1. Asynchronous Transfer Mode (ATM)
Chapter 5
Asynchronous Transfer Mode
(ATM)

2. Introduction ATM Protocol Architecture Logical connections
ATM cell structure
Service levels/categories
ATM Adaptation Layer (AAL)
Chapter 5 ATM

3. Introduction ATM evolved from B-ISDN development efforts
Frame Relay: high-speed WAN (1.5+ Mbps)
ATM: very high speed WAN (155 Mbps and 622Mbps)
ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible
minimal error and flow control capabilities
even more streamlined, therefore faster, than Frame Relay
Specifications developed by ITU-T and ATM Forum
Chapter 5 ATM

4. ATM Protocol Architecture
Fixed-size packets called cells
“cell switching” like packet switching
2 primary protocol layers relate to ATM functions:
Common layer providing packet transfers, logical connections (ATM)
Service dependent ATM adaptation layer (AAL)
AAL maps other protocols to ATM
like IP (AAL5)
Chapter 5 ATM

5. Protocol Model has 3 planes
User – provides for user information transfer and associated controls (flow control, congestion control)
Control – performs call control and connection control functions (signaling)
Management – provides plane management and layer management and coordination functions
Chapter 5 ATM

6. ATM Protocol Reference Model
Map data to the ATM cell
structure
Framing, cell structure
& Logical Connections
Various data rates ( Mbps, Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.)
Chapter 5 ATM

7. User Plane Layers … User information User information End system
AAL
AAL
ATM
ATM
ATM
ATM
PHY
PHY
PHY
PHY …
End system
Network
End system
Chapter 5 ATM

8. User Plane Layers
User information
User information
Chapter 5 ATM

9. Logical Connections
VCC (Virtual Channel Connection): a logical connection analogous to a virtual circuit in X.25, or Frame Relay data link connection
full-duplex flow between end users
user-network control signaling
network-network management/routing
VPC (Virtual Path Connection): a bundle of VCCs with the same end points (not necessarily same end-users)
and switched along the same path
Chapter 5 ATM

10. ATM Connection Relationships
Virtual Channel: basic logical communications channel
Virtual Path: groups of “common” virtual channels
Physical Transmission Path: physical communications link
Chapter 5 ATM

11. VCC (logical connection) Uses
Exchange between end users
user data
control signaling (more later)
Exchange between an end user and a network entity
Exchange between 2 network entities
traffic management
routing functions
Chapter 5 ATM

12. Advantages of Virtual Paths
Simplified network architecture – allows separation of functionality into into individual logical connections and related groups of logical connections
Increased network performance and reliability – network consists of fewer aggregated entities
Reduced processing and short connection setup time – complex setup tasks are in virtual paths, simplifies setup of new virtual channels over existing virtual path
Enhanced network services – supports user-specified closed groups/networks of VC bundles
Chapter 5 ATM

13. Virtual Path/Virtual Channel Terminology
Virtual Channel (VC) A generic term used to describe unidirectional transport of cells associated by a common unique identifier
Virtual Channel Identifier (VCI) A unique numerical tag for a particular VC link
Virtual Channel Link A means of unidirectional transport of cells between the point where a VCI is assigned and where it is translated or terminated
Virtual Channel Connection (VCC) A concatenation of VC links that extends between two connected ATM end-points
Chapter 5 ATM

14. Virtual Path/Virtual Channel Terminology
Virtual Path (VP) A generic term which describes unidirectional transfer of cells that are associated with a common unique identifier
Virtual Path Identifier (VPI) Identifies a particular VP
Virtual Path Link A group of VC links identified by a common identifier between the point where the identifier (VPI) is assigned and where it is translated or terminated
Virtual Path Connection (VPC) A concatenation of VP links that extends between ATM end-points where the VCIs are assigned and where they are translated or terminated
Chapter 5 ATM

15. ATM VPC/VCC Sw = switch DCC = Cross-connect switch a VP3 VP5 a b b c c1 c
ATM
DCC
ATM Sw 2
ATM Sw 3 b c d e
VP6
VP2
VP1
ATM Sw 4 d e
Sw = switch
DCC = Cross-connect switch
Chapter 5 ATM

16. ATM Connection Relationships
Chapter 5 ATM

17. VPC/VCC Characteristics
Quality of Service (QoS)
Switched and semi-permanent virtual channel connections
Cell sequence integrity
Traffic parameter negotiation and usage monitoring
average rate, peak rate, burstiness, peak duration, etc.
(VPC only) virtual channel identifier restriction within a VPC
Chapter 5 ATM

18. Call Establishment with Virtual Paths
Chapter 5 ATM

19. ATM Signaling X X X X X X X X X Private UNI Private NNI Public UNI
Q-2931
PNNI
Private NNI
Public UNI
Public ATM network A X
Private ATM network X X
PNNI
NNI
Public UNI X
B-ICI
Public ATM network B X
Public UNI
PNNI X X
Q-2931
Chapter 5 ATM

20. Control Signaling
A mechanism to establish and release VPCs and VCCs (per ITU-T Rec. I.150)
4 methods for VCCs:
Semi-permanent VCC: no control signaling required
Meta-signaling channel: permanent, low data rate channel for setting up signaling channels
User-to-network signaling virtual channel: set up between user and network
User-to-user signaling virtual channel: set up between users within a VPC, allowing users to set up and tear down VCCs, without network intervention
Chapter 5 ATM

21. Control Signaling 3 methods for VPCs
Semi-permanent: no control signaling required
Customer controlled: customer uses a signaling VCC to request VPC from the network
Network controlled: Network establishes VPC for its own control and signaling use
Chapter 5 ATM

22. ATM Cells
Fixed size
5-octet header
48-octet information field
Small cells may reduce queuing delay for high-priority cells (essential for low delay)
Fixed size facilitates more efficient switching in hardware (essential for very high data rates)
Chapter 5 ATM

23. ATM Cell Format (p. 98)
Chapter 5 ATM

24. Header Format Generic flow control (more ->)
Virtual path identifier (VPI)
Virtual channel identifier (VCI)
Payload type (3 bits: identifies cell as user data or network management cell, presence of congestion, SDU type)
Cell loss priority (0: high; 1: low)
Header error control (more ->)
Chapter 5 ATM

25. Generic Flow Control
Used to control traffic flow at user-network interface (UNI) to alleviate short-term overload conditions
Note: not employed in network core
When GFC is enabled at the UNI, two procedures are used:
Uncontrolled transmission: not subject to flow control
Controlled transmission: flow control constraints (using GFC mechanism) are in force
Chapter 5 ATM

26. Generic Flow Control (GFC) Field Coding
Chapter 5 ATM

27. Header Error Control
8-bit field - calculated based on the other 32 bits in the header
CRC based on x8 + x2 + x > generator is
error detection
in some cases, error correction of single-bit errors in header
2 modes:
Error detection
Error correction
Chapter 5 ATM

28. HEC Operation at Receiver
Based on recognition of fact that bit errors occur in bursts.
Chapter 5 ATM

29. Effect of Error at Cell Header
Chapter 5 ATM

30. Impact of Random Bits on HEC Performance
Chapter 5 ATM

31. ATM Service Categories
Protocols and the TCP/IP Suite
ATM Service Categories
Real-time service
Constant bit rate (CBR)
Real-time variable bit rate (rt-VBR)
Non-real-time service
Non-real-time variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
Guaranteed frame rate (GFR)
Slides with details for these?
Chapter 5 ATM
Chapter 2

32. ATM Bit Rate Service Levels
Chapter 5 ATM

33. ATM Adaptation Layer (AAL)
Support higher-level protocols and/or native applications
e.g., PCM voice, LAPF, IP
AAL Services
Handle transmission errors
Segmentation/reassembly (SAR)
Handle lost and misinserted cell conditions
Flow control and timing control
Chapter 5 ATM

34. ATM Adaptation Layer (AAL)
Chapter 5 ATM

35. Applications of AAL and ATM
Circuit emulation (e.g., T-1 synchronous TDM circuits)
VBR voice and video
General data services
IP over ATM
Multiprotocol encapsulation over ATM (MPOA)
LAN emulation (LANE)
Chapter 5 ATM

36. AAL Protocol and Services
Basis for classification:
requirement for a timing relationship between source and destination
requirement for a constant bit rate data flow
connection or connectionless transfer
Chapter 5 ATM

37. AAL Protocols AAL layer has 2 sublayers: Convergence Sublayer (CS)
Supports specific applications/protocols using AAL
Users attach via the Service Access Point (like a port number)
Common part (CPCS) and application service-specific part (SSCS)
Segmentation and Reassembly Sublayer (SAR)
Packages data from CS into ATM cells and unpacks at other end
Chapter 5 ATM

38. AAL Protocols and PDUs
Chapter 5 ATM

39. AAL Protocol Descriptions
Chapter 5 ATM

40. Segmentation and Reassembly PDUs
Chapter 5 ATM

41. AAL Type 1 Constant-bit-rate source
SAR simply packs bits into cells and unpacks them at destination
One-octet header contains 3-bit SC field to provide an 8-cell frame structure
No CS PDU structure is defined since CS sublayer primarily for clocking and synchronization
Chapter 5 ATM

42. AAL Type 1
Chapter 5 ATM

43. AAL Type 2
Intended for use with applications with variable bit-rate service on multiple channels (multiplexing), or low bit rate, short-frame traffic
AAL Type 3/4
Intended for variable bit rate applications that generate bursty data and demand low loss
Originally, connectionless (AAL4) or connection (AAL3) oriented, now combined into single format (AAL 3/4)
Provides comprehensive sequencing and error control mechanisms
Chapter 5 ATM

44. AAL 3/4 Example (message mode)
Chapter 5 ATM

45. AAL 3/4
Chapter 5 ATM

46. AAL Type 5 Streamlined transport for connection oriented protocols
Reduce protocol processing overhead
Reduce transmission overhead
Ensure adaptability to existing transport protocols
primary function is segmentation and reassembly of higher-level PDUs
Chapter 5 ATM

47. AAL5 Example
Chapter 5 ATM

48. AAL5
Chapter 5 ATM