Transcript Evolution of Data Networks

ATM Networks
• Introduction
• ATM Cells
• Virtual-circuit switching
1
ATM Multiplexing
Voice
Data
packets
MUX
Wasted bandwidth
Images
TDM
4
3
2
1
4
3
2
1
4
ATM
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2
1
2
1
`
4
3
1
3
2
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Cell Switching (ATM)
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•
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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
3
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
4
Big vs Small Packets
• Small Improves Queue behavior
– finer-grained pre-emption point for scheduling link
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•
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•
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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
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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
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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
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The ATM Cell
5 Bytes
Header
48 Bytes
Payload
7
Cell Format
• User-Network Interface (UNI)
4
8
16
3
1
8
384 (48 bytes)
GFC
VPI
VCI
Type
CLP
HEC (CRC-8)
Payload
–
–
–
–
–
–
–
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
8
ATM Switching
1
voice 67
1
video 67
2
data 39
3
video 25
5
video 61
N
1
75
32
61
3
2
39
67
67
…
6 data 32
voice 32
25
32
…
…
Switch
N
N
video 75
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Virtual Paths in an ATM Network
VP3
a
b
c
d
e
ATM
Sw
1
a
VP5
ATM
Sw
2
ATM
DCC
ATM
Sw
3
b
c
VP2
VP1
Sw = switch
ATM
Sw
4
d
e
10
ATM Virtual Connections
Virtual Paths
Physical Link
Virtual Channels
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Virtual circuit Switching
Packet
Packet
12
VC Setup Delays
t
Connect
request
CC
CR
CC
CR
Connect
confirm
1
2
3
1
2
t
Release
3
t
1
2
3
t
13
VC Signaling
SW
1
Connect
request
Connect
confirm
SW
2
…
Connect
request
SW
n
Connect
request
Connect
confirm
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VC Routing Table
Entry for packets
with identifier 15
Identifier
Output
port
Next
identifier
12
13
44
15
15
23
27
13
16
58
7
34
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Broadband ISDN reference model
Management plane
Higher layers
Higher layers
Plane management
User plane
Layer management
Control plane
ATM adaptation layer
ATM layer
Physical layer
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User Plane Layers
User
information
User
information
AAL
AAL
ATM
ATM
ATM
ATM
PHY
PHY
PHY
PHY
…
End system
Network
End system
17
Private ATM
network
Private
UNI
X
X
Private
NNI
Public ATM
network A
X
X
X
NNI
Public
UNI
X
B-ICI
Public ATM
network B
X
Public
UNI
X
X
ATM Network Interfaces
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ATM QOS Parameters
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Non-negotiable parameters: characteristics of networks
Cell error ratio
Cell misinsertion ratio: undetected header errors.
Severely-errored cell block ratio: Bursty errors
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Negotiable parameters: User can ask during setup
Cell loss ratio
Cell transfer delay
Cell delay variation
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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.
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AAL Sublayers
Service Specific
Convergence
Sublayer
AAL
Layer
Convergence
Sublayer
Common Part
Segmentation
and Reassembly
Sublayer
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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
AAL
…
…
ATM
ATM
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AAL1 Process
Higher layer
b1
b2
…
b3
Convergence
sublayer
User data stream
CS PDUs
47
47
47
SAR PDUs
SAR sublayer
ATM layer
H
H
H
1 47
1 47
1 47
H
H
5
48
5
ATM Cells
H
48
5
48
For Constant rate transfers, e.g., 64kbps PCM voice call
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AAL1 PDUs
(a) SAR PDU header
CSI
1 bit
SNP
Seq. Count
3 bits
4 bits
(b) CS PDU with pointer in structured data transfer
47 Bytes
AAL 1
Pointer
1 Byte
46 Bytes
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Application scenario for AAL2
AAL
2
ATM cells
Mobile
switching
office
Low bit rate
Short
voice packets
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
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single ATM connection, e.g., cell phone traffic, compressed audio.
AAL2 Process
Higher layer
This example assumes
24 byte packets
P3
P2
P1
Service specific
convergence
sublayer
Assume null
Common part
convergence
sublayer
H
H
H
3 24
3 24
3 24
SAR sublayer
PAD
1
ATM layer
Add 3-byte header to
each user packet
H
5
1
47
47
Segment into SAR
PDUs
H
48
5
48
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AAL 3/4 Process
Higher layer
Information
User message
Service specific
convergence
sublayer
Common part
convergence
sublayer
Assume null
H
2 44
T
4
4
…
SAR sublayer
ATM layer
PAD
Information
Pad message to multiple
of 4 bytes. Add header
and trailer.
2
2 44
2
2 44
2
Each SAR-PDU consists
of 2-byte header, 2-byte
trailer, and 44-byte
payload.
…
AAL3 for connection-oriented bursty data with low loss, no delay27
requirements. AAL4 for connection-less data transfers.
(a) CPCS-PDU format
Trailer
Header
CPI Btag BASize
1
1
2
(bytes)
CPCS - PDU Payload
1 - 65,535
(bytes)
Pad AL Etag Length
0-3 1 1
2
(bytes)
(b) SAR PDU format
Trailer (2
bytes)
Header
(2 bytes)
ST SN MID
2 4 10
(bits)
SAR - PDU Payload
44
(bytes)
AAL 3/4 CPCS and SAR formats
LI CRC
6 10
(bits)
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Higher layer
Service specific
convergence
sublayer
Common part
convergence and
SAR sublayers
P1
Assume two packets
from different users
P2
MID = a
MID = b
CPCS
SAR
CPCS
SAR


SPDUA2
SPDUB2
SPDUA1
SPDUB1
Each packet is
segmented separately.
SAR PDUs identified by
MID.
Interleaver
ATM layer
Interleaved cells
Multiplexing in AAL 3/4
Cells from two packets
are interleaved.
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AAL 3/4
• Convergence Sublayer Protocol Data Unit (CS-PDU)
–
–
–
–
8
8
16
CPI
Btag
BASize
< 64 KB
User data
0– 24
8
8
16
Pad
0
Etag
Len
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
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Cell Format
40
2
4
10
ATM header
T ype
SEQ
MID
352 (44 bytes)
Payload
6
10
Length
CRC-10
– 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
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AAL5
• CS-PDU Format
< 64 KB
0– 47 bytes
16
16
32
Data
Pad
Reserved
Len
CRC-32
– 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
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AAL 5 Process
Higher layer
Information
Service specific
convergence
sublayer
Assume null
Common part
convergence
sublayer
PAD
Information
T
…
SAR sublayer
48
(0)
48
(0)
48
(1)
…
ATM layer
PTI = 0
PTI = 0
PTI = 1
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AAL 5 PDU
Information
0 - 65,535
(bytes)
Pad
UU CPI
0-47
1
1
(bytes)
Length CRC
2
4
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