Transcript Modeling QoS with ATM

Modeling Performance and
QoS with Asynchronous
Transfer Mode (ATM)
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Why Discuss ATM??
ATM provides a good model to
discuss various QoS offerings and
high performance networking
 ATM provides a good example of how
circuit switching differs from
packet switching (i.e., IP)
 ATM is deployed in some backbone
networks as a “link layer” technology
in the Internet Protocol stack (“IPover-ATM” and wireless ATM)

The ATM QoS Model
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Evolution
ATM Forum (1991)
 Frame Relay Forum (1991)
 ADSL (later DSL) Forum (1994)
 MPLS Forum (2000)
 Frame Relay Forum and MPLS Forum
merge to form MPLS & Frame Relay
Alliance (2003)
 ATM Forum merges to form MFA Forum
(2005)
 DSL Forum merges to form Broadband
Forum (2008)

The ATM QoS Model
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Introduction
ATM Protocol Architecture
 Logical connections
 ATM cell structure
 Service levels/categories
 ATM Adaptation Layer (AAL)

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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)
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Protocol Model has 3 planes
User – provides for user/application
data 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

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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.)
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User Plane Layers
User data
User data
AAL
AAL
ATM
ATM
ATM
ATM
PHY
PHY
PHY
PHY
…
End system
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Network
End system
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ATM Layers
in End-Point
User
Plane
LayersDevices and Switches
User
information
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User
information
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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
network end-points (not necessarily
same end-users)
– switched along the same path
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ATM Connection Relationships
Virtual Channel: basic logical communications channel
Virtual Path: groups of “common” virtual channels
Physical Transmission Path: physical communications link
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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 userspecified closed groups/networks of VC bundles
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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
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Virtual Path/Virtual Channel
Terminology
Virtual Path
Virtual Path Identifier
Virtual Path Link
Virtual Path Connection
The ATM QoS Model
(VP) A generic term which describes
unidirectional transfer of cells that
are associated with a common unique
identifier
(VPI) Identifies a particular VP
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
(VPC) A concatenation of VP links
that extends between ATM endpoints where the VCIs are assigned
and where they are translated or
terminated
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ATM VPC/VCC
VP3
a
b
c
d
e
ATM
Sw
1
a
VP5
ATM
Sw
2
ATM
DCC
ATM
Sw
3
VP6
b
c
VP2
VP1
Sw = switch
ATM
Sw
4
d
e
DCC = Cross-connect switch
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ATM Connection Relationships
Figure 19-7
Connection Identifiers
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ATM – Second Session
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VPC/VCC Characteristics



Quality of Service (QoS) provisioning
Switched and semi-permanent virtual
channel connections
Cell sequence integrity
– i.e., cells are delivered in the order sent

Traffic parameter negotiation and usage
monitoring (policing)
– average rate, peak rate, burstiness, peak
duration, etc.

(VPC only) virtual channel identifier
restriction within a VPC
– e.g., a channel reserved for network management
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Call Establishment with Virtual
Signaling
Paths
Phase
Admission
Control Phase
Connection
Setup Phase
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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)

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ATM Cell Format (p. 98)
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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 ->)

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Generic Flow Control

Used to control traffic flow at usernetwork 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
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Generic Flow Control (GFC)
Field Coding
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Header Error Control

8-bit field - calculated based on the
other 32 bits in the header
– CRC based on x8 + x2 + x + 1
-> generator is 100000111
error detection
 in some cases, error correction of
single-bit errors in header
 2 modes:

– Error detection
– Error correction
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HEC Operation at Receiver
Based on recognition of fact that bit errors in fiber-based
networks are single-bit or occur in large bursts.
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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)
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ATM Bit Rate Service Levels
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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
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ATM Adaptation Layer (AAL)
Figure 19-22
AAL Types
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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
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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
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AAL Protocols and PDUs
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Segmentation and Reassembly
PDUs
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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

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Figure 19-23
AAL Type 1
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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 (such
as, perhaps, IP datagrams)
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AAL5 Example
e.g., IP datagram
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AAL5
Figure 19-26
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