Transcript good vpc reduction

371-1-0291: An Introduction to
Computer Networks


Homepage

http://help.cse.bgu.ac.il/cse/Courses/list.asp
Handout #15: ATM - Asynchronous Transfer
Mode
Additional Reading
Text book: Chap. 3.3,6.5.4
Lect-12: ATM
Computer Networks
1
Before ATM
POTS
Computer Network
TV Network
(Plain Old Telephone Service)
(LAN, MAN, WAN)
(CATV, Satellite, etc.)
VOICE
DATA
VIDEO
Circuit Switching
Delay Guarantee
QoS
Packet Switching
No Delay Guarantee
No QoS (best effort)
Circuit Switching
Delay Guarantee
QoS
Limited Speed Data
No Video
Not suitable for
Voice and Video
No data or voice
Lect-12: ATM
Computer Networks
2
Background
Motivation:
 To carry multiple types of traffic (with different
Qualities of Service - QoS) over a single network.
Problem:
 The Internet carries data efficiently, but carries
real-time traffic poorly.
 The telephone network carries voice clearly,
but data inefficiently.
ATM was first proposed in ~1985 as a way to integrate
data communication with telephony.
Lect-12: ATM
Computer Networks
3
Multi-Service Network
ATM Objective: Service and Vendor Integration into one
transport network
bulk
data
video
Multi-service
Network
voice
Lect-12: ATM
interactive
data
Computer Networks
4
Key Attributes of ATM
Virtual Circuits (VC):
• All “packets” (cells) in a virtual circuit follow the same path
through the network. Which means:
• Resources may be reserved along the path.
• Packet-switching is used at each ATM switch.
Fixed-length Cells:
• ATM cells are 53 bytes long: 48bytes of data (payload),
and 5 bytes of header.
• Cells are fixed-length to simplify switch design,
and to allow fine-grain control of delay through a switch.
• Short cells
• reduce packetization average delay for voice
• reduce delay variance (jitter)
• near cut-through behavior
Lect-12: ATM
Computer Networks
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Why delay is reduced ?
Router
I
n
p
u
t
A voice packet would
otherwise wait behind a
large data packet
O
u
t
p
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t
p
o
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t
s
Voice
packet (50)
Data packet
(2000)
p
o
r
t
s
Router
I
n
p
u
t
p
o
r
t
s
O
u
t
p
u
t
Data #40
(50)
Data #2
(50)
Voice
(50)
Data #1
(50)
p
o
r
t
s
Voice packet to be transmitted after Data #1
Lect-12: ATM
Computer Networks
6
Key Attributes of ATM (cont.)
Asynchronous (statistical multiplexing):
• Cells from different VCs are statistically and async
multiplexed on the same link
Quality of Service (QoS):
• Five service classes
• Each class is specified by a set of service parameters
High Speed Access:
• Consolidate traffic coming from many LAN’s
Lect-12: ATM
Computer Networks
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ATM: A 3-Layer Model
ATM terminal
ATM terminal
native appl
data
native appl
data
ATM switch
ATM Adaptation
Layer (AAL)
ATM Layer
ATM Layer
ATM Adaptation
Layer (AAL)
ATM Layer
Physical Layer
Physical Layer
Physical Layer
Lect-12: ATM
Computer Networks
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ATM Layering Example

End devices have three layers

ATM switches has only the bottom two layers
Lect-12: ATM
Computer Networks
9
ATM Topology – High View
NNI
Lect-12: ATM
Computer Networks
10
ATM Interfaces

User-Network Interface (UNI)


public and private
Network-Network Interface (NNI)
Lect-12: ATM
Computer Networks
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Virtual Circuits
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

ATM is connection oriented
An ATM connection is called virtual
channel/connection/circuit (VC)
A VC must be established between two end points
before data can be transmitted

Tens of thousands of VCs per fiber

Different VC, different bandwidth, QoS

User data and signaling on different VCs
Lect-12: ATM
Computer Networks
12
Virtual Paths

Virtual Private Network over public ATM
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
many individual VCs between two sites
switches need to switch VCs individually even if they start
and end at the same points
VCs starting and ending at the same end
points can be bundled in Virtual Paths (VP)
 Switches may switch only VPs, not aware of
individual VCs inside the VP

Lect-12: ATM
Computer Networks
13
Virtual Paths & Virtual Channels
Physical
Channel
Lect-12: ATM
VP
VC
VC
VC
VC
VP
VC
VC
VC
VC
VP
VC
VC
VC
VC
VP
VC
VC
VC
VC
Computer Networks
14
Virtual Path Connection (VPC)
Lect-12: ATM
Computer Networks
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Virtual Channel Connection (VCC)
Lect-12: ATM
Computer Networks
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VP and VC Labels





VPs and VCs are labeled with integer identifiers

Virtual Path Identifier (VPI)

Virtual Channel Identifier (VCI)
VPI/VCIs are assigned hop by hop
Same end-to-end ATM connection may have different
VPI/VCI in different hop
Hop-by-hop label assignment allows reuse of the same
number at different hops
The network maps VPI/VCIs at adjacent hops to
create an end-to-end virtual circuit
Lect-12: ATM
Computer Networks
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Virtual Channel Identifier (VCI)

Each VC is identified by a unique VCI on a given link

VCIs therefore have only local significance

An end-to-end ATM connection traversing several
links will be identified by different VCIs at different
links

VCI translation occurs at the link boundary

VCIs are assigned to a connection during call set-up
Lect-12: ATM
Computer Networks
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Virtual Circuit Setup
Virtual Circuits are established hop-by-hop from end-to-end.
A
VC Setup
Ack
Lect-12: ATM
VC Setup
VC Setup
Rqst
VC Setup
Rqst
Rqst
VC Setup
VC Setup
Ack
Ack
Computer Networks
B
19
Virtual Circuit Identifiers
VC Table at Port 3:
1
4
9
VC Table at Port 2:
VC in VC out Port out
1
3
1
4
12
1
9
2
2
VC in VC out Port out
3
4
1
12
1
1
3
3
2
2
2
12
1
4
1
VC in VC out Port out
2
1
Lect-12: ATM
1
2
7
1
7
Computer Networks
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Fixed-length cells
48 bytes
5 bytes
PAYLOAD DATA
VCI:
HEC:
HEC
VCI
10 bits
24 bits
Virtual Circuit Identifier: represents a flow between
two user processes.
Header Error Control: detects and corrects errors in
the cell header.
Lect-12: ATM
Computer Networks
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Cell Format

User-Network Interface (UNI)
4
8
16
3
1
GFC
VPI
VCI
Type
CLP








8
HEC (CRC-10)
384 (48 bytes)
Payload
host-to-switch format
GFC: Generic Flow Control
• Arbitrate access to the link at local site
VCI: Virtual Circuit Identifier
VPI: Virtual Path Identifier (see below)
Type: management, congestion control, AAL5 (see below)
CLP: Cell Loss Priority
HEC: Header Error Check (CRC-8); detect & 1-bit correction
Network-Network Interface (NNI)


switch-to-switch format
GFC becomes part of VPI field
Lect-12: ATM
Computer Networks
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VP and VC Switch





Two types of ATM switches
VP switch does not look at VCIs, switching is based on
VPIs only
VCI does not change when passing through a VP
switch; VPI may change
VC switch looks at both VPI and VCI
VCI (as well as VPI) may change when passing through
a VC switch
Lect-12: ATM
Computer Networks
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VP Switch
VPI=5
VCI=15
Lect-12: ATM
VP
Switch
VPI=7
VCI=15
VP
Switch
VPI=9
VCI=15
Computer Networks
VP
Switch
VPI=6
VCI=15
24
VC Switch
VPI=5
VCI=15
Lect-12: ATM
VP
Switch
VPI=7
VCI=15
VC
Switch
VPI=9
VCI=24
Computer Networks
VP
Switch
VPI=6
VCI=24
25
Permanent Virtual Circuit
VPI=0
VCI=51
VPI=0
VCI=39
No signaling required to setup a VC; the VC is pre-setup.
Lect-12: ATM
Computer Networks
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Permanent Virtual Circuit




VPIs and VCIs for the end-to-end connection are
pre-configured in hardware
Semi-Permanent PVC : VPIs and VCIs are preconfigured, but can be erased and reconfigured
later
PVC is good for Virtual Leased Line for VPN
PVC does not scale well for an ATM LAN/WAN with
many stations : too many PVCs required
Lect-12: ATM
Computer Networks
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Switched Virtual Circuit
Signaling messages
Signaling messages
VPI=0
VCI=5
VPI=0
VCI=5
Signaling required to setup a VC
Lect-12: ATM
Computer Networks
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Switched Virtual Circuit
User messages
User messages
VPI=x1
VCI=x2
VPI=Y1
VCI=Y2
X1,X2,Y1,Y2 may take different values at successive call setups.
Lect-12: ATM
Computer Networks
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Switched Virtual Circuit

Circuit on demand

Scalable solution to ATM connectivity

Different VPIs and VCIs at successive call
setups for the same source-destination.
Lect-12: ATM
Computer Networks
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Recall - Encapsulation




Outgoing:

Take chunk of data from layer above (e.g., IP)

Add a header on the front (e.g., Ethernet header)

May add something on the back (e.g., CRC)

Pass to next lower layer
But… ATM can only handle 48 byte data chunks!
Therefore need data Fragmentation and
Reassembly … at the link layer!
Question: where should this be done? (OS in
software, device card in hardware/firmware?)
Lect-12: ATM
Computer Networks
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ATM Adaptation Layer (AAL):
General encapsulation picture
PDU = “protocol data unit” -- network-speak
User data
AAL PDU
Segmentation and
Reassembly
ATM cell
Lect-12: ATM
Computer Networks
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Segmentation and Reassembly

ATM Adaptation Layer (AAL)

AAL 1 and 2 are designed for applications that need
guaranteed bit rate (e.g., voice, video) (const or var)

AAL 3/4 designed for packet data (connection (-less))

AAL 5 is an alternative “simple” std. for packet data
AAL
AAL
…
…
ATM
Lect-12: ATM
ATM
Computer Networks
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AAL 3/4
Main job: convert var length packet to fixedlength cells
 “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: common part indicator (version field)

Btag/Etag: beginning and ending tag

BAsize: hint on amount of buffer space to allocate

Length: size of whole PDU
Lect-12: ATM
Computer Networks
34
Special Cell Format for AAL 3/4
40
ATM header

2
4
10
Type
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
Lect-12: ATM
Computer Networks
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Efficiency
AAL 3/4 makes it even worse
 Best possible is 1 - 9/53 = 83%
 Often much worse due to CS-PDU header
and partial last cell

Lect-12: ATM
Computer Networks
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40
ATM header
2
4
10
Type
SEQ
MID
352 (44 bytes)
Payload
6
10
Length
CRC-10
(AAL 3/4)
AAL5: “Simple”



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 miss-ordered cells)
Cell Format

end-of-PDU bit in Type field of ATM header

This is the key
Stronger protection
Lect-12: ATM
Computer Networks
37
LAN Emulation
Many IP protocols assume some form of
broadcast/multicast on a LAN
 ATM link layer means you must either




Modify those protocols
Fake the *cast
LAN Emulation (LANE) is expensive and
complex
Lect-12: ATM
Computer Networks
38
ATM Service Classes

Constant Bit rate (CBR)



Variable Bit rate – real-time (VBR-rt)





Similar to Integrated Services Controlled Load.
No hard-bound on end-to-end delay.
Available Bit rate (ABR)


Leaky-bucket regulated source.
Leaky-bucket Spec (specification of the transmitted and
received traffic - (s,r) values) .
Network guarantees end-to-end delay.
Variable Bit rate – non-real-time (VBR-nrt)


Assumes source sends at constant rate.
Used for emulation of circuit-switching.
Includes Congestion-control mechanisms
Unspecified Bit rate (UBR)

Whatever is left-over: “best-effort”
Lect-12: ATM
Computer Networks
39
The adoption of ATM
ATM was designed to replace the Internet
by offering end-to-end QoS, highperformance switches, and integration of
the voice and data networks.
 It didn’t happen as planned.




Complex standards.
Momentum and simplicity of IP and the Internet.
Signaling to create Virtual Circuits complex and not robust.
But a large fraction of the core of the Internet today is ATM…
Lect-12: ATM
Computer Networks
40
How ATM (and other technologies) are
used in the Internet today
Logically, it is useful to think of the
Internet as a population of end-hosts
interconnected by routers.
 Logically (at the Network Layer) this is
true.
 But there is plenty of additional equipment
at the Link Layer and the Physical Layer.

Lect-12: ATM
Computer Networks
41
The Network Layer View of the
Internet
End hosts
Routers
Lect-12: ATM
Computer Networks
42
End-users connected via LANs
Ethernet switch (e.g. Ethernet)
Lect-12: ATM
Computer Networks
43
End-users connected via
Modems or DSL
Your local
Central Office
Lect-12: ATM
Computer Networks
44
ATM is frequently used as a Link
Layer
ATM switch
Packet Switched
(IP routers)
Lect-12: ATM
Circuit Switched
(SONET)
Computer Networks
45
ATM and IP traffic are carried over the
SONET (telephony) circuit switched network
SONET “Physical
Layer” Switched
switches carry
Packet Switched
Circuit
circuits between routers and ATM switches.
(IP routers)
(SONET)
Lect-12: ATM
Computer Networks
46