1. Computer Communication And Networks
Lecture 8
ATM: Asynchronous Transfer Mode By
Prof. L. K. Awasthi
CSED, N.I.T.- Hamirpur

2. Asynchronous Transfer Mode
Asynchronous Transfer Mode (ATM) is a standard switching technique designed to unify telecommunication and computer networks.
It uses asynchronous time-division multiplexing and it encodes data into small, fixed-sized cells.
ATM provides data link layer services that run over a wide range of OSI physical Layer links.
ATM has functional similarity with both circuit switched networking and small packet switched networking.

3. ATM
ATM standard (defined by CCITT) is widely accepted by common carriers as mode of operation for communication – particularly BISDN.
ATM is a form of cell switching using small fixed-sized packets.
Basic ATM Cell Format
5 Bytes
48 Bytes
Header
Payload

4. Cell Switched ATM Similar to frame relay. Difference?
Frame relay switches variable length frames within frame relay cloud from source to destination
ATM switches fixed-length cells (48 byte information field, 5 byte header)
Based on packet switching (connection-oriented)
Cell sequence integrity preserved via virtual channel
VCC – virtual channel connection – is set up between end users, variable rate, full duplex
VCC also used for control
Information field is carried transparently through the network, with minimal error control.

5. Protocol Architecture

6. Asynchronous Transfer Mode (ATM)
Voice
Data packets
MUX
Wasted bandwidth
Images
TDM
ATM `

7. ATM Protocol Architecture
ATM Adaptation Layer (AAL) – the protocol for packaging data into cells is collectively referred to as AAL.
Must efficiently package higher level data such as voice samples, video frames and datagram packets into a series of cells.
Design Issue: How many adaptation layers should there be?

8. Management plane
Plane management
Control plane
User plane
Layer management
Higher layers
Higher layers
ATM adaptation layer
ATM layer
Physical layer

9. User information User information … End system Network End system AAL
ATM
ATM
ATM
ATM
PHY
PHY
PHY
PHY …
End system
Network
End system

10. Original ATM Architecture
CCITT envisioned four classes of applications (A-D) requiring four distinct adaptation layers (1-4) which would be optimized for an application class:
Constant bit-rate applications CBR
Variable bit-rate applications VBR
Connection-oriented data applications
Connectionless data application

11. ATM Physical Layer
Transports cells via a communications channel (either optical or electrical)
LAN support: Mbps copper or fiber
WAN support: SONET rates over fiber
Physical Medium Sublayer: bit transfer, bit alignment, and copper/fiber conversions
Transmission Convergence Sublayer: bit/cell conversion at sending and receiving nodes

12. ATM Layer Handles functions of the network layer:
Connection-oriented without acknowledgements
Two possible interfaces:
UNI – User-Network Interface: Boundary between an ATM network and host
NNI – Network-Network Interface: Between two ATM switches

13. UNI/NNI Interface

14. ATM Adaptation Layer (AAL)
Maps higher-layer information into ATM cells to be transported over an ATM network
Collects information from ATM cells for delivery to higher layers

15. Virtual Connections
Virtual Channel Connection (VCC) – Full duplex virtual circuit with logical connection between source and destination – can be PVC or SVC.
Virtual Path Connection (VPC) – Semi-permanent (or customer controlled or network controlled) connection that provides a logical collection of virtual channels that have the same endpoint.
A single virtual path supports multiple virtual channels (analogy – highway = VPC, lane = VCC)

16. VCI vs VPI
VPI – Virtual Path Identifier – identified in cell’s header. Cannot establish a virtual channel before virtual path.
VCI – Virtual Channel Identifier – only have local significance – different virtual paths reuse VCIs (but VCIs on same path must be unique).

17. What is so special about a virtual path?
ATM is connection-oriented, so circuit must be established before transmission
As route established, VPIs and VCIs are assigned
VPI and VCI info suffices for addressing info
Simplified network architecture (based on VC or VP)
Increased network performance and reliability (fewer, aggregated entities because of simplified network architecture)
Reduces processing and short connection setup time
User may define closed user group or closed networks of virtual channel bundles

18. ATM Connection Relationships

19. ATM Cells Fixed size – 53 bytes 5 octet header
48 octet information field
Small cells reduce queuing delay for high priority cells
Small cells can be switched more efficiently
Easier to implement switching of small cells in hardware

20. ATM Cell Format

21. ATM Conceptual Model Four Assumptions
ATM network will be organized as a hierarchy.
User’s equipment connects to networks via a UNI (User-Network Interface).
Connections between provided networks are made through NNI (Network-Network Interface).
ATM will be connection-oriented.
A connection (an ATM channel) must be established before any cells are sent.

22. X X X X X X X X X Private ATM network Private UNI Private NNI
Public UNI
Public ATM network A X X X
NNI
Public UNI X
B-ICI
Public ATM network B X
Public UNI X X

23. ATM Connections two levels of ATM connections:
virtual path connections
virtual channel connections
indicated by two fields in the cell header:
virtual path identifier VPI
virtual channel identifier VCI

24. ATM Virtual Connections
Virtual Paths
Physical Link
Virtual Channels

25. ATM Conceptual Model Assumptions (cont.)
Vast majority of ATM networks will run on optical fiber networks with extremely low error rates.
ATM must support low cost attachments.
This decision lead to a significant decision – to prohibit cell reordering in ATM networks.
 ATM switch design is more difficult.

26. UNI Cell Format GFC (4 bits) VPI (4 bits) VPI (4 bits) VCI (4 bits)
ATM cell header
VCI (8 bits)
CLP (1 bit)
VCI (4 bits)
PT (3 bits)
HEC (8 bits)
Payload (48 bytes)

27. ATM Cell Switching Switch 1 voice 67 1 … video 67 2 25 N 75 5 video 2532 32 1 67
data 39 3 32 3 39
data 32
video 61 6 61 2 67 … …
video 75 N N

28. ATM Service Categories
ATM is designed to transfer many different types of traffic simultaneously, including real-time voice, video, and bursty TCP traffic
Way in which data flow is handled depends on the characteristics of the traffic flow and requirements of the application (ex. Real-time video must be delivered within minimum variation in delay)
Primary service categories – real time service, non-real time service

29. ATM Service Categories
Real time
Constant bit rate (CBR)
Real time variable bit rate (rt-VBR)
Non-real time
Non-real time variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
Guaranteed frame rate (GFR)

30. Real Time Services
If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR
CBR Fixed data rate continuously available
Commonly used for uncompressed audio and video
Video conferencing
Interactive audio
A/V distribution and retrieval
rt-VBR Best for time sensitive applications
Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that varies with time
e.g. compressed video
Produces varying sized image frames
Original (uncompressed) frame rate constant (isochronous)
So compressed data rate varies
Can statistically multiplex connections

31. Non-Real Time
Intended for applications with bursty traffic and limited constraints on delay and delay variation
Greater flexibility, greater use of statistical multiplexing

32. nrt-VBR May be able to characterize expected traffic flow
Improve QoS in loss and delay
End system specifies:
Peak cell rate
Sustainable or average rate
Measure of how bursty traffic is
e.g. Airline reservations, banking transactions

33. UBR Unused capacity of CBR and VBR traffic made available to UBR
For application that can tolerate some cell loss or variable delays
e.g. TCP based traffic
Cells forwarded on FIFO basis
Best efforts service

34. ABR
Application specifies peak cell rate (PCR) it will use and minimum cell rate (MCR) it requires
Resources allocated to give at least MCR
Spare capacity shared among ABR and UBR sources
e.g. LAN interconnection

35. Guaranteed Frame Rate (GFR)
Designed to support IP backbone subnetworks
Purpose: optimize handling of frame based traffic passing from LAN through router to ATM backbone
Used by enterprise, carrier and ISP networks
Consolidation and extension of IP over WAN
UNI establishes hand shaking between NIC and switch

36. ATM Adaptation Layers Voice s1 , s2 … Video … Data A/D AAL cells
Digital voice samples
Video
A/D …
Compression
AAL
cells
compressed frames
picture frames
Data
AAL
cells
Bursty variable-length packets
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 9.3

37. Revised ATM Service Categories
Class
Description
Example
CBR
Constant Bit Rate
T1 circuit
RT-VBR
Real Time Variable Bit Rate
Real-time videoconferencing
NRT-VBR
Non-real-time Variable Bit Rate
Multimedia
ABR
Available Bit Rate
Browsing the Web
UBR
Unspecified Bit Rate
Background file transfer

38. ATM versus Frame Relay Frame relay uses variable length frames
ATM fixed length cells
ATM has higher overhead, but faster speed and traffic management (better suited for video and voice).

39. Why is ATM so Efficient? Minimal error and flow control
Reduces overhead of processing ATM cells
Reduces number of required overhead bits
Fixed size simplified processing at each ATM node (can be switched more efficiently – more efficient use of router)
Small cells reduce queuing delay
Minimal addressing info on each cell
Efficient traffic management

40. QoS, PVC, and SVC
Quality of Service (QoS) requirements are handled at connection time and viewed as part of signaling.
ATM provides permanent virtual connections and switched virtual connections.
Permanent Virtual Connections (PVC)
permanent connections set up manually by network manager.
Switched Virtual Connections (SVC)
set up and released on demand by the end user via signaling procedures.

41. Questions