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Chapter 3
Asynchronous
Transfer Mode (ATM)
Synchronous vs.
Asynchronous
 ATM- cell switching technology (asynchronous)
 TDM – circuit switching technology
(synchronous)
 ATM is more efficient than TDM. (why?)
 TDM: dedicated time slots.
 ATM: time slots are available on demand with
information identifying the destination of the
transmission contained in the header of each
ATM cell.
ATM
 Multi-speed network environment that provides
a variety of complex network services
 Can carry voice, data, video separately or
simultaneously
 Can be used in LANs, MANs, or WANs
 Fixed-length packets (cells)
 Allows multiple logical connections to be
multiplexed
 Minimal error and flow control capabilities
 Connection-oriented virtual channel
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
ATM Network
(integrated voice, video, and data services)
Protocol Architecture
ATM Protocol Layers
ATM
Applications
ATM
Applications
ATM Adaptive
Layer (AAL)
ATM Adaptive
Layer (AAL)
ATM
Layer
ATM
Layer
ATM
Layer
Physical
Layer
Physical
Layer
Physical
Layer
ATM End Station
ATM Switch
ATM End Station
ATM Physical Layer
 Transports cells via a communications channel
(either optical or electrical)
 LAN support: 25-155 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
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
UNI/NNI Interface
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
ATM Adaptation Layer
 The AAL relays ATM cells between ATM
Layer and higher layer (Segmentation
and Reassembly (SAR) process)
 When relaying information received from the
higher layers, it segments the data into ATM
cells.
 When relaying information received from the
ATM Layer, it must reassemble the payloads
into a format the higher layers can
understand.
ATM Adaptation Layer
 Different AALs were defined in supporting different
traffic or service expected to be used. The service
classes and the corresponding types of AALs were as
follows:
 Class A - Constant Bit Rate (CBR) service:
 AAL1 supports a connection-oriented service in which the bit rate
is constant.
 Examples of this service include 64 Kbit/sec voice, fixed-rate
uncompressed video and leased lines for private data networks
 Class B - Variable Bit Rate (VBR) service:
 AAL2 supports a connection-oriented service in which the bit rate
is variable but requires a bounded delay for delivery.
 Examples of this service include compressed packetized voice or
video. The requirement on bounded delay for delivery is
necessary for the receiver to reconstruct the original
uncompressed voice or video.
ATM Adaptation Layer
 Class C - Connection-oriented data service:
 For connection-oriented file transfer and in
general, data network applications where a
connection is set up before data is transferred,
this type of service has variable bit rate and does
not require bounded delay for delivery.
 Two AAL protocols were defined to support this
service class, and have been merged into a
single type, called AAL3/4. But with its high
complexity, the AAL5 protocol is often used to
support this class of service.
ATM Adaptation Layer
 Class D - Connectionless data service:
 Examples of this service include datagram traffic and in general,
data network applications where no connection is set up before
data is transferred.
 Either AAL3/4 or AAL5 can be used to support this class of
service.
 Operation Administration and Maintenance (OA&M)
 Defined for supervision, testing, and performance monitoring. It
uses loop-back for maintenance and ITU TS standard CMIP, with
organization into 5 hierarchical levels:
 Virtual Channel (F5 - Between VC endpoints)
 Virtual Path (F4- Between VP endpoints)
 Transmission Path (F3- Between elements that perform assembling,
disassembling of payload, header, or control)
 Digital Section (F2 Between section end-points, performs frame
synchronization)
 Regenerator Section (F1- Between regeneration sections)
ATM Adaptation Layer
AAL Types
Connection
Mode
Bit Rate
Timing relationship
between source &
destination
AAL1
Connection
oriented
Constant
Yes
AAL2
Connection
oriented
Variable
Yes
AAL3
Connectionless
Variable
No
AAL4
Connectionless
Variable
No
AAL5
Connectionless
Variable
No
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) – Semipermanent (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)
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)
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
ATM PVC and SVC

PVC allows direct connectivity between sites, and is similar to a
leased line.

Advantages:



Disadvantages:



guarantees availability of a connection
does not require call setup procedures between switches.
requires manual setup between the source and the destination.
no network resiliency is available with PVC.
SVC is created and released dynamically and remains in use only as
long as data is being transferred. In this sense, it is similar to a
telephone call. Dynamic call control requires a signaling protocol
between the ATM endpoint and the ATM switch.

Advantages:



connection flexibility
call setup that can be handled automatically by a networking device.
Disadvantages:


the extra time and overhead required to set up the connection.
network management and troubleshooting
ATM Connection
Relationships
ATM Quality of Service (QoS)
 CBR: Constant Bit Rate

Guaranteed transmission rate
 VBR: Variable Bit Rate


rtVBR and nrtVBR
Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Max
Burst Size (MBR)
 UBR: Unspecified Bit Rate

No guarantee, best effort
Service
Bit Rate
CBR
PCR
SCR
VBR
UBR
Call Establishment Using VPs
VP/VC Characteristics
 Quality of service based on VCC
 Switched and semi-permanent channel
connections
 Call sequence integrity – packets arrive in
order
 Traffic parameter negotiation and usage
monitoring
 VPC only
 Virtual channel identifier restriction within VPC –
some VCCs reserved for network management
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




ATM Cell Format
Header Format
 Generic flow control (GFC)
 Only at user to network interface (UNI)
 Controls flow between user and local ATM switch
 2 sets of procedures: Uncontrolled and Controlled
transmission
 Virtual path identifier
 8 bits in UNI cell, 12 bits in NNI cell
 Addressing info for cell routing
 Virtual channel identifier
 16 bit
 Additional addressing info for cell routing
Header Format
 Payload type – 3 bits (user info or network
management)
 first bit: user data (0) or control data (1)
 second bit: 0 = no congestion, 1 = congestion),
 third bit: whether the cell is the last cell (1) in a series
of AAL5 frame
 Cell loss priority (CLP)
 Indicates whether the cell should be discarded (1) if it
encounters extreme congestion as it moves through
the network
 Header error control (HEC)
 Checksum on the first 4 bytes of the header
 Correct an isolated bit error and detect multiple bit
errors
ATM Service Categories
 ATM is designed to transfer many different
types of traffic simultaneously, including realtime 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
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)
Real Time Services
 If want to avoid or decrease variation of delay (jitter), use CBR or rtVBR
 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
 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
Non-Real Time
 Intended for applications with bursty
traffic and limited constraints on delay
and delay variation
 Greater flexibility, greater use of
statistical multiplexing
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
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
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
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 handshaking between NIC
and switch
ATM vs 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)
ATM vs SONET
 SONET is a transport mechanism,
transporting data over fiber.
 Can act as a transport carrier for ATM (or
FDDI, or ISDN, etc.)
 ATM is a technology and protocol
designed to use SONET as its carrier
service
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