Transcript CHAPTER 1

1. INTRODUCTION
NETWORK PARADIGMS
Bandwidth
(Mbps)
1000
100
ATM LANS
Gigabit Ethernet
ATM
Fast Ethernet
FDDI
Voice,
Image,
Video,
Data
SMDS
(DQDB)
10
1
Ethernet/
Token Ring/
Token Bus
LAN
Frame Relay
X.25
MAN/WAN
Distance
NETWORKING EVOLUTION
Traditionally
Disjoint Networks
• Voice
Telephone networks
• Data
Computer networks and LAN
• Video Teleconference
Private corporate networks
• TV
Broadcast radio or cable networks
* These networks are engineered for a specific application and
are ill-suited for other applications, e.g., the traditional telephone
network is too noisy and inefficient (capacity limited) for
bursty data communication.
* Data networks are not suitable for voice and video traffic because
they cannot satisfy the time-sensitivity of these traffic types.
GOAL
INTEGRATION
Old Fashioned
WIDE
AREA
LOCAL
PSTN
X.25
Internet
PBX
LAN
Voice
Data
Before Integration
Broadband ISDN
WIDE
AREA
B-ISDN
A Single
Unifying
Technology
LOCAL
Video, Image, Voice
and Data
After Integration
1.6-64 kbps => Narrowband
1.5-45 Mbps => Wideband
> 45 Mbps => Broadband
GOAL
INTEGRATION
(One Network Carrying Multimedia Traffic)
Source
Broadband
Integrated Services Network
(Data, Voice, Video, Still Image)
Destination
• WHY INTEGRATION?
Voice Traffic (based on AT&T data, 1993)
125 to 130 Million Calls/day x 5 min/call x 64 kbps = 28.8 Gbps
= 1 / 1000th of fiber capacity
GOAL
INTEGRATION
Updated statistics for 1998
• Average calls per business day = 272.2 million
• Average calls per day = 226.2 million
• Average length per business call = 2.5 min.
• Average length per consumer call = 8.0 min.
Suppose 200 Million x 24 hours/day x 64 kbps = 12.8 Tbps
Bottomline: Gigantic Capacity of fiber cannot be utilized only by Voice Traffic!!!
FURTHER REASONS:
• Convergence of computer and communications technologies
• Integration could offer efficiencies (lower cost) and
support of new applicatons
• Single network management & maintenance
• No duplication of cables, plants (since one physical network)
=> Less costs
INTEGRATION PROBLEMS
Integration is not easy because different
applications have different performance requirements.
Telemetry
Telecontrol
Telealarm
Voice
Telefax
Hifi Sound
Low Speed
Data
Video Telephony
High Quality Video
Video Library
Video Education
Medium
Speed Data
High Speed
Data
Very High
Speed Data
INTEGRATION PROBLEMS
•Voice
• 64 Kbps (Bandwidth Demand)
• 2 min. (End-to-End Delay on the Average)
(Request-Transfer Cycle)
•Video
• > 140 Mbps (BW Demand)
• 60 min. (E2E Delay on the Average)
•Data
• Extremely variable (BW Demand)
• Extremely variable (E2E Delay on the Average)
e.g., long “telnet” sessions; short “finger” requests
•Still
Image
• 1-50 Mbps (BW Demand)
• 1 Sec. (E2E Delay on the Average)
(Cannot be seen as data traffic because it may have real-time
nature, e.g. medical image retrieval; geographic databases)
ATM NETWORKS
Asynchronous Transfer Mode (ATM):
A new
multiplexing and a new switching
technique to realize the
Broadband Integrated Services Digital Networks (B-ISDN)
Asynchronous: Packet transmission is not synchronized to a global (network) clock!!!
Multiplexing
Defines the means by which multiple streams of information
share a common physical transmission medium.
Sources
Physical Link (Channel)
Mux.
N
Shares single output between many inputs.
Demux.
Sinks
(Destinations)
Demux has one input which must be distributed among outputs.
Multiplexing Techniques
Multiplexing Techniques
Frequency Division
Multiplexing
Synchronous TDM
(STM)
Time Division
Multiplexing
Asynchronous TDM
(Statistical Multiplexing)
(ATM)
STM
STM
Circuit Switching used for Telephone Networks (also for N-ISDN)
(Time Division Multiplexing) (Classical TDM)
Information is transferred with a certain repetition frequency.
e.g.,
8 bits
every 125sec for 64kbps
1000 bits
every 125sec for 8Mbps
From Nyquists Sampling Theorem
4kHz voice signal requires 8000 samples/sec
One 8 bit sample every 125sec = 64kbps
(Golden Rule of Tel. Networks) (DS0 Channel)
Basic unit of repetition frequency is called a TIME SLOT.
8 bits/sample
STM
Time Slot
1
2
...
n
1
2
...
n
...
Periodic Frame (one cycle)
Start of
each Frame
L sec ( bits can be
transmitted)
 Each slot is assigned to a particular call. The call is identified by the position of
the slot. When the user is assigned a slot, it owns a circuit. The user uses the
same slot within consecutive frames.
 If a user is not transmitting data in its own slot, that time slot remains
reserved (nobody else can transmit there).
ATM
ATM — In ATM, the MUX takes a packet (one or more packets) &
appends a header (5 Bytes) and transmits based on statistical
characteristics of the sources. MUX can take according number of cells
from a source & transmit.
Video
1
Header
Voice
11
21
31
2
Data
Physical Trunk
3
Video
4
5
MUX
41
42
ATM
e.g., from a video source 10 cells can be
taken, while from voice source 1 cell because
of their bandwidth demands.
Each packet must have a header (control
information)!!
No SYNCHRONIZATION!! (No network
clock!)
STM vs. ATM
Simple Example:
Sources
1
1 Bit
1 Bit
1 Bit
1 Bit
2
1 Bit
1 Bit
1 Bit
3
1 Bit
4
1 Bit
1 Bit
5
STM
ATM
1
1
2
3 I
I
I
4
1 cycle
5
1
2
2 3
4
5
1 I
2
1 cycle
3
2
3
4
5
3
I 5
4
1 cycle
1
3
5
STM vs. ATM
STM (Synchronous Transfer Mode)
Time Slot
Channel Channel
2
1
...
Channel
N
Channel Channel
1
2
FRAME (Cycle)
Frame
Header
...
Channel
N
FRAME (Cycle)
ATM (Asynchronous Transfer Mode)
Channel
1
Header
Data (cell)
Channel
5
Channel
1
Channel
unused
Channel
7
Channel
5
Channel
2
...
STM vs ATM
(STM)
Users
t0 t1 t2
(ATM)
t3 t4
MUX (Multiplexer)
A
B
C
D
To Network
Wasted Bandwidth
Synchronous
time-division
multiplexing
A1 B1 C1 D1 A2 B2 C2 D2
First Cycle
Asynchronous Statistical
time-division
multiplexing
A1
B1
First Cycle
Data
Second Cycle
B2
C2
Second Cycle
Address
Extra Bandwidth
Available
STM
Advantages



No overhead in
packetization.
Constant repetition of frames
low delay jitters.
Easy to maintain
synchronization between
sender & receiver.
Disadvantages



Limited flexibility
i)
BW (bandwidth)
allocation at 64kbps
modularity
ii)
Inefficient for VBR
(Variable Bit Rate)
traffic
Long connection set-up
delays.
Complex switching system.
ATM Advantages
Flexible BW-Allocation (sources with widely
different bit rates).

i)
ii)
Accommodate bursty sources (e.g., VBR)
Asymmetrical link bandwidths
High BW
Low BW





A wide range multimedia traffic types.
High efficiency due to statistical multiplexing.
Allows Quality of Service (QoS) guarantees.
Simple routing (small buffers).
Simple switching.
ATM Disadvantages

Overhead for cell header.
48 + 5 (bytes) 40Bits overhead  10% overhead






Requests fast switching technology (need new
switches).
Complex scheduling algorithms needed.
Connection set-up & signaling overhead.
Traffic management problem.
Difficult to reroute virtual circuits.
Jitter problem.
SWITCHING TECHNIQUE
Takes multiple instances of a physical
transmission medium containing
multiplexed info streams and
rearrange the info streams between
input & output.
In other words, information from a
particular physical link in a specific
multiplex position is switched to
another output physical link.
SWITCHING TECHNIQUE
Outlets = Outputs
Inlets = Inputs
1
1
2
2
N
…
…
Switch
M
Ports
Fabric
Switch Architectures




Single Bus
Self-Routing (Blocking)
Multiple Bus
Self-Routing (Non-Blocking Queueing
- Internal Queueing
- Input Queueing
- Output Queueing
(Shared Buffer 
Theoretically optimal 
Achieves maximum throughput!)

Actual ATM switches have combination of input, output, and
internal queueing.

The way how these functions are implemented, where in the
switch these functions are located, will distinguish one
switching solution from another.
SWITCHING
Question :
Why could existing switch architectures
(circuit-switches for voice, packet-switches for data )
NOT be used for ATM ?
Reasons :
1. High speed at which the switch must operate
(155-622 Mbps; now on Gigabit levels)
2. Statistical behavior of ATM streams passing
through the switch
3. ATM has small fixed cell size & limited
header functionality
ATM SWITCHING
Combines Space & Time Switching Principles!!
Space (Rerouting) Switching
x
y
z
w
…
Inlets
N
y
w
z
Switch
…
1
2
3
1
2
3
Outlets
x
N
Time Switching
t
Inlet
x
y
z w
Switch
w y
x
z
t
Outlet
ATM Basic Switching Principle
data
Header
d
c
a
a
b
b
b
c
Cell
Translation Table
O1
O2
data
…
e
Switch
…
t
I1
I2
Ij
On
Incoming
Headers
Outgoing
Headers
Incoming
Link
I1
…
Ij
Header /
link
translation
table
p
k
q
y
y
p
r
y
k
Routing Table
Header
Outgoing
Link
a
c
d
O1
p
k
q
On
k
y
r
c
b
e
Header
ATM Switching
• All cells which have header a or (c or d …) on incoming I1 are switched to O1 and
their header is translated (switched) to value p or (k or q …).
• All cells with a header c (or b or e) on link Ij are also switched to outlet On, but
their header gets values k (or y or r).
Remark: On each incoming & outgoing link individually, the values of the header
are unique, but identical headers can be found on different links, e.g., c on link I1
and Ij.
Realization:
• Routing info is contained in the header (label), not explicit address.
• Explicit addressing is not possible because of short fixed size cell.
• A physical Inlet/Outlet, characterized by a physical port number.
• A logical channel on the physical port characterized by a VCI and/or VPI.
Routing Tables
Must be set up in advance (signaling phase)
Either pre-defined or dynamically allocated
ATM SWITCHING
Header
Header
Switch
VPIa VCIb
VPIx VCIy
Input Port P
Routing Info:
Basic Functions
Output Port Q
Input Port
VPI
VCI
Output Port
VPI
VCI
P
a
b
Q
x
y
Space Switching (Routing)
Header Switching
Queueing
Why Queueing?
Suppose 2 cells from different Inlet (I1 & In) arrive simultaneously at ATM switch and are
destined to the same outlet O1.
Thus, they cannot be put on the output or the outlet at the same time  buffering, i.e., to
store the cells which cannot be served.
(No pre-assigned time slots, statistical multiplexing)
Remark: The way these 3 functions are implemented, where in the switch these functions
are located, will distinguish one switching solution from another.
ATM NETWORK
Host
Host
UNI
ATM
Switch
UNI
NNI
ATM
Switch
ATM
Switch
NNI
NNI
ATM
Switch
Backbone Network
ATM CELL STRUCTURE
8
7
6
5 4 3 2
1
1 Octet
2
3
4
5
:
:
:
53
HEADER
(5 octets)
PAYLOAD
(48 octets)
User Network Interface (UNI)
Cell Structure
8 7 6 5 4 3 2 1
1
2
3
4
5
GFC
VPI
VPI
VCI
VCI
VCI
PT
HEC
:
:
53
PR
• Octets are sent in increasing order
 1,2,3 …
• Within an octet the bits are sent
in decreasing order  8,7,6,5,4 ...
Network Network Interface (NNI)
Cell Structure
8 7 6 5 4 3 2 1
1
2
3
4
5
:
PAYLOAD
(48 octets)
:
53
VPI
VPI
VCI
VCI
VCI
PT
HEC
PAYLOAD
(48 octets)
PR
GFC : Generic Flow Control
VPI : Virtual Path Identifier
VCI : Virtual Channel Identifier
PT : Payload Type
PR : Priority
HEC : Header Error Control
ATM Interfaces
UNI / NNI
Host
Host
UNI
ATM
Switch
UNI
NNI
ATM
Switch
ATM
Switch
NNI
NNI
ATM
Switch
Backbone Network
ATM Network Interfaces (Detailed)
(Intra-LATA)
Regional Carriers
private
computer
UNI
Private
Switch
public
UNI
Private
(P-NNI)
NNI
Public
Switch
Public
NNI
Intersystem Switching
Interface (ISSI)
Long Distance Carrier
private
computer
UNI
computer
Private
Switch
B-ICI (Broadband
Inter-Carrier
Interface)
Public
Switch
DXI
Router
(Local Access
& Transp
Area)
Public
Switch
(Inter LATA ISSI)
B-ICI
Digital
Public
Public
Service
Switch
UNI
Unit
DXI: Data Exchange Interface, between packet routers & ATM Digital Service Units (DSU)
ATM Cell

Generic Flow Control (GFC) (4 Bits) (only at UNI)
It provides flow control information towards the network. It allows a multiplexer to control the rate of an
ATM terminal. Currently, no standard. 0’s are used for this field.

Routing Field (VPI/VCI)
24 Bits (8 Bits for VPI, 16 for VCI) at UNI.
28 Bits (12 for VPI, 16 for VCI) at NNI.
VPI/VCI have only local significance only; they identify the next destination.
Remark:
Each physical UNI to support not more than 28=256 VPs. NNI  212 = 4096 VPs. Each VP can support 216 =
65,636 VC on UNI and NNI.

Payload Type Field (PT) (3 Bits)
PT indicates whether the cell contains users data, signaling data or maintenance information.

Cell Loss Priority (CLP) (1 Bit)
CLP indicates the priority of the cell. Lower priority cells are discarded before higher priority cells when
congestion occurs.
Remark:
If CLP =1 Cell has low priority  dropped in heavy load.
If CLP =0 Cell has high priority  not discarded.

Header Error Control (HEC) (8 Bits)
HEC detects and corrects errors in the header. (i.e., single Bit Error Correction or Multiple-Bit Error
Detection). The info field is passed through the network intact, with no error checking or correction. ATM
relies higher protocols for this purpose.
ATM CELL

PAYLOAD TYPE (PT)
First Bit
 0

User Information
First Bit
 1

Network Management or Maintenance Function
Second Bit  Whether CONGESTION has been experienced or not.
Third Bit  known as AAU (ATM-User-to-ATM-User) used in AAL5 to convey
information between end users.
Contents:
[EFCI (Explicit Forward Congestion Indication)]
0 0 0  User Data Cell; Congestion No (EFCI=0); AAU=0
0 0 1  User Data Cell; Congestion No (EFCI=0); AAU=1
0 1 0  User Data Cell; Congestion Yes (EFCI=1); AAU=0
0 1 1  User Data Cell; Congestion Yes (EFCI=1); AAU=1
1 0 0  Segment Operation and Maintenance (OAM) (F5) Cell
1 0 1  End-to-End (OAM) Flow F5 Cell
1 1 0  Resource Management Cell
1 1 1  Reserved for Future Function
Pre-Assigned (Pre-Defined)
(Reserved) Header Values
Cell Types
• Idle Cell: Inserted and extracted by PHY in order to adapt the cell flow
rate at the boundary between ATM layer & PHY layer to the available
payload capacity of the transmission system.
• Valid Cell: has a header with no error or which has been corrected by the
HEC verification process.
• Invalid Cell: has a header that has errors that have not been modified by
the HEC verification process (discarded at PHY layer).
• Assigned Cell: provides service to an application using ATM layer service.
• Unassigned Cell: Not an assigned cell. Does not contain any useful
information.
Cell Types (Cont.)
Source
Destination
Upper Layers
Assigned
Cell
Assigned
Cell
Unassigned
Cell
Unassigned
Cell
ATM Layer
PHY Layer
Upper Layers
ATM Layer
SAP
SAP
PHY Layer
Idle Cell
Valid
Cell
Invalid Cell
Idle Cell
Network
Trash
Cell Types (Cont.)
Difference Idle Cells vs. Unassigned Cells
Unassigned Cells  Visible to ATM & PHY layer.
Idle Cells  Visible only to PHY layer  not to ATM layer.
Unassigned Cells are sent whenever there is no information available
at the sender. It allows full asynchronous operation of sender/receiver.
Idle Cells are inserted by the PHY layer in order to match the
transmission rate to the transmission system or for other PHY layer
purposes.
Octet 1 Octet 2 Octet 3 Octet 4 Octet 5
0...0 0...0 0...0 0...0 0...0
Each octet of Info. field of an “idle cell” is filled with 01101010.
The Size of the ATM Cell
(WHY 48 + 5 = 53 BYTES?)
1. Transmission Efficiency
2. Delays
3. Implementation Complexity
1) Transmission Efficiency
L
 
L
where L is the information size of the packet in bytes and
 is the header size of the packet in bytes.

100
Transmission
Efficiency 90
x
x
80
70 x
60 x
50
8
x
x
x
64
128
x
16
32
L
The longer the info. field, the higher is the efficiency for the same header size.
A header of 4 or 5 Bytes is typical value
for ATM cell.
x
(Assumption: All packets are completely filled.)
2. DELAYS

Packetization Delay (Segmentation)
Transmission Delay (depends on the distance between both endpoints). (Range
4-5sec per km; depends on the transmission medium).


Switching Delay

Queueing (Buffering) Delay

Depacketization Delay (Reassembly)
Queues are necessary to avoid massive loss of cells. Delay varies with the load of
the network and is determined by the behavior of queues.
Conclusions:
* The queueing delays increase with the size of information field.
* The end-to-end delay must be below 24 ms to avoid ECHO problems for voice
traffic!!!
EXAMPLE: [Packetization Delay (Segmentation)]
Transmission of 64 kbps voice traffic over ATM.
Voice signal is sampled 8000 times per second, which gives rise to 8000
bytes/sec or 1 byte every 125 usec.
* If the packet size is 16 bytes, then it will take (16*125) usec or 2ms to fill up
a packet.
* If the packet size is 64 bytes, then it will take (64*125) usec or 8ms to fill up
a packet.
So the smaller the packet size, the less the delay to fill up a packet.
The packetization delay could be kept small if a packet is partially filled;
however, this will lead to under-utilization of the network capacity.
EXAMPLE: [Time for Header Conversion]
The longer the packet, the more time the switch has to do the header
conversion.
Consider an ATM switch with OC-3 capacity, i.e., 155 Mbps.
If the cell size is 53 bytes, then a maximum of about 365 566 cells can
arrive per second. This translates to 2.7 usec per cell, i.e., assuming that
cells arrive back-to-back, a new cell arrives approximately every 2.7
usec. This means that the switch has 2.7 usec available to carry out the
header conversion.
Suppose a cell size of 10 bytes. A maximum of about 1 937 500 cells can
arrive per second, i.e., if cells arrive back-to-back, a new cell arrives
approximately every 0.5 usec. The switch has then only 0.5 usec for the
header conversion.
3) Implementation Complexity
Two parameters play a role in determining the complexity of a
system:
• The speed [Transmission (Processing) Time (P)
= (Cell Size/Data Rate) ]
• The number of required bits (MEMORY: M))
= The number of cells (BUFFER SIZE IN CELLS)
multiplied by the (CELL SIZE).
Tradeoff  Memory Size and Processing Speed
To guarantee a certain limit on the cell loss ratio, a number of
cells must be provided per queue. This number is independent of
the cell size. So the larger the cell size, the larger the queue in bits
will be (e.g., doubling the cell size will also double the memory
requirements).
IMPLEMENTATION COMPLEXITY
On the other hand, for every cell, the header must be processed. This processing
must be performed in one cell time, so the longer the cell size, the larger the
available time and the lower the speed requirements of the system.
In Figure we show the speed and memory size in function of the cell size, if the
system operates at 150 Mbps and if the queue is dimensioned for 50 cells (the
header is 4 Bytes).
P
(processing
time per cell
2
in s)
M
(memory
size in
bits)
P
M
64000
4
32000
8
16
16000
32
8000
16
32
64
128
256
cell size
Explanation of the FIGURE:
Assume 50 Cells; Cell Size: 16 + Header: 4 = 20 Bytes (Low)
Cell Size: 256 + Header: 4 = 260 Bytes (High)
Memory (M) =
= Cell Size * Buffer Size in Cells = 20*50 = 1000 Bytes = 8000 Bits (Low)
= 260*50=13000 Bytes=104000 Bits (High)
Processing Speed (P) =
Transmission Time = {Cell Size}/{Data Rate}=160 Bits/{150*10^6 bps} ~ 1 musec (L)
=2080/{150*10^6 bps} ~ 13.8 musec (H)
1. We see that for a cell of 16 bytes, we need only about 8000
bits for the memory, but the header processing of each cell
must be performed in less than 1 sec.
2. For a cell of 256 bytes, we need already more than 64,000
bits for a single queue. But we have about 15 sec for the
header processing of a single cell.
3. However, as seen in Figure, the speed is not the most critical
issue, since in 1 sec (in case of 16 Bytes) HIGH processing
can be achieved; so the limiting factor is the memory space
requirement.
FINALLY – THE RESULT:
Contradicting factors are contributing to the choice
of the cell size. However, a value between 32 and 64
bytes is preferable.
Europe was in favor of 32 bytes (because of the
requirement for echo cancellers for voice) where US
and Japan were in favor of 64 bytes because of higher
transmission efficiency.
Finally ==> a compromise of 48 Bytes reached at
CCITT meeting in June 1989.
Variable vs. Fixed Length Packets
Facts to consider in decision
– Transmission Bandwidth Efficiency
– Achievable Switching Performance
(i.e., the switching speed vs.
complexity)
– The Delay
Transmission Bandwidth Efficiency
The number of Informatio n Bytes ( L)

L  The number of Overhead Bytes

Fixed Packet Length,
F 
where
F
X
X
(L  H )
L
X
L
H
z
Number of useful information in bytes
Information Size of the Packet in bytes
Header size of the packet in bytes
represents the smallest integer larger than or equal to
z

This efficiency is optimal for all
information units which are multiples of
the packet information size, i.e.,
X
L  X
L

Optimal Case (Substitute the above
value into the prev. one)
F
OPT
L

LH
%
100
Transmission
Efficiency
V
90
F
80
F
OPT
70
60
50
40
30
Fixed Length
20
Variable Length
10
48

F
96
144
192
240
288
336 X[number of useful
information bytes]
has a sawtooth shape (Opt. L=48;H=5)




The efficiency depends very much on the
useful information bytes to be transmitted
If the number of useful information bytes is
large, the optimal achievable efficiency is
approached
Only if the number of useful information
bytes is small, this efficiency is rather low.
So, the distribution of the number of useful
information bytes to be transmitted largely
determines the efficiency.
Different Applications
Voice:
– Since voice is a CBR (Constant Bit Rate) service, we
can take the option at the sending terminal only to
transmit a packet when it is completely filled
(therefore, introducing a packetization delay).
– So, the efficiency can reach the optimal achievable
value, if packets are completely filled which then
puts limitation on the packet size in order to limit
the packetization delay.
Different Applications
Video:
• Where fixed bit rate video coding techniques are
used, this service can be considered as a CBR
service, again reaching the optimal efficiency
• Where variable bit rate video coding techniques
are used, it may occasionally happen that packets
are not completely filled.
• However, a typical video image contains
thousands of bytes, so the optimal achievable
efficiency will be very closely approached.
Different Applications
Data:
• Distinguish low speed and high speed data.
• Low speed applications, e.g., keyboard input:
small information units must be considered, so
the efficiency is rather small (around 10%)
• High speed applications, e.g., file transfer, image
transfer for CAD, etc: the very long information
field (e.g., file, image, etc) can be sent into fixed
packets giving rise to an efficiency very close to
the optimal efficiency,
e.g., for 1000 bytes, the efficiency is 89%,
instead of an  FOPT = 90.5% in the figure)
Remark:
•Since traffic in a broadband network will
largely be composed of video, high speed
data, and voice, the overall transmission
efficiency approaches the optimal, even if
fixed length packets are used.
Variable Length Packets
Here the overhead is determined by the header and the
flags to delimit the packets, e.g., 6 bits in HDLC, plus in
addition, some stuffing bits to ensure proper flag
recognition.
Also, add to the header, a length indicator, determining the
length of the packet
X
v 
X  H  hv
where hv is the specific packet header overhead
mentioned above


In Figure, we assume 5 bytes H and 2
bytes of hv . We see the transmission
efficiency can be very high (close to
100%) for very long packets
Remark: For practical reasons such as
buffer dimensioning, delay, the max
variable length packets must be limited
to a certain threshold.
CONCLUSIONS
– The transmission efficiency of variable
length packets is better than that of fixed
length packets
– However, in broadband networks, this gain
of transmission efficiency is rather limited
since the main traffic constituting
broadband services will consist of a
combination of voice, video, bulk data
transfer.
SWITCHING SPEED AND COMPLEXITY
Two factors for complexity of ATM switch implementation
• Speed of Operation
• Queue Memory Size Requirements
A) Speed of Operation
Header processing
Let us assume that header functions are the same for fixed and variable
length case.
For fixed length packets, available time to perform all functions is fixed,
(e.g., 2.8 sec in the 48 + 5 bytes solution at 150 Mbps.)
For variable length packets, the available time depends on the worst case
(i.e., the smallest packet), so the speed requirements are much higher (e.g.,
to perform same functions for a 5 + 5 byte packet at 150 Mbps only 553 ns
are available.).
SWITCHING SPEED AND COMPLEXITY
B) (Queue) Memory Management
Fixed Length Packets  Memory management system can assign
memory stocks with the same size, namely, the same size of the
packets. This operation is simple and management of free memory list
is easy.
Variable Length Packets  Memory management system must be able to
assign memory stocks in multiples of bytes so that algorithms like “find best
fit”, “find first fit”,…, can be used. Memory management is complex.
CONCLUSION: Regarding “speed of operation” and “queue memory size”
“Fixed Length Packet Size” is preferred!!!!
In 1988, fixed packet size has been accepted for ATM.
FACTS on ATM TECHNOLOGY
* Provides a way of linking a wide range of devices (from
telephones to computers) using the seamless network
* Also removes the distinctions between LAN, MAN and
WAN)
* Combines packet and circuit switching
* It can be sent on any physical media (copper, fiber). Wide
range of transmission speed.
* Scalable
* Allows QoS parameters (voice, video, still image, etc.)
* Supports any type of traffic
* Allows sources of different bit rates
* Uses fixed size packets called “CELLS”.
FACTS on ATM TECHNOLOGY
*
*
*
*
*
No error protection or flow control on hop base
Header functionality is reduced
Information field is very small.
Operates in Connection-Oriented Mode
Supports Connectionless Mode
HISTORY of ATM
1980
Narrowband ISDN adopted
Early 80’s
Research on fast switches
1985
B-ISDN Study Group formed
1986
ATM approach chosen for B-ISDN
1987
ATM is standardized by ITU-T
June 1989
Cell Size (48+5) chosen
Oct. 1991
ATM Forum formed
July 1992
UNI V2 released by ATM Forum
October 1999
AAL Layers finalized
1993
First Gen. ATM Switches
October 1995
Traffic Management Finalized
1996
Second Gen. ATM Switches
1999
Third Gen. ATM Switches
Currently: Heavily used in
Backbone Networks (ISP: Internet Service Providers);
ADSL (Residential Access Networks)
Passive Optical Networks (PONs) deployed in Residential Access Networks
ATM FORUM TECHNICAL COMMITTEES
*
*
*
*
*
*
*
*
*
*
*
Traffic Management
Signaling
Physical Layer
Testing
B-ICI
LAN Emulation
SAA (Service Aspects & Applications) (VTOA)
Network Management
P-NNI
Multiprotocol over ATM (MPOA)
Residential Broadband
ATM NETWORKS
ATM
Switch
End
Sources
ATM
Switch
End
Destinations
ATM
Switch
(Native ATM )
PSTN
End
Sources
LAN/MAN
ATM
Switch
ATM
Switch
ATM
Switch
(Non-native ATM )
End
Destinations
Internet