Network technology - 2
A Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication
Network
Broadcast Communication
Network
Circuit-Switched Communication
Network
Packet-Switched Communication
Network
Datagram Network
Virtual Circuit Network
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 3
Asynchronous Transfer Mode (ATM)The technology supported by the telecom operators as the solution for Broadband ISDN
Unified solution for LAN, MAN, WAN
Was fully standardized by the end of 90s, but …Internet was already very widely spread
Now mainly serves for the provision of backbone connections
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 4
Key Features of ATMAsynchronous: non-periodic transfer of information between ports in switches
Short fixed size cells
frame
STM (Synchronous Transfer Mode):
ATM:
...
...
header(VPI/VCI)
data
5 bytes 48 bytes
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 5
Key Features of ATM (cont.)Connection oriented: Virtual Channel (VC) connection is setup prior to information transfer
Labels vs. addresses: labels have local significance scalability
...
⇒
7 2
3 7
Network technology - 6
Switching in Packet NetworksDatagram (e.g. Internet): packet header contains full address information
Address has global significance
Virtual circuit (e.g. ATM, Frame Relay): packet header contains virtual circuit identifier
Address has local significance
Destination address: 147.52.16.2
VC: 16
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 7
Cell Switching
25 25 25
34
30
ATM switchCell
Header(VPI/VCI)
Payload
30 49
input port 1
output port1
output port 2
input port 2
Input Port VPI/VCI Output Port VPI/VCI1 25 2 341 30 1 49
Internal Routing Table
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Virtual Channel Connection
30
49
17
7 Virtual Channel Link
Virtual Channel Connection
Two types of Virtual Channels:Switched Virtual Channels (SVC): connection setup with signallingPermanent Virtual Channels (PVC): connection setup with management
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Routing and Call Admission Control (CAC)
Xsource
destination
Routing: find path from source to destination that fulfils user requirements (bandwidth, QoS)Call Admission Control (CAC): performed at every switch, determines whether there are enough resources to accept a call
CAC CAC
CAC
CAC
CAC
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 10
Virtual Channels and Virtual PathsCell label:
First 8 (12) bits: Virtual Path IdentifierLast 16 bits: Virtual Circuit Identifier
Virtual Path: group of VCs treated similarly
TRANSMISSIONPATH
VP
VPVP
VP VC
VCVC
VC
Network technology - 11
ATM Cell Header Format
GFC = Generic Flow ControlVPI = Virtual Path IdentifierVCI = Virtual Channel IdentifierPTI = Payload Type IdentifierCLP = Cell Loss PriorityHEC = Header Error Control
GFC/VPI VPI
VPI VCI
VCI
VCI PTI CLP
HEC
1
2
3
4
5
byte
GFC: only in UNI CLP=1 => cell has low priorityPTI: identifies user cell/data cell, congestion control
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 12
Input Port VPI Output Port VPI1 2 2 51 9 1 102 4 1 9
2/1 5/1
2/7 5/7
9/3 10/3
4/2 9/2
VC 1VC 7
VC 1VC 7
VP 2
VP 5
VP 9
VP 4
VP 10
VP 9
input port 1
input port 2
output port 1
output port 2VC 2
VC 2
VC 3
VC 3
VP Switching
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VP and VP/VC Switching in a Network
source destination
26 / 43 33 / 57 63 / 127 47 / 52 39 / 41 45 / 3856 / 29
destinationsource
VPI/VCI
26 / 43 33 / 29 63 / 29 47 / 52 39 / 52 45 / 5256 / 29VPI/VCI
VP/VC switchVP switchor cross-connect
VPI: 56 VPI: 63 VPI: 47 VPI: 45
26/43
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 14
B-ISDN Interface Terminology
ATM switchATM station
User-Network Interface (UNI)
Network-Node Interface (NNI)
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B-ISDN Interface Terminology (cont.)
B-ICI(B-ISDN Inter-exchangeCarrier Interface)
Public ATM
: Private ATM switch : Public ATM switch
Private ATM
Private UNI Private NNIPublic UNI
Public ATM
NNI
Network technology - 16
ATM Layer Service Categories
Offered at ATM UNI (User-Network Interface)
ATM UNI
ATMNetwork
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Service Category TypicalApplication
Constant Bit Rate (CBR) Circuit emulation,videoconferencing,
Real-Time Variable Bit Rate(rt-VBR)
Compressedvideo/audio
Non-Real-Time Variable Bit Rate(nrt-VBR)
Critical data
Available Bit Rate (ABR) LANinterconnection,
Unspecified Bit Rate (UBR) File transfer,message transfer
ATM Forum Service Categories
Real-Time :
Non-Real-Time:
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ATM Forum Real-Time Service CategoriesConstant Bit Rate (CBR):
real-time applications requiring a static amount of bandwidthGuaranteed Quality of Service (QoS) in terms of delay, delay variation, cell loss
Real-Time Variable Bit Rate (rt-VBR):real time applications with “bursty” trafficGuaranteed Quality of Service (QoS) in terms of delay, delay variation, cell loss
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 19
ATM Forum Non-Real-Time Service CategoriesNon-Real-Time Variable Bit Rate (nrt-VBR):
non-real-time applications with bursty trafficGuaranteed cell loss bound but no delay bounds
Available Bit Rate (ABR):“elastic” applications which can adapt their traffic rateclosed loop flow control supported
Unspecified Bit Rate (UBR):non-real-time applications, no service guarantees
Network technology - 20
Bandwidth Usage
Bandwidth unallocated
LinkBandwidth
time
Bandwidth allocatedand used
Bandwidth allocatedbut not used
CBR + VBR traffic
ABR + UBR traffic
LinkBandwidth
Network technology - 21
Traffic Contract Negotiation
ATM UNI
ATMNetwork
QoS parameters: cell delay, cell delay variation, cell loss ratio
Traffic parameters: peak rate, sustainable cell rate, burst size, minimum cell rate
Traffic Contract:
Conformance definition: Generic Cell Rate Algorithm (GCRA)
Network technology - 22
Traffic Parameters
Peak Cell Rate (PCR): Maximum instantaneous rateSustained Cell Rate (SCR): Average cell rate measured over some long intervalMaximum Burst Size (MBS): Maximum burst size (# of cells) that can be sent at the peak rateMinimum Cell Rate (MCR): Minimum cell rate user is guaranteed to have
Traffic parameters describe inherent characteristics of a traffic source
Network technology - 23
Traffic parameters (cont.)
time
Peak Cell Rate (PCR):
1/PCR
time
Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), Maximum Burst Size (MBS)
...
...
Ti (minimum burst interarrival time)=MBS/SCR
cell
Tb (maximum burst durationat PCR)=(MBS-1)/PCR
SCR is also met with more “sparse” cells
...
Network technology - 24
ATM Quality of Service (QoS) Parameters
Max Cell Transfer Delay (CTD): Delay experienced by a cell between network entrance and exit pointspeak-to-peak Cell Delay Variation (CDV): max - min cell transfer delayCell Loss Ratio (CLR): Percentage of cells that are lostThe above are user negotiable
ATM QoS parameters characterise performance of an ATM layer connection
Network technology - 25
ATM QoS parameters (cont.)
fixed delay (min) peak-to-peak CDV
area ≤
CLR
max CTD
probabilitydensity
cell transferdelay
Network technology - 26
Conformance Definition
GCRA: Generic Cell Rate AlgorithmAbove applies to CBR, VBR, and UBRABR is a special case (includes feedback)
Conformance definition defines conformity at an interface with respect to traffic contract according to one or more instances of GCRA
Network technology - 27
Service Category Attributes and Guarantees
ServiceCategory
TrafficDescription
Guarantees FeedbackControl
Min Loss(CLR)
Delay/Variance
Bandwidth
CBR PCR
rt-VBR PCR,SCR,MBS
nrt-VBR PCR,SCR,MBS
ABR PCR,MCR+behavior
UBR (PCR) NO NO
NO
NO
NO
NO
NO
NO
NO
Network technology - 28
Traffic Control in ATMConnection Acceptance Control (CAC)RoutingUsage Parameter Control (UPC) or Source PolicingShapingPriorities, schedulingFeedback controlOther (e.g. pricing)
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 29
Connection interarrival time
Time scales of network control
Selective cell discard, frame discard,priority control and scheduling, Usage
Parameter Control (UPC), traffic shaping
Feedback controls
Routing, Call Admission Control (CAC)
Network management control
Cell time
Round-trip propogation time
Traffic & Congestion Control Functions Response time
order of minutes
Pricing months, years
Network technology - 30
Network control and pricingSet of feasible services depends on network control mechanismsEconomic incentives influence network control mechanismsNetwork control: guarantees contractsPricing: influences demand in order to increase efficiency
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Network Control for Congestion
Guaranteed servicesCall Acceptance Control - CAC user-network contractUse charging as incentive for users to provide more accurate information about traffic profile
Best effort services (ABR):no CAC, except for MCR (Minimum Cell Rate)Flow controlUse charging to control congestion
acceptable
n1
n2
CAC:
(n1,n2: number of multiplexed sources of type 1,2)
non-acceptable
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 32
Prototype Problem of Call Acceptance (for Guaranteed QoS)
k traffic classesclass i contains
sources ni
n1
nkB: buffer
C: capacity... ......
What do not violate QoS constraints ?
StatisicalStatisical QoSQoS constraint: e.g. CLP less than p (e.g. p=10-8 )
( , , )n nk1 K
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Prototype Problem of Call Acceptance (cont.)
Peak rate allocation: sufficient conditionacceptance region:
accomplishes zero cell losssimple acceptance criterion (linear in )dedicated capacity per source (reminds circuit switching)conservative: assumes worst-case demand arises simultaneouslyno multiplexing gain less revenue, more expensive to customers
n h n h Ck k1 1⋅ + ⋅ <K
n1
nkB: buffer
C: capacity... ......traffic
ni
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 34
Prototype Problem of Acceptance Control (cont.)
Mean rate allocation: only necessary conditionacceptance region:
ultimate multiplexing gainsimple (linear in )no performance guarantees (only CLP<1 !)
n m n m Ck k1 1⋅ + ⋅ <K
ni
n1
nkB: buffer
C: capacity... ......traffic
Network technology - 35
Effective Bandwidth and Many Sources Asymptotic
If , C=Nc and B=Nb, then
and thus
is the effective bandwidth of sources of type j,
Xj[0,t] = number of cells in [0,t]s = space scale parametert = time scale parameter
lim log Pr[ ] sup inf ( , ) ( )N t s j
k
jNI st a s t s ct b
→ ∞= − = − +
⎧⎨⎩
⎫⎬⎭
∑11
overflow ρ
a s tst
EejsX tj( , ) log [ , ]=
1 0
Pr[ ]overflow ≈ =− −e eNI γ
a s tj ( , )
n Ni i= ρ
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 36
Simplifying the Problem of Acceptance Control
Use Effective Bandwidths acceptance region:
depends only on source type, traffic mix, capacity, buffer and QoS requirements, summarises each source’s effect on QoSimmediate guarantee of QoSlinearizes a very complex problem of QoSstill allows easy estimation of available capacity
n eb n eb Ck k1 1⋅ + ⋅ <K $
ebi
iii hebm ≤≤
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ON/OFF Source Model
h
ON/OFF source: ON & OFF periods are exponentially distributed
on
off
ab
timeon off
1/h
mean ON period = 1/a, mean OFF period = 1/b
mean rate m = b*h/(a+b)
effective bandwidth ( )ebs
mh
eish= + −
⎡⎣⎢
⎤⎦⎥
11 1log
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 38
CAC: Peak Rate vs. Effective Bandwidth
Effective Bandwidth CAC constraints correspond to tangents of the acceptance region boundary curve
The right selection of a tangent depends on the operating point
acceptable
n1
n2
non-acceptable
peak-rate constraint
(n1,n2: number of multiplexed sources of type 1,2)
Effective-bandwidth constraints
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 39
Usage Parameter Control (UPC) or Source Policing
UPCATM
NetworkATM UNI
Traffic Contract
UPC actions:
UPC
CLP bit
Cell passing:
Cell tagging:
Cell discarding:
UPC
UPC
0
0
0
0
1
Network technology - 40
UPC and Traffic Shaping
Traffic shaping at source prevents loss due to policing
UPCATM UNI
Shaping
Traffic Contract
ATMNetwork
nonconforming cellsMBS
No shaping
MBS
With shaping
MBS: maximum burst size
time time
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 41
UPC (or Source Policing) FunctionsAlgorithm implemented at UNI for ensuring that user traffic adheres to traffic contract.Generic Cell Rate Algorithm (GCRA):
“Leaky bucket” type algorithmOpen loop algorithmSpecified by ATM Forum
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Generic Cell Rate AlgorithmGCRA(T, τ)t: cell arrival timetat: theoretical arrival time
tat0
Tτ
cell nonconforming cell conforming
Ttat
if (t<tat- τ)cell nonconforming
elsecell conformingtat=max(t,tat)+T
“Virtual Scheduling Algorithm”
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 43
GCRA examplesGCRA(10,2)
Cell arrival times: 0 10 18 28 42 49
Cell arrival times: 0 10 15 25 35
t 0 10 15 25 35tat-τ 0-2 10-2 20-2 20-2 35-2result C C NC C Ctat after 10 20 20 35 45
if (t<tat- τ)cell nonconforming
elsecell conformingtat=max(t,tat)+T
t 0 10 18 28 42 49 tat-τ 0-2 10-2 20-2 30-2 40-2 52-2result C C C C C NC tat after 10 20 30 40 52 52
0 10 20 30 40 50
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 44
Leaky Bucket AlgorithmBucket size=1+τ/TLeak rate=1/T cells/sBucket contents increased by 1 for each conforming cell
Is provably equivalentto GCRA
B: bucket contents
If B+1> 1+τ/Tcell nonconforming
elsecell conformingB=B+1
virtual cell1+τ/T cells
1/T cells/s
actual cell
conforming ornonconforming
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 45
Traffic Contract for CBRFor each CBR connection:
PCR (Peak Cell Rate)CDVT (Cell Delay Variation Tolerance): takes care of slotted nature of ATM, physical overhead, ATM layer function overhead
– E.g. only with CDVT we can have PCR=2/3*R, where R is ATM layer rate
Connection must conform to GCRA(R/PCR,CDVT), PCR=15 Mbps, CDVT=2, ATM Layer Rate=150 Mbps =>
GCRA(10,2)
Customer Premises Network (CPN) UPC
Public ATM network
Private UNI Public UNI
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Traffic Contract for VBR For each VBR connection:
PCR, CDVTSCR, MBS, CDVTBT=(MBS-1)(R/SCR-R/PCR), where R: ATM Layer Rate
Connection must conform to GCRA(R/PCR,CDVT) ANDGCRA(R/SCR, BT+CDVT)
Equivalent description: Dual leaky bucket
© C.Courcoubetis, V.Siris, G.D.StamoulisNetwork technology - 47
Leaky buckets for SCR and PCR policing
virtual cell1+(BT+CDVT) x SCR cells
SCR cells/s
arriving cell1+CDVT x PCR cells
PCR cells/s
Network technology - 48
Deterministic multiplexing of leaky bucket streams
Assume leaky bucket (r,b) and access rate h
Maximum time t user can transmit at rate h
Deterministic QoS constraints: no losses, delay < D for all cells
rhbtbrtht−
=⇒+≤
NB: buffer
C: capacity...
h: access rate
Network technology - 49
Multiplexing leaky bucket streams (cont.)Maximum backlog Q if N users (r,b) multiplexed on link with capacity C
Delay constraint D
Must also have:
Finally: CAC criterion , where is the effective bandwidth
rhbCNhCtNhtQ−
−=−= )(
CDrhb
bhNDCrh
bCNhDCQ
≤⎟⎟⎠
⎞⎜⎜⎝
⎛−+
⇒≤−
−⇒≤)(
)(
CNr ≤
CNa ≤
⎭⎬⎫
⎩⎨⎧
−+= r
Drhbbha ,
)(max
a
Network technology - 50
Available Bit Rate (ABR) Services within ATMIntended for elastic sources (i.e., sources which can increase/decrease their traffic rate)For each ABR connection:
PCR (Peak Cell Rate)MCR (Minimum Cell Rate)
No specific QoS parametersCLR (Cell Loss Ratio) is expected to be low for compliant sourcesfair share of available bandwidth
Rate-based closed loop congestion controlbinary feedback (Explicit Forward Congestion Indication - EFCI)rate-based (Explicit Rate - ER), Resource Management cell
Network technology - 51
ABR Congestion Control
a) EFCI marking mode
b) Relative Rate marking mode
c) Explicit Rate marking mode
source destinationCI bit setdata cell
RM cell
RM cell
CI bit set
source destination
RM cell
RM cell
ER set
source destination
Network technology - 52
Switch Operation with ABR Congestion ControlEach switch computes Explicit Rate (ER)
e.g., ER=C/N, where C is link capacity and N number of ABR connections
Sets minimum of value in ER field (in RM cell) and computed ER
RM cell
RM cell
ER
source destination
ER=min(ER_cell, ER_computed)
Network technology - 53
ABR Congestion Control: Source Behavior Same algorithm for any network (internal) modesIf no congestion ⇒ can increase sending rate (additive)If congestion ⇒ decrease sending rate (multiplicative)At all times: Sending Rate < Explicit Rate (ER)
Actually a bit more complicated (NI: No Increase, ACR: Allowed Cell Rate)If CI=1
Reduce ACR by amount proportional to current ACR but not less than MCR
Else if NI=0Increase ACR by amount proportional to PCR but not more than PCR
If ACR>ER set ACR=max{ER,MCR}
Network technology - 54
ABR versus TCPABR feedback controls rate of transmission
Rate control
TCP feedback controls window sizeWindow control
ABR feedback explicit from switches or destinationECN (Explicit Congestion Notification)
TCP feedback implicit (losses)ATM switch must perform:
Congestion control: Monitor queue lengthFair capacity allocation: Throttle back connections using more than fair share
Active Queue Management schemes similar to aboveRED (Random Early Detection)
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