Transcript 沒有投影片標題 - National Tsing Hua University

Asychronous Transfer Mode (ATM)
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ATM is originally the transfer mode for implementing
Broadband ISDN (B-ISDN).
In 1988, CCITT (from ITU) issued the first two
recommendations relating to B-ISDN:
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I.113 Vocabulary of Terms for Broadband Aspects of ISDN
I.121 Broadband Aspects of ISDN
In 1911, the ATM Forum was created with the goal of
accelerating the development of ATM standards.
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Broadband Services
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Information Characteristics
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Voice:
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Data:
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1Kbps ~ 10 Mbps, loss  10-9 , Bursty or Stream
HDTV:
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8Kbps ~ 64Kbps, round trip delay  8ms ,loss  10-6
140Mbps, loss  10-6, Variable Bit Rate, delay  5ms, Distributive
Graphics / Image:
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10Mbps (10Mb/picture, 1 sec), Bursty, Delay  5s (interactive)
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ATM Characteristics
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No error protection or flow control on a link-bylink basis.
ATM operates in a connection-oriented mode.
The header functionality is reduced.
The information field length is relatively small
and fixed.
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Physical Layer Interface Specification
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SONET STS-3C
SONET STS-12
DS3
STP for ATM LAN
etc.
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Physical Layer Functions for SONET STS-3C
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Physical Layer Functions for DS3
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DLL in ATM
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Transmission convergence sublayer of the ATM physical
layer.
Header error control: correcting single-bit error of a cell
header.
Cell delineation: cell framing.
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Header Error Control
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HEC checksum: Remainder of the 32 header bits divided
by the polynomial x**8+x**2+x+1, added with 01010101
to provide robustness in face of headers containing mostly
0 bits.
Correcting all single-bit errors and detecting many multibit errors.
Suitable for optical fiber: 99.64% of all errors are singlebit errors.
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Cell Delineation
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Using the HEC to delineate a cell.
Cell delineation heuristic: see Fig. 3-30.
To protect against malicious users, payload bits
are scrambled on transmission and descrambled on
reception.
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ATM Layer in ATM networks
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Connection-oriented.
No acknowledgements.
Cells arriving destinations in order.
Tow-level connection hierarchy.
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Advantages for the Use of Virtual Paths
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Simplified network architecture.
Increased network performance and reliability.
Reduced processing and short connection setup
time.
Enhanced network services.
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ATM Layer Services
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The ATM layer provides for the transparent transfer of
cells between communicating upper layer entities.
This transfer occurs on a pre-established ATM connection
according to a traffic contract.
A traffic contracts is comprised of a QoS class, a vector of
traffic parameters, a conformance definition etc.
Each ATM end-point is expected to generate traffic which
conforms to these parameters.
Two service primitives: ATM-DATA.Request and ATMDATA.Indicate.
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ATM Cell Format
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ATM Cell Header
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Payload Type
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Connection Setup/Release
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ATM Routing
To route on the VPI field except at the final hop.
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ATM Network-Network Interface
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Private Network-Network Interface (PNNI)
Specification version 1.0, defined in March 1996
Function of PNNI routing protocol
Discovery of neighbors and link status
 Synchronization of topology databases
 Flooding of topology state elements
 Summarization of topology state information
 Construction of the routing hierarchy
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ATM Traffic Management
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Defined in Traffic Management Specification Version 4.0,
April 1996
Five services categories:
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CBR, rt-VBR, nrt-VBR, UBR, ABR
Generic functions:
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Connection Admission Control
Feedback Controls
Usage Parameter Control
Cell Loss Priority Control
Traffic Shaping
Network Resource Management
Frame Discard
ABR Flow Control
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Quality of Service Categories
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QoS Parameters: CLR, CTD, CDV
 CBR: CDV, CTD, CLR
 rt-VBR: CDV, CTD, CLR
 nrt-VBR: CLR
Traffic Parameters:
 CBR: PCR, CDVT
 rt-VBR: PCR, CDVT, SCR, MBS
 nrt-VBR: PCR, CDVT, SCR, MBS
UBR: PCR, CDVT
 ABR: PCR, CDVT, MCR
where MBS is the maximum burst size.

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Quality of Service Parameters
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The following QoS parameters are negotiated
between the end-systems and the network
Peak-to-Peak Cell Delay Variation(peak-to-peak CDV)
 Maximum Cell Transfer Delay(maxCTD)
 Cell Lose Ratio(CLR)
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The following QoS parameters are not negotiated
Cell Error Ratio(CER)
 Severely Errored Cell Block Ratio(SECBR)
 Cell Misinsertion Rate(CMR)
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Traffic parameters
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A traffic parameter describes an inherent characteristic of
a traffic source
Traffic parameters include
Peak Cell Rate(PCR)
 Sustainable Cell Rate(SCR)
 Maximum Burst Size(MBS)
 Minimum Cell Rate(MCR)
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Traffic Contract
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A traffic contract specifies the negotiated characteristics of
a connection.
Traffic contract specification consists of
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A connection traffic descriptor.
A set of QoS parameters for each direction of the connection.
The definition of a compliant connection.
Connection traffic descriptor includes
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A set of traffic parameter of the ATM source.
The CDVT.
The conformance definition that specifies the conforming cells of
the connection.
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The Generic Cell
Rate Algorithm
(GCRA)
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GCRA (I, L)
Where I: Increment
L: Limit
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Traffic Contract Conformance Definition
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Traffic Management specification,Version 4.0
The ATM Forum, April 1996
CBR Service:
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VBR Services
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GCRA (1/PCR, CDVT)
GCRA (1/PCR, CDVT)
GCRA (1/SCR, BT+CDVT), where BT = (MBS-1)(1/SCR-1/PCR)
UBR Services
 PCR
ABR
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DGCRA
MCR
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, PCR 1 , ICR 1 , 1 , 2 , 3 
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Usage Parameter Control
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Usage Parameter Control(UPC) is defined as the set of actions
taken by the network to monitor and control traffic
The monitoring task for UPC is performed for VCCs and VPCs
respectively by the following two actions:
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Checking the validity of VPI and VCI and monitoring the traffic entering
the network in order to ensure that parameters agreed upon are not
violated.
Checking the validity of VPI and monitoring the traffic entering the
network in order to ensure that parameters agreed upon are not violated.
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Congestion Control
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Admission Control with resource reservation.
Rate-based Congestion Control for ABR traffic.
The sender has a current cell rate ACR (Allowed Cell Rate), MCRACRPCR.
ACR is reduced, if congestion occurs.
For each RM-cell, ER (Explicit Rate) is set by the source to a requested rate
(such as PCR) and may be subsequently reduced by any network element in
the path to a value that the element can sustain. ER is then used to limit the
source ACR to a specific value.
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ABR Flow Control
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In ABR service, the source adapts its rate to changing
network conditions.
Information about the state of the network like bandwidth
availability, state of congestion, and impending congestion, is
conveyed to the source through special control cells called
Resource Management Cells (RM-cells).
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ABR Service Parameters
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ABR Service Parameters(cont.)
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RM Cell
Structure
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Message Type Field
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In-rate and Out-of-rate Cell Types
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For ABR,CLP=0 cells are called “in-rate” cells.
For ABR ,CLP=1 cells are called “out-of-rate” cells.
One use of out-of-rate RM-cells is to enable a rate increase
for a connection that has an ACR of zero.
The source would use the out-of-rate cells as probes to
learn when it may increase its rate.
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Source Behavior
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The following items define the source behavior for
CLP=0 and CLP=1 cell streams of a connection.
By convention, the CLP=0 stream is referred to as inrate, and the CLP=1 stream is referred to as out-ofrate.Data cells shall not be sent with CLP=1.
1 .The value of ACR shall never exceed PCR, nor shall
it ever be less than MCR. The source shall never send
in-rate cells at a rate exceeding ACR. The source may
always send in-rate cells at a rate less than or equal to
ACR.
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Source Behavior(cont.)
2. Before a source sends the first cell after connection setup, it
shall set ACR to at most ICR. The firstin-rate cell sent shall
be a forward RM-cell.
3. After the first in-rate forward RM-cell, in-rate cells shall be
sent in the following order:
a) The next in-rate cell shall be a forward RM-cell if and only if, since
the last in-rate forward RM-cell was sent, either:
i) at least Mrm in-rate cells have been sent and at least Trm time
has elapsed,
or
ii) Nrm-1 in-rate cells have been sent.
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Source Behavior(cont.)
b) The next in-rate cell shall be a backward RM-cell if condition
(a) above is not met, if a backward RM-cell is waiting for
transmission, and if either:
i) no in-rate backward RM-cell has been sent since the last
in-rate forward RM-cell,
or
ii) no data cell is waiting for transmission.
c) The next in-rate cell sent shall be a data cell if neither
condition (a) nor condition (b) above ismet, and if a data cell
is waiting for transmission.
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Source Behavior(cont.)
4. Cells sent in accordance with source behaviors #1, #2, and
#3 shall have CLP=0.
5. Before sending a forward in-rate RM-cell, if ACR > ICR and
the time T that has elapsed since the last in-rate forward
RM-cell was sent is greater than ADTF, then ACR shall be
reduced to ICR.
6. Before sending an in-rate forward RM-cell, and after
following behavior #5 above, if at least CRMin-rate forward
RM-cells have been sent since the last backward RM-cell
with BN=0 was received, then ACR shall be reduced by at
least ACR*CDF, unless that reduction would result in a rate
below MCR, in which case ACR shall be set to MCR.
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Source Behavior(cont.)
7. After following behaviors #5 and #6 above, the ACR value
shall be placed in the CCR field of the outgoing forward
RM-cell, but only in-rate cells sent after the outgoing
forward RM-cell need to follow the new rate.
8. When a backward RM-cell (in-rate or out-of-rate) is
received with CI=1, then ACR shall be reduced by at least
ACR*RDF, unless that reduction would result in a rate
below MCR, in which case ACR shall be set to MCR. If the
backward RM-cell has both CI=0 and NI=0, then the ACR
may be increased by no more than RIF*PCR, to a rate not
greater than PCR. If the backward RM-cell has NI=1, the
ACR shall not be
increased.
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Source Behavior(cont.)
9. When a backward RM-cell (in-rate or out-of-rate) is
received, and after ACR is adjusted according to source
behavior #8, ACR is set to at most the minimum of ACR
as computed in source behavior #8,and the ER field, but
no lower than MCR.
10. When generating a forward RM-cell, the source shall
assign values to the various RM-cell fields as specified
for source-generated cells in Table 5-4.
11. Forward RM-cells may be sent out-of-rate (i.e., not
conforming to the current ACR). Out-of-rate forward RMcells shall not be sent at a rate greater than TCR.
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Source Behavior(cont.)
12. A source shall reset EFCI on every data cell it sends.
13. The source may implement a use-it-or-lose it policy
to reduce its ACR to a value which approximates the
actual cell transmission rate. Use-it-or-lose-it policies
are discussed in Appendix I.8.
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Destination Behavior
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The following items define the destination behavior for
CLP=0 and CLP=1 cell streams of a connection.By
convention, the CLP=0 stream is referred to as in-rate,
and the CLP=1 stream is referred to as out-of-rate.
1. When a data cell is received, its EFCI indicator is saved
as the EFCI state of the connection.
2. On receiving a forward RM-cell, the destination shall
turn around the cell to return to the source. The DIR bit in
the RM-cell shall be changed from “forward”
to ”backward”, BN shall be set to zero, and CCR, MCR,
ER, CI, and NI fields in the RM-cell shall be unchanged
except:
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Destination Behavior(cont.)
a) If the saved EFCI state is set, then the destination shall set CI=1
in the RM-cell, and the saved EFCI state shall be reset. It is
preferred that this step is performed as close to the transmission
time as possible;
b) The destination (having internal congestion) may reduce ER to
whatever rate it can support and/or set CI=1 or NI=1. A
destination shall either set the QL and SN fields to zero, preserve
these fields, or set them in accordance with ITU-T
Recommendation I.371-draft.
The octets defined in Table 5-4 as reserved may be set to 6A
(hexadecimal) or left unchanged.The bits defined as reserved in
Table 5-4 for octet 7 may be set to zero or left unchanged. There
maining fields
shall be set in accordance with Section 5.10.3.1
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(Note that this does not preclude looping fields back from the
Destination Behavior(cont.)
3. If a forward RM-cell is received by the destination while
another turned-around RM-cell (on the same connection)
is scheduled for in-rate transmission:
a) It is recommended that the contents of the old cell are overwritten
by the contents of the new cell;
b) It is recommended that the old cell (after possibly having been
over-written) shall be sent out-of-rate; alternatively the old cell may
be discarded or remain scheduled for in-rate transmission;
c) It is required that the new cell be scheduled for in-rate
transmission.
4. Regardless of the alternatives chosen in destination
behavior #3 above, the contents of an older cell shall not
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be transmitted(C)after
the
contents
of a newer
cell have been
Destination Behavior(cont.)
5. A destination may generate a backward RM-cell without
having received a forward RM-cell. The rate of these
backward RM-cells (including both in-rate and out-of-rate)
shall be limited to 10 cells/second, per connection. When
a destination generates an RM-cell it shall set either CI=1
or NI=1, shall set BN=1, and shall set the direction to
backward. The destination shall assign values to the
various RM-cell fields as specified for destination
generated cells in Table 5-4.
6. When a forward RM-cell with CLP=1 is turned around it
may be sent in-rate (with CLP=0) or out-of-rate (with
CLP=1).
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Switch Behavior
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The following items define the switch behavior for
CLP=0 and CLP=1 cell streams of a connection. By
convention, the CLP=0 stream is referred to as in-rate,
and the CLP=1 stream is referred to as out-of-rate.Data
cells shall not be sent with CLP=1.
1. A switch shall implement at least one of the following
methods to control congestion at queuing points:
a) EFCI marking: The switch may set the EFCI state in the data
cell headers;
b) Relative Rate Marking: The switch may set CI=1 or NI=1 in
forward and/or backward RM-cells;
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Switch Behavior(cont.)
c) Explicit Rate Marking: The switch may reduce the ER field of
forward and/or backward RM-cells (Explicit Rate Marking) ;
d) VS/VD Control: The switch may segment the ABR control loop
using a virtual source and destination.
2. A switch may generate a backward RM-cell. The rate of
these backward RM-cells (including both in-rate and outof-rate) shall be limited to 10 cells/second, per
connection. When a switch generates an RM-cell it shall
set either CI=1 or NI=1, shall set BN=1, and shall set the
direction to backward. The switch shall assign values to
the various RM-cell fields as specified for switchgenerated cells in Table 5-4.
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Switch Behavior(cont.)
3. RM-cells may be transmitted out of sequence with
respect to data cells. Sequence integrity within the
RM-cell stream must be maintained.
4. For RM-cells that transit a switch (i.e., are received
and then forwarded), the values of the various fields
before the CRC-10 shall be unchanged except:
a) CI, NI, and ER may be modified as noted in #1 above
b) RA, QL, and SN may be set in accordance with ITU-T
Recommendation I.371-draft
c) MCR may be corrected to the connection MCR if the
incoming MCR value is incorrect.
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Switch Behavior(cont.)
5. The switch may implement a use-it-or-lose-it policy to
reduce an ACR to a value which approximates the actual
cell transmission rate from the source. Use-it-or-lose-it
policies are discussed in Appendix I.8.
Notes:
1. A switch queuing point is a point of resource contention
where cells may be potentially delayed or lost. A switch
may contain multiple queuing points.
2. Some example switch mechanisms are presented in
Appendix I.5.
3. The implications of combinations of the above methods is
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beyond the scope
of this specification.
Virtual Source and Virtual
Destination Behavior
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VS/VD Behavior
1. Each ABR control segment, except the first, is
sourced by a virtual source. A virtual source assumes
the behavior of an ABR source end point. Backward
RM-cells received by a virtual source are removed
from the connection.
2. Each ABR control segment, except the last, is
terminated by a virtual destination. A virtual
destination assumes the behavior of an ABR
destination end point. Forward RM-cells received by
a virtual destination shall be turned around as defined
in destination behavior #2, and shall not be forwarded
to the next segment
of the connection.
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VS/VD Behavior(cont.)
3. The coupling between two adjacent ABR control
segments associated with an ABR connection is
implementation specific.
4. MCR shall be conveyed across VS/VD boundaries.
5. Setting of other parameters at VS/VD is network
specific.
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ATM Adaptation Layer
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AAL1,2,3,4 for class A, B,C, and D respectively.
AAL3 and AAL4 are combined as AAL3/4.
AAL5, originally called SEAL (Simple Efficient Adaptation Layer), is adopted
by the ATM Forum to replace AAL3/4.
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
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The message is transmitted by passing it to the SAR sublayer, which does not
add any headers or trailer. It breaks the message into 48-byte units and passes
to the ATM layer for transmission.
The Internet is expected to transport IP packets over ATM networks with the
AAL5 payload field. RFC 1483 and 1577.
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ATM in LAN
Support multiple,guaranteed classes of service.
 Provide scalable throughput.
 Facilitate the interworking between LAN and WAN
technology.
ATM LAN emulation by ATM Forum
 LAN Emulation over ATM Specification,Version 1.0 1995

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ATM in Internet
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Multiple Protocol Over ATM,Version 1,July 1997
For efficient transfer of inter-subnet unicast data in a LAN
emulation LANE environment.
MPOA integrates LANE and NHRP(Next Hop Resolution
Protocol)to allow inter-subnet,internetwork layer protocol
communication over ATM VCCs without requiring routers in
the data path.
NHRP is currently an Internet-Draft to determine the
internetworking layer address and Non-Broadcast,MnHi-Access
(NBMA)subnetwork addresses of the “NBMA next hop”
towards a destination station
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