Download ATM Layer Functions

B-ISDN Protocol Reference Model
Management Plan
User Plane Control Plane
Applications
Signaling
Protocol Native TCP/IP
AAL
ATM Layer
Physical Layer
SNMP: Simple
Network
Management
Protocol
CMIP:
Common
Management
Information
Protocol
 Control Plane
 Supports Signaling
 Call Setup, Call Control, Connection Control
 User Plane
 Data Transfer, Flow Control, Error Recovery
 Management Plane
 Operation, Administration, & Maintenance
Management Plane
(Provides control of ATM switch)
Layer Management
(Layered)
– Use to manage each of the ATM
layers with entity corresponding to
each ATM layer
– OAM issues
Plane Management
(No Layered)
– Concerned with management of all
the planes
– All management functions (Fault,
Performance, Configuration,
Operation, & Security) which
relates to the whole system are
located in the Plane Management
– Provides coordination between all
planes
Broadband Networking with
SONET and ATM
Video
Image
Data
etc…
USER
USER
ATM SW
ATM SW
NNI
UNI
USER
USER
UNI
Higher Layers
Higher Layers
•Flow Control
•Error Handling
•Message Segmentation
Convergence
Sublayers (CS)
Convergence
Sublayers (CS)
•Segmentation Type
•Message Number
•Message ID
Segmentation
Reassembly
Sublayer (SAR)
•5 Byte Header
•48 Byte Payload
•Handles cont. and
bursty traffic
•SONET
Adaptation
Layer
Segmentation
Reassembly
Sublayer (SAR)
ATM Layer
ATM Layer
ATM Layer
ATM Layer
Physical Layer
Physical Layer
Physical Layer
Physical Layer
USER
USER
ATM Protocol Reference Model in the User Plane
Abbreviations
Upper Layers
class A
1
class B
2
class C
3
class D
4
CS
•Handling lost / misdelivered cells
•Timing recovery
•Interleaving
SAR
•Split frames / bit stream info cells
•Re-assemble frames / bit stream
AAL
•Cell routing
•Multiplexing / demultiplexing
•Generic flow control
ATM
TC
PL
PM
•Cell header verification and cell
delineation
•Rate decoupling (insert idle cells)
•Transmission frame adaptation
•Bit timing
•Physical medium
Remark: See next page
Cell
Information
Field
AAL = ATM Adaptation Layer
SAR = Segmentation and Reassembly
CS = Convergence Sublayer
PL = Physical Layer
TC = Transmission Convergence
PM = Physical Medium
Service Classes for AAL
Class
Cell
Header
A
B
C
D
Type
Constant Bit Rate
Variable Bit Rate
Connection Oriented Data
Connectionless Data
• SEAL = Simple and Efficient Adaptation Layer
•Type 5 AAL
•Acknowledged info transfer
Remarks: PMD
TC
Physical Medium Dependent
Transmission Convergence
Sublayer
It separates transmission from the physical
interface and allows ATM interfaces to be built
on a large variety of physical interfaces
Physical Layer Functions
a) Physical Medium (PM)
– PM sublayer provides the bit transmission
capability including bit alignment
– Line coding and, if necessary, electrical/optical
conversion is performed in this sublayer
– Optical fiber is used for the physical medium.
Other media, coax cables are also possible
– Bit rates  155 Mbps or 622.080 Mbps.
Physical Layer Functions
b) Bit Timing
– Generation and reception of waveforms which
are suitable for the medium, the insertion, and
extraction of bit timing information and the line
coding if required
– CMI (Code Mark Inversion) (CCITT G.703)
proposed for 155.520 Mbps interface.
– NRZ “Nonreturn to Zero” code proposed for
optical interface.
Line Coding
Electrical Interface: Coded Mark Inversion (CMI)
– For binary 0  always a positive transition at the
midpoint of the binary unit time interval.
– For binary 1  always a constant signal level for the
duration of the bit time. This level alternates between
high and low for successive binary 1s.
0
Level A2
Level A1
1
0
0
1
1
0
0
0
1
1
Line Coding (cont.)
Optical Interface: Nonreturn to Zero (NRZ)
– For binary 0  Emission of light
– For binary 1  No emission of light
– Transition: 0  1 or 1  0
Otherwise no transition
0
Level A2
Level A1
1
0
0
1
1
0
0
0
1
1
ATM INTERFACES
• SONET/SDH : 155 Mbps and 622 Mbps over OC-3 (single mode fiber)
• Cell Based
• PDH Based (ATM cells mapped into PDH signals)
(59 columns and 9 rows
frame). Frame at 34.368 Mbps.
• FDDI based or 100 Mbps (same as in FDDI PMD uses multimode fiber and
line coding of 4B/5B). (called TAXI interface). Early private UNI interfaces
were based on TAXI interfaces.
• DS-3 (45 Mbps) Transfer of ATM cells on T3 (DS-3) public carrier interface.
It is cheaper than SONET links.
• STS-3 (155 Mbps) over Multimode fiber uses line coding of 8B/10B.
• STS-3 (155 Mbps) over Twisted Pair (using Taxi interface) uses line coding
of 8B/10B.
• D1-T1 carriers (1.5 Mbps)
CELL BASED INTERFACE
•This interface consists of a continuous stream of cells where
each cell contains 53 octets.
26
0
1
26
0
1
Physical layer OAM cell
•Synchronization achieved through HEC basis.
•Maximum spacing between successive physical layer cells is
26 ATM layer cells.
•After 26 consecutive ATM layer cells, a physical layer cell (idle
cells or OAM cells) is enforced to adapt transfer capability to
the interface rate.
Transmission Convergence Sublayer (TC)
1) Transmission Frame Adaptation
•Adapts the cell flow according to the used
payload structure of the transmission system in
the sending direction.
•In the opposite direction, it extracts the cell flow
out of the transmission frame.
2. Header Error Control (HEC)
Multiple-bit errror (Cell discarded)
No
Error
Correction Mode
No error
Detection Mode
Error detected
Cell discarded
Correction
Single-bit error
• After initialization receiver is in the “Correction Mode”
• Single bit error detected  corrected
• Multiple bit error detected  cell discarded
• Receiver switches to “Detection Mode”
• In “Detection Mode”, each cell with a detected single-bit
error is discarded.
• If a correct header is found, receiver switches to
“Correction Mode”
Example:
p
Probability that a bit is in error
(1-p)
Probability that a bit is NOT in error
p40
Probability that 40 bits are in error
(1-p)40
Probability that 40 bits are correct
a) With what probability a cell is rejected when the HEC state
machine is in the "Correction Mode"?
Correction Mode
Probability of a cell being rejected
Different Perspective:
When is a cell accepted?
* Probability of having no errors in cell header
OR
* Probability of having a single bit error in cell header
b) With what probability a cell is rejected when the HEC state
machine is in the "Detection Mode"?
Detection Mode
HEC will only accept ERROR-FREE cells.
Different Perspective:
What is the probability that a cell header is correct?
c) Assume that the HEC state machine is in the
“Correction Mode.” What is the probability that n
successive cells will be rejected, where n >= 1 ?
Correction Mode
Probability of n successive cells being accepted (n>1)
n=1:
Probability that 1 cell is accepted, i.e., the entire header
is error-free.
What is that probability?
OR
There is at most one bit error in the header.
What is that probability?
2
1
n=2:
Probability that the cell header (2) is correct AND
Previous case for cell 1
OR
Probability that the cell header (2) has at most
1 bit error AND
Probability that the cell header (1) is correct
(error free)
3
2
1
n=3:
Probability that the cell header (3) is correct AND
Previous case for cell n=1
OR
Probability that the cell header (3) has at most
1 bit error AND
Probability that the cell header (2) is correct AND
The case for n=1
d) Assume that the HEC state machine is in the
“Correction Mode.” What is the probability p(n) that n
successive cells will be accepted, where n >= 1 ?
First cell is rejected:
What is the probability that a cell is rejected?
 Case a)
Different Perspective:
 Probability that all header bits of a cell are correct
 Probability that one single bit error in a cell header
Remaining n-1 successive cells:
Now, HEC is in Detection Mode
What is the probability that (n-1) successive
cells are rejected, i.e., there will be errors in the
headers for the remaining (n-1) cells
EFFECT OF ERROR IN CELL HEADER
Incoming Cell
Error in
Header?
No
Valid cell
(intended service)
Yes
Error
detected
No
Yes
Current
mode?
Detection
Apparently valid cell
With errored header
(unintended service)
Discarded Cell
Correction
Error
incorrectable?
Yes
No
Correction
attempt
Unsuccessful
Successful
HEC Generation Algorithm (I.432)
• Every ATM cell transmitter calculates the HEC value across the first 4
octets of the cell header and inserts the result in the fifth octet (HEC
field) of the cell header.
• The HEC value is defined as “the remainder of the division (modulo
2) by the generator polynomial x8+x2+x+1 of the product x8 multiplied
by the content of the header excluding the HEC field to which the
fixed pattern 01010101 will be added modulo 2.”
• The receiver must subtract first the coset value of the 8 HEC bits
before calculating the syndrome of the header.
• Device always preset to 0s.
[Key Word: CRC (Cyclic Redundancy Check Algorithm)]
ATM CELL STRUCTURE
8
7
6
5 4 3 2
1
1 Octet
2
3
4
5
:
:
:
53
HEADER
(5 octets)
PAYLOAD
(48 octets)
8
1
2
3
4
5
7
6
5 4 3 2
GFC
VPI
VPI
VCI
VCI
VCI
PT
HEC
:
:
53
1
PAYLOAD
(48 octets)
PR
HEC Generation Algorithm
• The HEC field contains the 8-bit FCS (Frame
Check Sequence) obtained by dividing the first 4
octets (32 bits) of the cell header multiplied by 2^8
by the CRC code (generator polynomial)
(x8+x2+x+1)
HEC Generation Algorithm (I.432)
• This HEC code can
1) Correct single bit errors
2) Detect multiple bit errors
Purpose:
•Protects the header control information
•Helps to find a valid cell (cell delineation and boundaries)
CELL DELINEATION
(This process allows identification of cell boundaries)
Correct HEC
Bit-by-Bit
Cell-by-Cell
HUNT
PRESYNC
Incorrect HEC
 consecutive
incorrect HEC
 consecutive
correct HEC
SYNCH
Cell Delineation (cont.)
• In Hunt State  a cell delineation algorithm is performed
bit-by-bit to determine if the HEC coding law is observed
(i.e., match between received HEC and calculated HEC).
• Once a match is achieved, it is assumed that one header has
been found and the method enters the PRESYNCH state.
• The HEC algorithm is performed cell-by-cell. If 
consecutive correct HECs are found, SYNCH state is
entered; if not the system goes back to HUNT state.
• SYNCH is only left (to HUNT) state if  consecutive
incorrect HECs are identified.
Cell Delineation (cont.)
•  and  are design parameters that influence the
performance of cell delineation process.
(=7 and =6).
• Greater values of  result in longer delays in
recognizing a misalignment but in a greater
robustness against false alignment.
• Greater values of  result in longer delays in
establishing synchronization but in greater
robustness against false delineation.
Cell Delineation (cont.)
Remarks:
•
A 155.520 Mbps ATM system will be in SYNCH state for more than 5349
years even when the bit error probability is BER=10-4.
•
This method may fail if the header HEC occurs in the info field
(maliciously or accidentally)  Cell Payload Scrambling.
•
To overcome  the info field contents scrambled using a selfsynchronizing scrambler with polynomial X43 + 1. Header itself is not
scrambled.
The probability of 7 consecutive incorrect HEC with
BER=10-4
A= The probability that 7 consecutive cells are in error.
[1- (1-10-4)40 ]7 =
1.616*10-17 = A
1/A  The number of cells sent in order to have a 7
consecutive error cells; (Unit Cells);
How often does event A occur in terms
of ATM cells.
{53 * 8} / {155.52 Mbps} = C
(53*8) = # of bits/cell ; Link Speed = # of bits/sec
How long does it take to send one ATM cell through
the 155 Mbps link.
{[1 / { A}] * C =
{6.187*106} * 53 * 8} / {155.52 Mbps} =
1.6868*1011 = 5349 Years
End Result  in terms of seconds
End result/(365*24*60*60)  approx. 5349 years..
Cell Rate Decoupling
(Speed Matching)
• Adapts cell stream into Transmission Bit Rate
(Insertion / Discarding idle cells; in particular for
SONET Interface). SONET uses synchronous cell
time slots!
Note: Cell Based Interface  No need for this
function.
Cell Rate Decoupling (cont.)
(Speed Matching)
ATM Transmitter
ATM Receiver
VPI/VCI
VPI/VCI
B
u
f
f
e
r
+
-
VPI/VCI
Insert Idle or Unassigned cells
Remove the Idle or Unassigned cells
Transmitter multiplexes multiple streams; queueing them if an
ATM cell is not immediately available. If the queue is empty,
when the time arrives to fill the next synchronous cell time slot,
then the Transmission Convergence Sublayer inserts an Idle cell
(or the ATM layer inserts an Unassigned cell.)
ATM Layer Functions
•
Cell Multiplexing/Demultiplexing
• Cell VPI/VCI Translation
• Cell Header Generation/Extraction
• GFC Function
ATM Layer Functions (Cont’d)
•
Cell Multiplexing/Demultiplexing
 In the transmit direction, cells from individual VPs
and VCs are multiplexed into one resulting stream.
 At the receiving side  the cell demultiplexing
function splits the arriving cell stream into the individual
cell flows appropriate to the VP or VC.
ATM Layer Functions (cont.)
ii) Cell VPI/VCI Translation
- At ATM switching nodes, the VPI and VCI translation
must be performed.
- Within VP switch, the value of the VPI field of each
incoming cell is translated into a new VPI value for
the outgoing cell.
- At a VC switch, the values of the VPI as well as the
VCI are translated into new values.
ATM Layer Functions (cont.)
iii) Cell Header Generation / Extraction
- This function is applied at the termination points of the ATM
layer.
- Transmit Side: After receiving the cell information from the
AAL, the cell header generation adds the appropriate ATM
cell header except for the HEC values. HEC is done at
Physical Layer. VPI/VCI values could be obtained by a
translation from the SAP identifier.
- Receive Side: The cell header extraction function removes
the cell header. Only the cell information is passed to the AAL.
- This function could also translate a VPI/VCI value into a
SAP identifier.
ATM Layer Functions (cont.)
iv) GFC functions
- Supports the control of the ATM traffic flow
in a UNI. It can be used to alleviate short
overload conditions.
- Control of cell flows toward the network
but not flow control from the network.
- No effect within the network.
Virtual Path and Virtual Circuit Concept
•ATM cells flow along entities known as VIRTUAL CHANNELS. A VC
is identified by its virtual circuit identifier (VCI).
VC
set up between 2 end-users (like VC in X.25 => Indiv. Log
connection).
VP
Bundle of VCs
having the same end points (Group
logical connection; reserved trunk of connections).
•All cells in a given VC follow the same route across the network and
are delivered in the order they were transmitted.
•VCs are transported within Virtual Paths (VPs). A VP is identified
by its virtual path identifier (VPI). VPs are used for aggregating VCs
together or for providing an unstructured data pipe.
Virtual Path and Virtual Circuit Concept
• Optical links will be capable of transporting hundreds of
Mbps where VCs fill kbps. Thus, a large number of
simultaneous channels have to be supported in a
transmission link. Typically 10K simultaneous channels
are considered (thus, VCI field up to 16bits).
• Since ATM is connection oriented, each connection is
characterized by a VCI which is assigned at Call-Set-Up.
• When connection is released, VCI values on the involved
links will be released or can be reused by other
components.
VIRTUAL PATH / VIRTUAL CIRCUIT CONCEPT
VP
TRANSMISSION PATH
VC
Virtual Path
Text
VCI =1 (text)
Voice
VCI =2 (voice)
Video
VCI =3 (video)
ATM Network
Interface
VIRTUAL PATH/VIRTUAL CIRCUIT CONCEPT
• Each VP has a different VPI value and each VC within a VP
has a different value.
• Two VCs belonging to different VPs at the same interface
may have identical VCI values.
• VPI is changed at points where a VP link is terminated.
• VCI is changed at points where a VC link is terminated.
Goal  Multimedia Communication
 Video & Voice  Time Sensitive (Delay bounds)
 Data  Loss Sensitive (Loss bounds)
 Allows the network to add or remove
components during the connection
e.g. Video Telephony  Start with voice (only single VC)
 Add video later (on another VC)
 Add data (on another VC)
 Signaling (on another VC)
EXAMPLE
• Three VP connections exist from A to B. They are seen by A as corresponding to the
values p, q, r of the VPI field, and by B as corresponding to the values p2, q2, r2.
Whenever A wants to send some information to B on the VP connection seen as p, it writes
the value p in the VPI field of the cell.
• The VP switches T1, T2 and T3 swap the VPI labels according to the lookup tables. The
VCI field is not changed by the VP switches, so it can be used by A to multiplex several
VC connections on any one of the three VP connections. Therefore, at the VC level, A has
at its disposal three direct links to B.
VP Level
A
B
p
VC Level
A
p2
p
B
p2
p1
T1
T2
q
q
r
T3
q2
r2
r
q2
r2
SWITCHING OF VCs and VPs
• Routing functions for VPs are performed at a VP switch.
• This routing involves translation of the VPI values of the
incoming VP links to the VPI values of the outgoing VP links.
VCI values remain unchanged.
• VC switches terminate both VC links and necessarily VP links.
• VPI and VCI translation is performed.
VCI 23
VCI 21
VPI1
VPI4
VCI 22
VCI 24
VCI 23
VCI 25
VPI2
VPI5
VCI 24
VCI 24
VCI 25
VCI 21
VPI3
VPI6
VCI 24
VP Switch/Cross Connect
VP Switching
VCI 22
VP and VC SWITCHING
VCI 25
VCI 25
VPI 4
VCI 21
VPI 5
VCI 21
VCI 23
VCI 23
VPI 2
VCI 24
VCI 24
VC Switch/Cross Connect
MORE ABOUT VCs and VPs
A VP Connection:
• Contains multiple VC connections.
• VC connections may be built up of multiple VP connections.
• Use of VPI simplifies routing table lookup.
Virtual Channel Connection
Virtual Path Connection x
Virtual Path Connection y
VCI = a1
A
D1
VPI=x1
VCI = a2
T
D2
VPI=x2
VPI=x3
D3
VPI=y1
B
D4
VPI=y2
VPI=y3
Virtual Channel View
B
T
A
VCI=a1
Other VCI
VCI=a1
VCI=a2
Other VCI Other VCI
VCI=a2
Other VCI
VCs and VPs (Cont.)
• The inter-networking of the VP and VC switches is illustrated in Figure.
• There exist VP connections (x and y) between A and T; T and B.
• Assume now that A wants to setup a VC connection to B using those two
VP connections.
• The network has to provide a VCI value, say a1, for the A to T link, and
a VCI value, say a2, for the T to B link.
• The VC connection from A to B is thus made of two VC links only.
• At switching points D1 through D4, only the VPI field is swapped.
• At the switching point T, both VPI and VCI fields are swapped.
• The situation is thus similar to that where A and B would be access
nodes in a circuit switched network, T would be a transit node, and D1
through D4 would be cross-connects.
Example for VCIs and VPIs
• A VP is established between Subscriber A and Subscriber C
transporting 2 individual connections, each with a separate VCI.
• Remark: The VCI values used (1,2,3 and 3,4 in the example) are NOT
translated in the switches, which are only switching on the VPI field.
A
VPI=8
VCI=3,4
VPIIN
6
8
VPI=6
VCI=1,2,3
ATM
Node 1
VPIOUT
4
6
VPI=6
VCI=1,2,3
VPI=4, VCI=1,2,3
B
ATM
Node 2
VPIIN
4
VPI=6
VCI=3,4
ATM
Node 3
VPI=2
VCI=3,4
C
VPIIN
6
VPIOU
2T
VPIOU
6T
Namings
• VC
Virtual Channel
 Virtual Circuit
• VC Link
A point where a VCI value is assigned to another where
that value is translated or terminated.
• VC Identifier
A value which identifies a particular VC link for a
given VP Connection.
• VCC (Virtual Channel Connection)
A concatenation of VC links that extends between 2
points. (cell sequence integrity preserved)
• VP
Bundle of VCs.
• VP Link
A group of VC links, identified by a common value of
VPI, between a point where a VPI value is assigned and
the point where that value is translated as terminated.
• VP Identifier
Identifies a particular VP Link.
• VPC (Connection)
A concatenation of VP Links.
PVC and SVC
• Permanent Virtual Circuits (PVC)
Established by a network operator in which
appropriate VPI/VCI values are programmed for a
given source and destination (for long time).
VPs  0, …, 256 (manually configured)
PVCs are established by provisioning & usually last a
long time (months/years).
• Switched Virtual Circuits (SVC)
Established automatically through a signalling protocol
(Q.2931B) and lasts for short time (minutes/hours).
VCs  0, …, 65535 (automatically configured)
SOFT PVC (addendum)
• Part of the connection is permanent and part of
it is switched.
• Hybrid of PVC and SVC!!!
• VCC  0 - 31
• 0, 5  Call set up (Signalling)
• 0, 16  Network Management
(Integrated Local Management Interface ILMI)
• 32 - 65535  User Data
• 0, 17  For LAN Emulation Configuration Server
(LECS)
• 0, 18  For Private NNI (PNNI)
• 0, 19 or 0, 20  Reserved for future use.
Advantages of VP/VC Concept
• Simplified Network Architecture: Network transport functions can
be separated into those related to an individual logical connection
(VC) and those related to a group of logical connections (VP).
• Increased Network Performance and Reliability: The network
deals with fewer, aggregated entities.
• Reduced Processing and Short Connection Setup Time: Much of
the work is done when the VP is set up. The addition of new VCs to
an existing VP involves minimal processing.
• Enhanced Network Services: The VP is used internal to the
network but is also visible to the end user. Thus, the user may
define closed user groups or closed networks of VC bundles.