congestion experienced, SDU type=0
User data cell, congestion experienced, SDU type=1
OAM segment associated cell
OAM end-to-end associated cell
Resource management cell
Reserved for future function
SDU = Service Data Unit
OAM = Operations, Administration, and Maintenance

8. ATM Cells Small size, may reduce queuing delay of high priority cells
Fixed size, more efficient switching
UNI
NNI
GFC
VP identifier
VP identifier
VP identifier
5-byte header
VC identifier
VC identifier
PT CLP
PL type CLP
Header error control
Header error control
53 byte cell
Info field, 48 bytes
Info field, 48 bytes

9. VC’s are not always VC’s
Virtual Channel
Transmission is connection-oriented
VC set up by some signaling protocol before any cells can be sent
Virtual Path Connection (VPC), bundle of VCC’s
Virtual channels
Virtual path
Physical channel

10. Logical connections
VPC = bundle of VCC’s with the same endpoints  all switched together
Network management of group of connections, not many individual ones
Setup time is for a VP, adding VC’s to it involves minimal processing

11. Block VCC or request more capacity
Request for VCC originates
Request for VCC originates
VPC exists?
Yes
Can QoS be satisfied?
Yes No No
Block VCC or request more capacity
Establish new VPC
Make connection
Request granted?
Yes No
Reject VCC request

12. ATM Adaptation Layer (AAL)
Layer above ATM
Service dependent
Mask ATM specifics from user; universality
SAR: segmentation and reassembly. “Translates” service data from a non-ATM format into ATM cells, then back again at destination
CS: convergence sublayer. Takes care of delay jitter, error checking, remove corrupted cells
AAL user CS
AAL
SAR sublayer
ATM layer
Physical layer

13. ATM Service Categories
Real-time services
Constant bit rate – uncompressed audio/video info
Videoconferencing, TV, pay-per-view, VOD, etc.
rt-Variable bit rate
Non-real-time services
nrt-VBR – high end system QoS, critical response time
Bank transactions, airline reservations, etc.
Unspecified bit rate – best-effort service
e.g. text/image messaging, telecommuting
Available bit rate – bursty apps requiring reliable end-to-end connection
e.g. LAN, router-to-router reliability

14. ATM advantages Universality Scalability
Mixed traffic types, real-time and non-real-time
Scalability
LANs, MANs, WANs, WLANs
Efficient use of network resources Bandwidth on demand concept
Simplified network infrastructure

15. ATM challenges
In-network mux/buffering can lead to cell delay or loss  QoS guarantees
Many types of traffic
Large geographic distribution
Traffic modeling, control