Asynchronous Transfer Mode

1
Chapter 5

• Asynchronous Transfer Mode
• (ATM)

2
Introduction

• ATM Protocol Architecture
• Logical connections
• ATM cell structure
• Service levels/categories
• ATM Adaptation Layer (AAL)

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

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)

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

6
ATM Protocol Reference Model
Map data to the ATM cell structure
Framing, cell structure Logical Connections
Various data rates (155.52 Mbps, 622.08 Mbps)
over various physical media types (Fiber Optic,
SONET, UTP, etc.)
7
User Plane Layers
User information
User information
AAL
AAL
ATM
ATM
ATM
ATM
PHY
PHY
PHY
PHY

End system
End system
Network
8
User Plane Layers
User information
User information
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

10
ATM Connection Relationships
Virtual Channel basic logical communications
channel Virtual Path groups of common virtual
channels Physical Transmission Path physical
communications link
11
VCC (logical connection) Uses

• Exchange between end users
• user data
• control signaling (more later)
• Exchange between an end user and a network entity
• control signaling (more later)
• Exchange between 2 network entities
• traffic management
• routing functions

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

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
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
15
ATM VPC/VCC
a
VP3
VP5
a
b
ATM Sw 1
b
c
ATM Sw 2
ATM Sw 3
ATM DCC
c
d
VP6
e
VP2
VP1
d
ATM Sw 4
e
Sw switch DCC Cross-connect switch
16
ATM Connection Relationships
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

18
Call Establishment with Virtual Paths
19
ATM Signaling
Private UNI
X
X
PNNI
Q-2931
Private NNI
Public ATM network A
Public UNI
X
Private ATM network
X
X
PNNI
NNI
X
Public UNI
B-ICI
Public ATM network B
X
Public UNI
PNNI
X
X
Q-2931
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

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

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)

23
ATM Cell Format (p. 98)
24
Header Format

• Generic flow control (more -gt)
• 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 -gt)

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

26
Generic Flow Control (GFC) Field Coding
27
Header Error Control

• 8-bit field - calculated based on the other 32
  bits in the header
• CRC based on x8 x2 x 1 -gt
  generator is 100000111
• error detection
• in some cases, error correction of single-bit
  errors in header
• 2 modes
• Error detection
• Error correction

28
HEC Operation at Receiver
Based on recognition of fact that bit errors
occur in bursts.
29
Effect of Error at Cell Header
30
Impact of Random Bits on HEC Performance
31
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)

32
ATM Bit Rate Service Levels
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

34
ATM Adaptation Layer (AAL)
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)

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

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

38
AAL Protocols and PDUs
39
AAL Protocol Descriptions
40
Segmentation and Reassembly PDUs
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

42
AAL Type 1
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

44
AAL 3/4 Example (message mode)
45
AAL 3/4
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

47
AAL5 Example
48
AAL5