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munchdrabNetworking and Communications

Oct 30, 2013 (3 years and 5 months ago)

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1

ATM Networks


Introduction


ATM Cells


Virtual
-
circuit switching

2

MUX

`

Wasted bandwidth

ATM

TDM

4 3 2 1 4 3 2 1 4 3 2 1

4 3 1 3 2 2 1

Voice

Data
packets

Images

ATM Multiplexing

3

Cell Switching (ATM)


Connection
-
oriented packet
-
switched network


Used in both WAN and LAN settings


Signaling (connection setup) Protocol: Q.2931


Specified by ATM forum


Packets are called
cells


5
-
byte header + 48
-
byte payload


Commonly transmitted over SONET


other physical layers possible

4

Variable vs Fixed
-
Length Packets


No Optimal Length


if small: high header
-
to
-
data overhead


if large: low utilization for small messages


Fixed
-
Length Easier to Switch in Hardware


simpler


enables parallelism

5

Big vs Small Packets


Small Improves Queue behavior


finer
-
grained pre
-
emption point for scheduling link


maximum packet = 4KB


link speed = 100Mbps


transmission time = 4096 x 8/100 = 327.68us


high priority packet may sit in the queue 327.68us


in contrast, 53 x 8/100 = 4.24us for ATM


near cut
-
through behavior


two 4KB packets arrive at same time


link idle for 327.68us while both arrive


at end of 327.68us, still have 8KB to transmit


in contrast, can transmit first cell after 4.24us


at end of 327.68us, just over 4KB left in queue

6

Big vs Small (cont)


Small Improves Latency (for voice)


voice digitally encoded at 64KBps (8
-
bit samples at 8KHz)


need full cell’s worth of samples before sending cell


example: 1000
-
byte cells implies 125ms per cell (too long)


smaller latency implies no need for echo cancellors


ATM Compromise: 48 bytes = (32+64)/2

7

Header

Payload

5 Bytes

48 Bytes

The ATM Cell

8

Cell Format


User
-
Network Interface (UNI)




host
-
to
-
switch format


GFC: Generic Flow Control (still being defined)


VCI: Virtual Circuit Identifier


VPI: Virtual Path Identifier


Type: management, congestion control, AAL5 (later)


CLPL Cell Loss Priority


HEC: Header Error Check (CRC
-
8)



Network
-
Network Interface (NNI)


switch
-
to
-
switch format


GFC becomes part of VPI field

GFC

HEC (CRC
-
8)

4

16

3

1

8

VPI

VCI

CLP

Type

Payload

384 (48 bytes)

8

9

2

3

N

1

Switch

N

1



5

6

video

25

video

voice

data

32

32

61

25

32

32

61

75

67

39

67

N

1

3

2

video

75

voice

67

data

39

video

67





ATM Switching

10

c

ATM

Sw

1

ATM

Sw

4

ATM

Sw

2

ATM

Sw

3

ATM

DCC

a

b

d

e

VP3

VP5

VP2

VP1

a

b

c

d

e

Sw = switch

Virtual Paths in an ATM Network

11

Physical Link

Virtual Paths

Virtual Channels

ATM Virtual Connections

12

Packet

Packet

Virtual circuit Switching

13

t

t

t

t

3

1

2

3

1

2

3

2

1

Release

Connect
request

CR

CR

Connect
confirm

CC

CC

Figure 7.19

VC Setup Delays

14

SW
1

SW
2

SW
n

Connect
request

Connect
request

Connect
request

Connect
confirm

Connect
confirm



VC Signaling

15

Identifier

Output

port

15

15

58

13

13

7

27

12

Next

identifier

44

23

16

34

Entry for packets

with identifier 15

VC Routing Table

16

Plane management

Management plane

Control plane

User plane

Physical layer

ATM layer

ATM adaptation layer

Higher layers

Higher layers

Layer management

Broadband ISDN reference model

17

AAL

ATM

User
information

User
information

AAL

ATM

PHY

PHY

ATM

PHY

ATM

PHY



End system

End system

Network

User Plane Layers

18

X

X

X

X

X

X

X

X

X

Private
UNI

Public
UNI

NNI

Private
NNI

Private ATM
network

Public
UNI

B
-
ICI

Public ATM
network A

Public ATM
network B

ATM Network Interfaces

19

ATM QOS Parameters


Non
-
negotiable parameters: characteristics of networks


Cell error ratio


Cell misinsertion ratio: undetected header errors.


Severely
-
errored cell block ratio: Bursty errors



Negotiable parameters: User can ask during setup


Cell loss ratio


Cell transfer delay


Cell delay variation

20

ATM Service Categories


Constant bit rate (CBR): Rigorous timing requirements,
voice, circuit emulation.


Real
-
time Variable Bit Rate: VBR traffic such as video.


Non
-
real
-
time VBR: Bursty data sources, no rigorous
timing requirments


Available bit rate (ABR): Sources that can adapt to
available bandwidth in the network. Low cell loss ratio.


Unspecified bit rate (UBR): No QOS guarantees.

21

Segmentation
and Reassembly
Sublayer

AAL
Layer

Convergence
Sublayer
Common Part

Service Specific
Convergence
Sublayer

AAL Sublayers

22

Segmentation and Reassembly


ATM Adaptation Layer (AAL)


AAL 1 and 2 designed for applications that need
guaranteed rate (e.g., voice, video)


AAL 3/4 designed for packet data


AAL 5 is an alternative standard for packet data

AAL

A

TM

AAL

A

TM





23



Higher layer

User data stream

Convergence
sublayer

SAR sublayer

ATM layer

CS PDUs

SAR PDUs

ATM Cells

47

47

47

1 47

1 47

1 47

H

H

H

5 48

H

5 48

H

5 48

H

b
1
b
2
b
3


AAL1 Process

For Constant rate transfers, e.g., 64kbps PCM voice call

24

(a) SAR PDU header

(b) CS PDU with pointer in structured data transfer

CSI

SNP

AAL 1
Pointer

1 Byte

46 Bytes

47 Bytes

Seq. Count


1 bit


3 bits







4 bits

AAL1 PDUs

25


AAL
2



ATM cells

Low bit rate Short
voice packets

Mobile
switching
office

Application scenario for AAL2

Originally, for variable bit rate (VBR) applications with end
-
to
-
end
delay requirements, e.g., compressed video.

Now, BW
-
efficient transfer of low
-
bit
-
rate short
-
packet traffic with
low
-
delay requirements. Allows multiplexing of multiple users on a
single ATM connection, e.g., cell phone traffic, compressed audio.

26

Higher layer

This example assumes
24 byte packets

Common part
convergence
sublayer

SAR sublayer

ATM layer

1 47

5 48

H

P
3

Service specific
convergence
sublayer

P
2

P
1

Assume null

1 47

3 24

3 24

3 24

PAD

5 48

H

Add 3
-
byte header to
each user packet

Segment into SAR
PDUs

H

H

H

AAL2 Process

27

Higher layer

Common part
convergence
sublayer

SAR sublayer

ATM layer

Service specific
convergence
sublayer

Information

Assume null

T

PAD

User message

Pad message to multiple
of 4 bytes. Add header
and trailer.

Each SAR
-
PDU consists
of 2
-
byte header, 2
-
byte
trailer, and 44
-
byte
payload.

H

4

4

2 44 2


2 44 2


2 44 2






Information

AAL 3/4 Process

AAL3 for connection
-
oriented bursty data with low loss, no delay
requirements. AAL4 for connection
-
less data transfers.

28

(a) CPCS
-
PDU format

(b) SAR PDU format

CPI Btag BASize

CPCS
-

PDU Payload


1 1 2

1
-

65,535




0
-
3 1 1 2

(bytes)



(bytes)




(bytes)

AL Etag Length

Pad

Header

Trailer

ST SN MID

SAR
-

PDU Payload


2 4 10 44







6 10

(bits)



(bytes)





(bits)

LI CRC

Header
(2 bytes)

Trailer (2
bytes)

AAL 3/4 CPCS and SAR formats

29

Higher layer

Assume two packets
from different users

Common part
convergence and
SAR sublayers

ATM layer

Service specific
convergence
sublayer

P
2

P
1

Each packet is
segmented separately.
SAR PDUs identified by
MID.

Cells from two packets
are interleaved.

Interleaved cells

MID = b

MID =
a

CPCS
SAR

CPCS
SAR

Interleaver



SPDUA2

SPDUA1



SPDUB2

SPDUB1

Multiplexing in AAL 3/4

30

AAL 3/4


Convergence Sublayer Protocol Data Unit (CS
-
PDU)




CPI: commerce part indicator (version field)


Btag/Etag:beginning and ending tag


BAsize: hint on amount of buffer space to allocate


Length: size of whole PDU

31

Cell Format


Type


BOM: beginning of message


COM: continuation of message


EOM end of message


SEQ: sequence of number


MID: message id


Length: number of bytes of PDU in this cell

32

AAL5


CS
-
PDU Format




pad so trailer always falls at end of ATM cell


Length: size of PDU (data only)


CRC
-
32 (detects missing or misordered cells)


Cell Format


end
-
of
-
PDU bit in Type field of ATM header


A more efficient alternative to AAL3/4


message and stream modes


assured and nonassured delivery

33

Higher layer

Common part
convergence
sublayer

SAR sublayer

ATM layer

PTI = 0

Service specific
convergence
sublayer

Assume null

48
(1)

Information

T

PAD





Information

48
(0)

48
(0)

PTI = 0

PTI = 1

AAL 5 Process

34

Information


0
-

65,535


0
-
47 1 1 2 4

(bytes)




(bytes)

UU CPI Length CRC

Pad

AAL 5 PDU