Transcript 5. ATM Adaptation Layer 2001 Copyright 5

Principle of ATM
（2）
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5. ATM Adaptation Layer
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5. ATM Adaptation Layer
1. QoS Service Catagories
 CBR
Constant Bit Rate
 VBR-RT
Variable Bit Rate - Real Time
 VBR-NRT
Variable Bit Rate - Non-Real Time
 ABR
Available Bit Rate
 UBR
Unspecified Bit Rate
 GFR
Guaranteed Frame Rate (later)
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5. ATM Adaptation Layer
1. QoS Service Catagories (cont.)
CBR has been defined to support constant bit rate connectionoriented traffic where end-to-end synchronisation is
required. This is otherwise known as ITU-T Class A
performance requirements.
VBR-RT has been defined to support variable bit rate
connection-oriented traffic where end-to-end
synchronisation is required. This is otherwise known
as Class B performance requirements.
VBR-NRT is for types of traffic which are predictable, yet do
not require a timing relationship to be maintained end-toend.
ABR service is designed for economical support of
applications with vague requirements for throughputs and
delays.
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5. ATM Adaptation Layer
1. QoS Service Catagories (cont.)
UBR operates on a 'best effort' basis, with no reservation of
bandwidth.
Signalling used to set up and clear down calls is normally
transmitted as UBR,as is Local Area Network Emulation
(LANE) traffic.
GFR is a new service category which is still being defined. It is
intended to provide a mechanism that will deliver frames (as
cells).
If one cell is lost they are all lost. What is guaranteed is a
frame rate rather than a cell rate.
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5. ATM Adaptation Layer
Classes as defined by ITU-T rec. I 362
Class A
Timing between
source and destination
Bit rate
Class C
Required
Constant
Connection mode
Relevant
Adaptation Layer
Class B
Not required
Variable
Connection-oriented
AAL 1
Class D
AAL 2
Connectionless
AAL 3
AAL 4
AAL 5
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5. ATM Adaptation Layer
2. General Principles of Adaptation
The use of a CS
is not required by
all AALs
Higher layer data
Etc.
CS
T
H
H
T
H
T
Adaptation
Layer
SAR
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5. ATM Adaptation Layer
3. Usage of Adaptation Layer
AAL is used to adapt a source application to ATM
 ATM switching takes place in the ATM Layer.
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5. ATM Adaptation Layer
4. AAL1
• A function of the AAL associated with Class A data, AAL1, is
to ensure that there is timing integrity between the sending
and the receiving end.
• Another function is to carry out clock recovery at the
destination.
• The AAL also provides a mechanism to detect lost cells, and
inserts a dummy into the cell stream to ensure that the
timing information is not lost.
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5. ATM Adaptation Layer
4. AAL 1 (cont.)
To format Class A data into cells, the data stream at the
defined operating speed is simply chopped up into 47-byte
chunks. Each 47-byte SDU is preceded by a one-byte
header, resulting in a 48-byte payload.
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5. ATM Adaptation Layer
4. AAL 1 (cont.)
The SN field is then split into two parts: the Convergence
Sublayer Indication bit (CSI) which is normally set to 0, and
three bits for the Sequence Number.This cycles through from 0
to 7 and back to 0 again, and is suitable for identifying missing
or misinserted cells.
To ensure the integrity of the SN field, it is protected by the
SNP (Sequence Number Protection) field, which is a three-bit
CRC check with an additional even-parity bit.
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5. ATM Adaptation Layer
5. AAL2
• AAL2 defines the transport of VBR traffic that is timingsensitive, such as VBR audio and video.
• A feature of AAL2 is the ability to accept several streams of
traffic and multiplex them together.
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5. ATM Adaptation Layer
5. AAL 2 (cont.)
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5. ATM Adaptation Layer
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5. ATM Adaptation Layer
5. AAL2 (cont.)
Initial AAL 2 header
• CID Field The channel identifier field identifies the individual
user channels within the AAL2, and allows up to 248
individual users within each AAL2 structure.
• LI Field The length identifier identifies the length of the
packet payload associated with each individual user, and
assures conveyance of the variable payload.
• UUI Field One current use for the User-to-user field is to
negotiate a larger Maximum Transfer Unit (MTU) size for IP.
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5. ATM Adaptation Layer
5. AAL2 (cont.)
Secondary AAL 2 header
The Offset Field identifies the location of the start of the next
packet within the flow.
For robustness the Start Field is protected from errors by the
Parity bit (P) and data integrity is protected by the
Sequence Number (SN).
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5. ATM Adaptation Layer
6. AAL3/4
• The two workgroups, AAL3 and AAL4,discovered that they
had produced near-identical processes. The two work
groups subsequently joined forces to produce the single
adaptation known as AAL 3/4.
• AAL 3/4 has a relatively high overhead. In this case, 4
octets are consumed by the header and trailer fields. After
subtracting this overhead, the payload has been reduced to
44 octets.
• Although originally designed to carry all manner of
traditional data traffic, AAL 3/4 was seen as overly complex
to implement and also as inefficient due to its high
overheads. Consequently, most data traffic is carried in AAL
5.
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5. ATM Adaptation Layer
6. AAL 3/4 (cont.)
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5. ATM Adaptation Layer
6. AAL 3/4 (cont.)
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5. ATM Adaptation Layer
6.1 AAL3/4 CS
• Type Indicates the units used by the BA and Length fields.
• BTag/Etag: These two 'tags' are a numerical value (the
same value), which help to ensure that it is a single CS unit
that has been received and not a damaged CS unit created
by joining together parts of two CS units.
• BA Size Length of the user information subfield of the CS
payload.
• Pad Padding added to ensure that the total length of the
CS is divisible by 4 (32 bits).This is an engineering
consideration to simplify processing by 32-bit processors.
• Length of the user information subfield. Other fields and
subfields are reserved for future definition.
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5. ATM Adaptation Layer
6.2 AAL 3/4 SAR
• Segment Type Indicates whether a cell is the first (at the
beginning) of a message (BOM), a continuation of a
message (COM), or the last (at the end) in a message
(EOM).
• Sequence Indicates the position in a convergence PDU of a
SAR PDU.
• MID: This is multiplexing ID field. This can be used to allow
the multiplexing of several traffic streams into a single
connection.
• Len: This is the length of the actual data in the last cell of a
message.
• CRC: A 10-bit Cyclic Redundancy Check computed over the
SAR PDU.
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5. ATM Adaptation Layer
7. AAL5
• AAL5 has significantly lower overheads than AAL 3/4 and is,
therefore, very widely adopted.
• In practice, since AAL 2 is not yet widely used and AAL 3/4
is seen as overly complex and cumbersome, only AAL1 and
AAL5 are widely used.
• AAL1 is used for CBR traffic and AAL5 for all others: VBR,
UBR and ABR.
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5. ATM Adaptation Layer
7. AAL5 (cont.)
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5. ATM Adaptation Layer
7. AAL5 (cont.)
AAL5 Frame Format
• AAL5 simply takes the network layer packet and adds a
single trailer.
• The PAD field is there to pad out the complete PDU so that
it can be divided into an integer number of 48-byte
segments for loading into the cells.
AAL5 Trailer
The AAL5 8-byte trailer consists of:
• Two 1-byte fields which are unused
• A 2-byte length field which indicates the length of the data,
not including the trailer and pad
• A 4-byte CRC
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5. ATM Adaptation Layer
7. AAL5 (cont.)
AAL5 Trailer
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5. ATM Adaptation Layer
7. AAL5 (cont.)
AAL5 Transmission
•
The PTI field in the header is used. Bit 1 is set to 1 when
the last cell representing the PDU is assembled, and all
other cells have the bit set to 0.
•
When the receiver sees the PTI field with bit 1 set to 1, it
assumes that the next cell with the same VPI/VCI number
will be the first cell of a new PDU.
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5. ATM Adaptation Layer
7. AAL5 (cont.)
AAL5 Transmission
AAL5 makes use of the PTI field in ATM cell header
Bit 1 = 1 indicates this cell carries the AAL5 trailer
GFC
VPI
VPI
VCI
VCI
VCI
PTI
CLP
HEC
48-byte data field
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6. Signalling
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6. Signalling
1. Signalling Functions
• If connections are to be set up on demand, a form of
signalling is essential. Connections set up in this way are
referred to as switched virtual circuits (SVCs).
• it is necessary to adopt a signalling system which is
internationally accepted together with an addressing
scheme which operates on a global basis.
• The ITU-T standard for signalling in ATM public networks is
known as Q.2931.
• The ATM Forum derived two separate standards from this
for private networks, known as V3.0 and V3.1.
• UNI 4.0 has also been released. This brings the ATMF
signalling subset closer to Q.2931.
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6. Signalling
1. Signalling Functions (cont.)
• All signalling within ATM is carried over a standard reserved
channel: VPI=0, VCI=5.
• Signalling is separately defined for use across the UNI
(Q.2931, UNI 3.0, UNI3.1, UNI 4.0) and for use at the NNI
for setting up the calls (PNNI).
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6. Signalling
2. Signalling Control Functions




Establishing a virtual circuit
Status report for a virtual circuit
Maintaining a virtual circuit
Clearing a virtual circuit
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6. Signalling
3.
Address Formats
• Work is still proceeding on defining the most effective
addressing structures for use in ATM. Below are listed three
formats that are used in private networks.
• The carriers have already declared their intent to use E.164
addresses.
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6. Signalling
3. Address Formats (cont.)
D.C.C. Format
AFI
High order DSP
DCC
IDI
E.S.I.
Sel.
DSP
IDP
I.C.D. Format
AFI
High order DSP
ICD
IDI
E.S.I.
Sel.
E.S.I.
Sel.
DSP
IDP
E.164 Format
AFI
E.164 ISDN Number
High order DSP
IDI
DSP
IDP
48-bit ‘MAC’ address
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6. Signalling
• DCC ATM Address Format
Authority Format Indicator (AFI) = 39
Data Country Code (DCC).
• ICD ATM Address
Authority Format Indicator (AFI) = 47
International Code Designator (ICD)
• NSAP Encapsulated E. 164 Address Format
Authority Format Indicator (AFI) = 45
E.164 - An E.164 format (telephone) number
(NSAP: Network System Access Point)
General
Domain Specific Part (DSP)
End System (or Station) Identifier (ESI)
Sel Selector
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6. Signalling
4. Call Set-up
• A call set-up message is sent by the calling party into the
network to initiate a connection.
• It is also passed from the network to the called party to
initiate the connection.
• Assuming successful call establishment, the called party will
respond with a connect message.
• With ATM we need to specify a list of characteristics that
the network must support, for example, the quality of service
(QoS).
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6. Signalling
4. Call Set-up (cont.)
Calling Party
UNI
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Called Party
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6. Signalling
4. Call Release
• The release message can be sent by either party to clear
down the connection.
• If one party clears, then the network will send a clear
message to the other party.
• The network may also initiate the clear-down if, for example,
a network failure occurs, or in the absence of traffic for a
pre-determined time period.
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6. Signalling
4. Call Release (cont.)
Calling Party
UNI
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Called Party
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6. Signalling
5. Point-to-Multipoint Connections
• Multipoint connections are a feature of ATM networks. They
are used in all LAN techniques.
• They will be a most important feature of broadcast networks
such as those providing video on demand.
• The process of setting up a point-to-multipoint connection
involves first of all setting up a point-to-point connection. It
must be specified that this connection is to be multipoint
(This must be done as multipoints are uni-directional.单向
的)
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6. Signalling
5. Point-to-Multipoint Connections
• Once the initial point-to-point is set up additional
destinations (leaves) can be added. There are two
alternative mechanisms that can be used here:
(1) Send a request to the root (the originator of the original
point-to-point);
(2) With signalling version 4.0 issue a Leaf Initiated Join
(LIJ) request to the network.
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6. Signalling
5. Point-to-Multipoint Connections (cont.)
ROOT Party
UNI
UNI
NEW Leaf
Point-to-Point Connection
Point-to-Multipoint Connection
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6. Signalling
6. The Traffic Contract
• The traffic contract is the sum total of all the parameters
required to define the characteristics of a connection.
• The contract includes an indication of how the network is to
verify that the user does not use more resources than were
requested at set-up time.
• The contract consists of a series of requirements that are
encoded for transmission to the network at the ingress
switch to the network (this includes a value of required
bandwidth and delay).
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6. Signalling
6. The Traffic Contract (cont.)
• The set-up message carries the destination 20-byte ATM
addresses, plus the basic bandwidth parameters forward
and reverse, and the QoS class.
• The set-up message may also carry the source ATM
address.
• The traffic contract between user and network establishes:
– Virtual bandwidth reserved in each of the forward and
backward directions;
– QoS class for cells in each of the forward and reverse
directions.
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6. Signalling
6. The Traffic Contract (cont.)
• The Connection Admission Control (CAC) algorithm of
the switch will then assess the network in the light of the
request, before allowing the connection to proceed to setup.
• The ingress switch will retain a copy of the pertinent
parameters (such as PCR,SCR and MBS) and will use this
information to check that the connection stays within its
contracted bounds (a policing function).
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6. User-Network Interface
(UNI) Signalling
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6. UNI Signalling
1. The User-Network Interface
• The user-network Interface (UNI) is that point between the
end-point ATM equipment and the first ATM switch.
• There have been several versions of the UNI specification,
defined by the ATM forum: UNI 2.0, UNI 3.0, UNI 3.1 and
UNI 4.0 (also known as Sig 4.0).
• Of these specifications UNI 2.0 supports only PVCs, while
the latter three versions also support SVCs.
• The ATMF signalling (from UNI 3.1 onwards) was aligned
with the ITU-T Q.2931 signalling standard.
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6. UNI Signalling
1. The User-Network Interface (cont.)


The set-up message will carry the source and destination
ATM addresses, plus the bandwidth and the QoS parameters
The call set-up message is chopped up using AAL5 and sent
on reserved channel (VPI= 0, VCI=5).
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6. UNI Signalling
2. Q.2931 Signalling Format
• Signalling under ATM consists of joining together a variety
of basic building blocks containing the necessary
information.
•
These building blocks are known as Information Elements
(IEs) and each element has a standard 4-byte header
followed by the IE content.
• IEs are built as required by the message type and service
type.
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6. UNI Signalling
2. Q.2931 Signalling Format
Message Types:
Call Establishment:
CALL PROCEEDING
CONNECT
CONNECT ACKNOWLEDGE
SETUP
Call Clearing:
RELEASE
RELEASE COMPLETE
RESTART
RESTART ACKNOWLEDGE
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6. UNI Signalling
2. Q.2931 Signalling Format
Message Types:
Miscellaneous:
STATUS
STATUS ENQUIRY
Point-to-Multipoint:
ADD PARTY
ADD PARTY ACKNOWLEDGE
ADD PARTY REJECT
DROP PARTY
DROP PARTY ACKNOWLEDGE
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7. Private Network-to-Network
Interface (PNNI)
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7. PNNI
1. PNNI Overview
• Although the PNNI specification has been issued for use in
private networks, PNNI proves to be sufficiently scalable
and robust to be used in public networks.
• It is likely that the official NNI signalling standard, when it is
eventually released, will be strongly based on PNNI.
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1. PNNI Overview (cont.)


7. PNNI
Signalling protocol to set up connections based on routing
information
Routing protocol to distribute reachability, capacity and QoS
information
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7. PNNI
2. PNNI Targets






To distribute among all participating switches the topology of
the ATM network
To operate at the network-network interface
To allow for scalability by the creation of groups of switches
To allow switches to build routing tables from the topological
information
To allow for ‘crank back’ to last the confirmed point and a
search for an alternate route
On end-to-end route confirmation, interface with Connection
Admission Control to accept ATM call set-up
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7. PNNI
2. PNNI Targets (cont.)
PNNI Standards
• PNNI is an interface specification that uses a Link State
process for the distribution of routing information.
• The ATM Forum standard is P-NNI version 1.0 af-pnni0055.000 March 1996.
• Error corrections issued as af-pnni-0081.000 July 1997.
• PNNI supersedes an earlier version from December 1994
called Interim Inter Switch Protocol (IISP). This is
sometimes referred to as PNNI phase 0.
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7. PNNI
3. PNNI Base Level
• All ATM switch ports have an ATM address of 20 bytes
(1) The first 13 bytes are normally fixed for each switch
(2) The final 7 bytes are the physical address of the
attached device (6 bytes, referred to as the MAC
address) plus the selector field which is one byte.
• In the example used here, we have only shown the last few
hex digits of an ATM address.
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7. PNNI
3. PNNI Base Level (cont.)
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7. PNNI
3. PNNI Base Level (cont.)
Forming Groups
• On start-up, PNNI nodes send 'hello' packets on all
interfaces to discover neighbours.
• As part of this process, neighbouring nodes exchange their
ID numbers. In this example, a 13-digit match is required.
• All nodes with matching numbers form a logical peer group
using the matching digits as a group identifier, for
example, group number 202.
• Nodes with at least one link terminating at a switch in a
'foreign' group are considered 'Border Nodes'.
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7. PNNI
3. PNNI Base Level (cont.)
Information Exchange
• Nodes within a group exchange and relay information about
link status including virtual bandwidth, availability and next
hop.
• A reliable transport mechanism is used to ensure that all
nodes ultimately share the same database.
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3. PNNI Base Level (cont.)
7. PNNI
PNNI Groups
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7. PNNI
4. Peer Group Leader
• As part of this process the nodes within a group select a
group leader based on a configured priority number, or by
selecting the node with the lowest address.
• Group leaders establish logical connections with each
other and exchange a summary of information about their
groups.
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4. PNNI Peer Group Leaders (cont.)
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7. PNNI
4. PNNI Logical Network
• The network, viewed from the perspective of a group leader,
will appear to have the topology as shown in the next
diagram.
• Locally, a group leader will retain the detailed view of its
own group including border nodes and therefore 'real' links
to neighbouring groups.
• Group leaders pass this logical network map to the
members of their own group. Each PNNI node, therefore,
has a detailed description of its own group and a logical
map on how to get to any other group.
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7. PNNI
PNNI Logical Network
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7. PNNI
5. PNNI Operation
• When a UNI signalling request comes in from the endstation on reserved channel 0,5 its contents will be
analysed within the switch.
• The switch first performs a Connection Admission
Control (CAC) algorithm which determines whether or
not the switch has the resources necessary to handle
the incoming call.
• A Generic Connection Admission Control (GCAC) is
then performed.
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7. PNNI
5. PNNI Operation
• This GCAC algorithm determines whether or not the
switches between the source and destination can
handle the call.
• Following the GCAC algorithm, the switch prepares a
Designated Transit List (DTL) which is an entire route
through the network for the signalling request.
• This DTL is added onto the call set-up message and
sent to the next node along the intended signalling
route.
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Global topology as seen by node 10126
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PNNI Designated Transit List
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7. PNNI
6. PNNI Packets
Standard Header
There are many types of packet used for the PNNI
protocol. However, they all start with a standard layout
header.
PNNI Signalling
The format of PNNI signalling packets is based on UNI
4.0 (Q.2931) with additions to cater for the transit lists
and crankback.
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7. PNNI
6. PNNI Packets (cont.)
All PNNI packets have a common header:
0
2 Bytes
Packet Type
4 Bytes
Length
Version
Packet Type:
1 = Hello
2 = PTSP (PNNI Topology State Packet)
3 = PTSP Acknowledge
4 = Database Summary
5 = PTSE Request
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6 Bytes
Supported
Most recently supported
protocol version, used to
align protocol versions
between different nodes
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8. Network Management
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8. Network Management
1. General Management Model
The management protocols used are typically the Simple
Network Management Protocol (SNMP) and, in more
recent devices, the Hypertext Transfer Protocol (HTTP).
Local Management
With local management, the management terminal is
plugged directly into the ATM switch, typically into an
Ethernet port or a serial port.
Initially an ATM switch may be configured in this manner.
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8. Network Management
1. General Management Model (cont.)
In band Management
• In managing an ATM network, management traffic is sent in
band that is over the ATM network itself on an ATM
connection.
• An SVC, or more typically a PVC, may be used for this
purpose.
ILMI
• The Interim Local Management Interface Protocol (ILMI)
is a standard ATM management protocol.
• ILMI works only across the UNI interface, that is between
the end ATM station and the first ATM switch.
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General Management Model
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8. Network Management
2. Interim Local Management Interface (ILMI)
• The ILMI standards produced by the ATM Forum enable a
number of network management functions to be performed
across the UNI.
• ILMI is a model based on the use of SNMP for the
interchange of data, which is adapted via AAL5 and
transmitted over a predefined VPI/VCI=0/16.
• The important difference between SNMP, as discussed in
the previous section, and ILMI is that ILMI is SNMP over
AAL5 directly, that is, without using IP.
• ILMI is positioned at the public and private UNIs. ILMI also
runs between the public and private network.
• If one wishes to access this information remotely then one
must run a management agent locally to access the local
MIB.
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8. Network Management
SNMP and ILMI
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8. Network Management
3. ILMI Functions
• ILMI was intended originally to handle only the address
registration and de-registration process for each end station
in an ATM network.
• Now its functions have grown and now include many other
housekeeping operations, including control information,
switch configuration details, statistics relating to the ATM
connections, and the physical and ATM layer data.
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ILMI Basic Requirements
8. Network Management
Single MIB for each ATM device
A Management Information Base (MIB), which contains
data relating to the status of each end station or
intermediate switch, is set up for the ATM system.
ILMI provides:
Status information
Configuration information
Control information
ILMI handles:
Address registration
Address de-registration
Switch configuration
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8. Network Management
4. ILMI Agents
• The ILMI requires a management entity at each end of the
interface.
• The UNI Management Entity (IME) acts as the server to
the network.
• management station client, and performs all necessary
communications tasks via AAL5 and ATM cells.
• The IME also controls access to the MIB.
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8. Network Management
4. ILMI Agents (cont.)
 The Interface Management Entity (IME) in each ATM device
does the following:
Handles the communication
Provides access to the MIB
Co-ordinates between ATM and physical layer information

There are two types of Interface Managed Entity (IME):
uIME - User IME (ATM end device)
nIME - Network IME (ATM switch)
nIME
uIME
ILMI
(SNMP over AAL5)
Private
switch
Private
UNI
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8. Network Management
5. ILMI ATM UNI MIB Tree
ATM UNI ILMI MIB
Physical
Layer
ATM
Layer
Virtual
Virtual
Path
Channel
Connection Connection
ATM
Layer
Statistics
Network
Prefix
Address
Interface
Index
Interface Interface
Index +
Index +
Prefix
Address
Common Specific
Interface
Index
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Interface
Index
Interface
Index +
VPI
Interface
Index +
VPI + VCI
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8. Network Management
6. Address Registration
• All ATM addresses consist of 20 bytes of data, made up of
two distinct parts:
The end-station address, which is 6 bytes of MAC data plus
a selector byte;
The network prefix, which is 13 bytes of data.
• The registration of end-station devices is carried out by ILMI
using a cold start trap from either the end-station or the
ATM switch.
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8. Network Management
6. Address Registration (cont.)
Operation
• When the ATM interface in the end station is enabled, a cold
start trap is transmitted out along VCI 16.
• The ATM switch receives this start trap and replies with the
prefix associated with that ATM switch.
• The end station then adds its own MAC address and
selector field to the prefix to form a full ATM address.
• This address is sent to the switch where it is registered.
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8. Network Management
6. Address Registration (cont.)
Cold start trap
nIME
uIME
ILMI SNMP set message Network Prefix
ILMI SNMP Response ACK / NACK
ILMI SNMP set message Host Address
(Prefix + MAC + Selector )
ILMI SNMP Response ACK / NACK
SNMP in AAL5 on VCI 16
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8. Network Management
7. Device Check
• uIMEs are declared ‘down’ if they do not respond after four
consecutive polls.
• A uIME is de-registered after the nIME declares that the
uIME is down.
• The uIME is de-registered by removing its address entry
from the nIME address table.
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8. Network Management
7. Device Check (cont.)
uIME
nIME
ILMI SNMP get message connectivity poll
ILMI SNMP response message connectivity ACK
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9. ATM Traffic Descriptors
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9. ATM Traffic Descriptors
1. Traffic Management



Traffic Management is essential for the proper operation of
ATM.
The aim is to ensure that all the different classes of traffic
receive the appropriate handling.
Main features of traffic management:
Traffic Contract
Connection Admission Control
Traffic Shaping
Traffic Policing
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9. ATM Traffic Descriptors
2. Traffic Descriptor Parameters
These are the parameters requested at connection set-up
time:
Peak Cell Rate (PCR)
Sustainable Cell Rate (SCR)
Maximum Burst Size (MBS)
Minimum Cell Rate (MCR)
Cell Delay Variation Tolerance (CDVT)
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9. ATM Traffic Descriptors
3. Required Parameters for each Service
Category
Service
Category
PCR
SCR
MCR
CDVT
CBR
VBR-RT
VBR-NRT
ABR
GFR
UBR
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9. ATM Traffic Descriptors
4. Peak Cell Rate

Peak Cell Rate (PCR) is the absolute maximum rate at
which the network guarantees cell delivery
 A user may send cells at this rate for a short period of
time
 Rate is reduced to maintain an average (SCR)

PCR is used by CBR, VBR and ABR service categories
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9. ATM Traffic Descriptors
5. Sustainable Cell rate



Sustainable Cell Rate (SCR) is the average rate that a
network guarantees cell delivery
Users may burst above the SCR to the PCR (up to a
maximum of BT) as long as they reduce their rate of flow to
maintain this rate
SCR is only used by the VBR QoS category
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9. ATM Traffic Descriptors
6. Burst Tolerance

Burst Tolerance (BT) is the maximum time that the
network will accept cell rates of PCR

BT is only used by the VBR QoS category
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9. ATM Traffic Descriptors
7. Minimum Cell Rate



Minimum Cell Rate (MCR) is the highest rate at which the
network guarantees delivery of cells
A user may attempt to send at higher rates at the risk of
losing cells
This parameter is used to support an ABR service
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8. Cell Delay Variation and Tolerance



Variation in Cell Delay is a fact of life
Delays are caused by :
 Multiplexing
 Queuing
 OAM cell insertion
 Physical Layer overhead
An application may need a guaranteed limit on the degree
of variation, a specified tolerance
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9. Quality of Service Parameters
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9. Quality of Service Parameters
1. Quality of Service Parameters


Negotiable QoS Parameters

Cell Loss Ratio (CLR)

Maximum Cell Transfer Delay (Max CTD)

Peak to peak Cell Delay Variation (peak-to-peak CDV)
Non-negotiable QoS Parameters

Cell Error Ratio (CER)

Severely Errored Cell Block Ratio (SECBR)

Cell Misinsertion Rate (CMR)
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9. Quality of Service Parameters
2. Cell Loss Ratio
Ratio of cells successfully delivered to cells presented per
VPI/VCI
Lost cells
CLR 
Total transmitted cells
Cells in
Cells out
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9. Quality of Service Parameters
3. Maximum Cell Transfer Delay

Cell Transfer Delay (CTD) is the time a cell takes to traverse
the network

CTD is made up of
 Propagation Delay
 Transmission Delay
 Switching Delay
 Queuing Delay

The Maximum Cell Transfer Delay (maxCTD) is the
maximum allowable CTD
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CDT = time
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9. Quality of Service Parameters
4. Peak-to-Peak Cell Delay Variation
Cell Delay Variation (CDV)
CDV is a measure of the difference between actual time of
delivery of a cell and expected time
CDV highlights bursts of cells
typical of LAN-generated traffic
Without delay variation
Cells in
Real
CDV
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9. Quality of Service Parameters
5. MaxCTD, Peak-to-Peak CDV and CLR
Cell Arrival Distribution
Fixed Transit
Delay
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The CLR requested at
connection setup time
actually places a limit
on the value of the
percentage of the cell
arrival probability
distribution lying
outside the maxCTD
arrival times.
Cells delivered late
peak-topeak
CDV maxCTD
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9. Quality of Service Parameters
6. Accumulation of QoS Parameters
CDVs + maxCTDs
CDV
+
maxCTD
CDVs1
+
maxCTD
+
CDVs2
+
maxCTD
+
CDVs3
+
=
P2P-CDV
maxCTD = Total maxCTD
The maxCTD and CDV parameters passed with signalling SETUP
calls are accumulated as the call progresses through the network.
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9. Quality of Service Parameters
7. One Point CDV
Reference
Cell Stream
Actual
Cell Stream
RC0
AC0
RC1
AC1
RC2
AC2 AC3
RC3
RC4
AC4
RC5
AC5
CDV
Cell Delay Variation = Reference arrival time - Actual arrival time
= RCn - ACn
Negative values = gaps in cell stream
Positive values = cell “clumping”
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9. Quality of Service Parameters
8. Non-negotiable QoS Parameters

Cell Error Ratio, where :
Errored cells
CER 
Successful ly transf ered cells  Errored cells

Severely Errored Cell Block Ratio, where :
Severely errored cell blocks
SECBR 
Total transmi tted cell blocks

Cell Misinsertion Rate, where :
Misinserte d cells
CMR 
Time interval
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9. Quality of Service Parameters
9. Factors affecting QoS Parameters
CDV
CTD

Propagation Delay
Media Errors
Switch Design
Buffer Capacity
Traffic Load
Number of Nodes
Network Failures
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






CLR
CER







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CMR
SECBR









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9. Quality of Service Parameters
10. Required Traffic Descriptors and QoS
Parameters
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10. Traffic Control
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1. Connection Admission Control
Included in the process of establishing a virtual connection
(VPI/VCI):

Traffic descriptors are included either in UNI signalling
(SVC) or management setup (PVC)

Network switches check the traffic requirements against
handling ability

Admission of the connection is rejected if the network
cannot guarantee that it will meet the requirements
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1. Connection Admission Control – CAC (cont.)
NNI signalling
Attempt to find a path to B
Able to currently support this request
Inform CAC of result
UNI signalling
I wish to connect to B
with these QoS parameters
CAC
UNI signalling
Call proceeding
B
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2. Virtual Bandwidth

Traffic descriptors such as CDV, PCR may be summarised
as a connection with a specific virtual bandwidth (Vbw)
requirement

Switches along the intended path of the connection check
for Vbw

If a switch does not have sufficient Vbw:
 A new route is selected
 The request is denied
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2. Virtual Bandwidth (cont.)
A separate Virtual Bandwidth algorithm is used for each
Quality of Service Category.
Service Category
Bandwidth Allocated
CBR
VBR-NRT
VBR-RT
ABR
UBR
GFR
PCR <= Vbw <= Link rate
SCR <= Vbw <= PCR
SCR <= Vbw <= PCR
Vbw = MCR
Under Review
Vbw = MCR
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3. Traffic Shaping



Traffic shaping is the name given to any technique at the
user site to ensure that outgoing cells conform to the
traffic contract
This makes the customer premises equipment (CPE) a
well-behaved user
Throttling back to agreed rates can increase throughput as
the need to retransmit network discarded cells is removed
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4. Traffic Policing



A traffic contract exists across the UNI when a call’s
descriptors are accepted
Not all user devices will be well-behaved
 Traffic policing is necessary to ensure that badlybehaved devices do not interfere with other users
Cells outside the limits of the contract will:
 Be discarded
 Have CLP set for discard at busy switches
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5. Generic Cell Rate Algorithm

Policing traffic is performed by applying the Generic Cell
Rate Algorithm (GCRA)
 GCRA is a continuous state ‘leaky bucket’ algorithm

It checks that cell streams conform to PCR, CDVT, SCR and
BT
 PCR & SCR require separate instances of the leaky
bucket, hence switches employ a ‘dual state leaky
bucket’
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6. Leaky Bucket Algorithm
cells
Peak Cell Rate
Bucket
Tolerance factor to account for
jitter - caller the Cell Delay Variation
Tolerance (CDVT), measured in ms
Cells which overflow will be dropped
or have their CLP set to 1
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The End of Part 2
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