Data and Computer
Communications
Chapter 11 – Asynchronous Transfer
Mode
Eighth Edition
by William Stallings
ATM
• a streamlined packet transfer interface
• similarities to packet switching
– transfers data in discrete chunks
– supports multiple logical connections over a
single physical interface
• ATM uses fixed sized packets called cells
• with minimal error and flow control
• data rates of 25.6Mbps to 622.08Mbps
Protocol Architecture
Reference Model Planes
• user plane
– provides for user information transfer
• control plane
– call and connection control
• management plane
– plane management
• whole system functions
– layer management
• Resources and parameters in protocol entities
ATM Logical Connections
• virtual channel connections (VCC)
– analogous to virtual circuit in X.25
• basic unit of switching between two end users
– full duplex
– fixed size cells
• also for
– user-network exchange (control)
– network-network exchange (network mgmt & routing)
ATM Virtual Path Connection
• virtual path connection (VPC)
– bundle of VCC with same end points
Advantages of Virtual Paths
• simplified network architecture
• increased network performance and
reliability
• reduced processing
• short connection setup time
• enhanced network services
Call
Establishm
ent
Using VPs
Virtual Channel Connection
Uses
• between end users
– end to end user data
– vontrol signals
– VPC provides overall capacity
• VCC organization done by users
• between end user and network
– control signaling
• between network entities
– network traffic management
– routing
VP/VC Characteristics
• quality of service
• switched and semi-permanent channel
connections
• call sequence integrity
• traffic parameter negotiation and usage
monitoring
• VPC only
– virtual channel identifier restriction within VPC
Control Signaling - VCC
• to establish or release VCCs & VPCs
• uses a separate connection
• methods are:
1. semi-permanent VCC
2. meta-signaling channel
3. user to network signaling virtual channel
4. user to user signaling virtual channel
Control Signaling - VPC
• methods for control signalling for VPCs:
1. Semi-permanent
2. Customer controlled
3. Network controlled
ATM Cells
ATM Header Fields
• generic flow control
• Virtual path identifier
• Virtual channel identifier
• payload type
• cell loss priority
• header error control
Transmission of ATM Cells
• I.432 specifies several data rates:
– 622.08Mbps
– 155.52Mbps
– 51.84Mbps
– 25.6Mbps
• two choices of transmission structure:
– Cell based physical layer
– SDH based physical layer
Cell Based Physical Layer
• no framing imposed
• continuous stream of 53 octet cells
• cell delineation based on header error
control field
Cell Delineation State Diagram
ATM Service Categories
• Real time - limit amount/variation of delay
– Constant bit rate (CBR)
– Real time variable bit rate (rt-VBR)
• Non-real time - for bursty traffic
– Non-real time variable bit rate (nrt-VBR)
– Available bit rate (ABR)
– Unspecified bit rate (UBR)
– Guaranteed frame rate (GFR)
Constant Bit Rate (CBR)
• fixed data rate continuously available
• tight upper bound on delay
• uncompressed audio and video
– video conferencing
– interactive audio
– A/V distribution and retrieval
Real-Time Variable Bit Rate
(rt-VBR)
• for time sensitive applications
– tightly constrained delay and delay variation
• rt-VBR applications transmit data at a rate that
varies with time
– eg. compressed video
– produces varying sized image frames
– original (uncompressed) frame rate constant
– so compressed data rate varies
• hence can statistically multiplex connections
Non-Real-Time Variable Bit Rate
(nrt-VBR)
• if can characterize expected bursty traffic flow
– eg. airline reservations, banking transactions
• ATM net allocates resources based on this
– to meet critical response-time requirements
• giving improve QoS in loss and delay
• end system specifies:
– peak cell rate
– sustainable or average rate
– measure of how bursty traffic is
Unspecified Bit Rate (UBR)
• may be additional capacity over and above that
used by CBR and VBR traffic
– not all resources dedicated to CBR/VBR traffic
– unused cells due to bursty nature of VBR
• for application that can tolerate some cell loss or
variable delays
– eg. TCP based traffic
• cells forwarded on FIFO basis
• best effort service
Available Bit Rate (ABR)
• application specifies peak cell rate (PCR)
and minimum cell rate (MCR)
• resources allocated to give at least MCR
• spare capacity shared among all ARB
sources
– eg. LAN interconnection
ATM Bit Rate Services
Summary
• Asynchronous Transfer Mode (ATM)
• architecture & logical connections
• ATM Cell format
• transmission of ATM cells
• ATM services
