Transcript ECE544Lec3_15

ECE544: Communication Networks-II,
Spring 2015
Profs. Raychaudhuri & Reininger
Lecture 3
Includes teaching materials from J. Kurose, L. Peterson and ATM Forum tutorials
Today’s Lecture
• Switched Networks
– Switching Concepts
– Ethernet Switches
• Learning bridge
• Spanning tree
• Multicast
– Asynchronous Transfer Mode (ATM) Network
•
•
•
•
•
Overview
Virtual Circuit Switching
Virtual Circuit and Virtual Path
ATM AAL
ATM Quality of Service (QoS)
Intro to Switching
•
•
•
Build a large network by interconnecting a number of switches
Easily add new hosts
Switching Techniques
– Datagram or connectionless (Ethernet)
• Unique address
• No need to setup connection
– Virtual circuit or connection-oriented (ATM)
• Set up connection and maintain connection state
– Source routing
• Source specify the whole or partial route to the destination
Ethernet Hub
• Hub is just a repeater
– Receive signal from one port and broadcast to all other ports
• Extends max distance between nodes, but collisions
are propagated
– Individual segment collision domains become one large
collision domain
• Cannot interconnect different LAN technologies, e.g.
10BaseT & 100BaseT
hub
hub
hub
hub
Bridges/LAN switches
• Link layer device
– stores and forwards frames
– examines frame header and selectively forwards frame based
on MAC dest address
– when frame is to be forwarded on segment, uses the
corresponding MAC to access segment (e.g. CSMA/CD for
Ethernet)
Switch
hub
hub
hub
Bridges/LAN switches (Cont.)
• Interconnect multiple LANs, possibly even
support different IEEE 802.x types, e.g. 802.3
and 802.5, 802.11, but NOT 802.x with ATM
Tokenring
Bridge
IP
IP
Bridge
LLC
802.3 MAC
LLC
LAN
802.3 MAC
LLC
802.5 MAC
LAN
802.5 MAC
Ethernet Hubs vs. Ethernet Switches
• An Ethernet switch is a packet switch for Ethernet
frames
• Buffering of frames prevents collisions
• Each port is isolated and builds its own collision domain
– Break subnet into LAN segments
• Host can directly connect to switch, no collision, full duplex
• An Ethernet Hub does not perform buffering:
• Collisions occur if two frames arrive at the same time.
Hub
Switch
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
HighSpeed
Backplane
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
Input
Buffers
Output
Buffers
A Switched Enterprise Network
Internet
Router
Switch
Forwarding
Forwarding Table
• Which port to forward a frame?
– Use forwarding database/table
< MAC address, port, Time-to-Live
(TTL)>
• How to build the forwarding table???
– A routing problem
Host D
Host E
Switch 1
Destination
Port
Time-to-Live
(TTL)
A
3
2
B
0
4
C
3
2
D
3
4
E
2
5
F
1
1
G
0
4
H
0
5
0
1
Host C
2
3
3
2
Host F
1
Switch 2
0
0
Host A
Host G
1
2
Host H
Host B
3
Switch 3
Transparent Bridges
Three parts to transparent bridges:
(1) Learning of Addresses
(2) Forwarding of Frames
(3) Spanning Tree Algorithm
Self Learning (Learning Bridges)
•
Forwarding tables entries are set automatically with a simple heuristic:
The source address field of a frame that arrives on a port tells which
host is reachable from this port.
– When a frame received, switch “learns” location of sender
– records sender/location pair in forwarding table with TTL = MAX_TTL
• TTL reset to MAX_TTL every time a frame with the same source
addr is received to refresh the existing table entry
• Entry removed when TTL counts down to 0
Src=x, Dest=y
Port 1
Port 4
x is at Port 1
y is at Port 5
Port 2
Port 5
Port 3
Port 6
Src=x, Dest=y
Frame Forwarding/Filtering
When switch receives a frame:
index forwarding table using MAC dest address
if entry found for destination
then{
if dest on the same port from which frame arrived
then drop the frame (filtering)
else forward the frame on port indicated
}
else flood
Forward on all but the port
on which the frame arrived
Example
•Consider the following packets:
(Src=A, Dest=F),
(Src=C, Dest=A), (Src=E, Dest=C)
•What have the bridges learned?
Bridge 2
Port1
Bridge 2
Port2
LAN 1
A
B
Port2
Port1
LAN 2
C
LAN 3
D
E
F
Danger of Loops
• Consider the two LANs that are
connected by two bridges.
• Assume host n is transmitting a
frame F with unknown destination.
What is happening?
F
• Bridges A and B flood the frame
Bridge A
to LAN 2.
F
• Bridge B sees F on LAN 2 (with
unknown destination), and copies
the frame back to LAN 1
• Bridge A does the same.
• The copying continues
LAN 2
F
Bridge B
F
LAN 1
F
host n
Spanning Trees / Transparent Bridges
• A solution is to prevent loops in the
topology
LAN 2
d
Bridge 4
Bridge 3
• IEEE 802.1d has an algorithm that
organizes the bridges as spanning
tree in a dynamic environment
Bridge 1
LAN 5
– Note: Trees don’t have loops
Bridge 5
• Bridges that run 802.1d are called
transparent bridges
LAN 1
Bridge 2
LAN 3
LAN 4
Spanning Tree Protocol (STP)
• Each bridge has a unique ID (MAC addr + priority level)
• Select the bridge with the smallest ID as the root of the spanning
tree, called “root bridge”
– All the ports on the root bridge are active (forwards the frames)
• Each bridge determines the minimum-cost path from itself to the root
and nodes which of its port is on the path (root port)
– Link cost: the cost of traversing a single network segment (link)
– Path cost: the sum of the costs of the segments (links) on the path
• an administrator can configure the cost of traversing a particular
segment (link)
• E.g. set the cost for every segment to 1, the path cost is a count of
the number of bridges along the path.
– Root path cost: the cost of the minimum-cost path from this bridge to
the root
– Root port: the port connecting to the minimum-cost path on this bridge
– Breaking ties: When multiple paths from a bridge are min-cost paths,
choose the path using the neighbor bridge with the lower bridge ID.
– If the multiple ports connects this bridge and the neighbor bridge on the
root path, choose the port with the lowest port ID as the root port.
Spanning Tree Protocol (Cont.)
• Select a single “designated bridge” and its designated
port on each LAN segment
– Designated bridge: the bridge on that LAN segment with the
minimum-cost path to the root. Only designated bridge allowed to
forward frames to and from this LAN segment.
• If two or more bridges have the same root path cost, choose the one
with the lowest bridge ID
– Designated port: the port connecting the designated bridge to
this LAN segment
• If the designated bridges has two or more ports attached to this LAN,
choose the port with the lowest port ID
• Any port that is not a root port or a designated port is
blocked.
Spanning Tree Protocol (Cont.)
• Bridges exchange messages to configure the bridge (Configuration
Bridge Protocol Data Unit, CBPDUs) to cut the loop and build the
tree.
– Source addr: port MAC addr, Dest. addr: STP multicast address
– <sending bridge ID, root bridge ID, root path cost>
• At the beginning, each bridge considers itself to be the root, sends
CBPDU identifying itself as root
• Upon receiving a CBPDU, check if the new path is better
–
if better, update its STP record, forward the message after updating
the root path cost in the message
– After stabilization, only the root bridge generates new CBPDUs
regularly, others stops generate CBPDUs once learning it is not a root
• From a non-root port, receives a CBPDU indicating it is not the
designated bridge for that segment, goes to blocking state
• BPDU is still received in blocking state.
Spanning Tree Protocol
Example
A
B
K
B3
B5
B7
1.
2.
3.
4.
C
D
B2
F
E
5.
B1
6.
H
G
7.
B4
B6
J
I
B3 receives (B2,0,B2)
Since 2<3, B3 accepts B2 as root
B3 adds 1 to distance advertised
by B2 and sends (B2,1,B3) to B5
Meanwhile B2 accepts B1 as
root because it has lower ID and
sends (B1,1,B2) to B3
B5 accepts B1 as root and sends
(B1,1,B5) towards B3
B3 accepts B1 as root and it
notes that both B2 and B5 are
closer to the root than it is.
B3 stops fowarding messages on
both its interfaces
Virtual LAN
•
•
•
•
•
Group the stations in a broadcast domain, regardless of their physical
location.
A VLAN ID (VID) in the frame
A frame is not forwarded/broadcasted from one VLAN to another VLAN
Each VLAN establishes its own spanning tree
Assign a port to one or multiple or all VLANs (static or dynamic)
Host B
Host A
VLAN 100
VLAN 100
B2
B1
VLAN 200
Host C
VLAN 200
Host D
Asynchronous Transfer Mode
(ATM) Network
•
•
•
•
•
•
•
Overview
Virtual Circuit Switching
Virtual Circuit and Virtual Path
ATM AAL
ATM Quality of Service (QoS)
Leaky Bucket Algorithm
Switch Implementation
ATM Introduction
• 1990’s standards for high-speed (155Mbps to 622
Mbps and higher) Broadband Integrated Service
Digital Network (BISDN) architecture
• Goal: integrated, end-end transport of carry voice,
video, data
• meeting timing/quality of service (QoS) requirements
of voice, video (versus Internet best-effort model)
• “next generation” telephony: technical roots in
telephone world
• packet-switching (fixed length packets, called “cells”)
using virtual circuits
ATM Vision
The Ultimate Integrated Services Network
Voice
Voice
Data
Voice
Video
ATM
Network
Data
Data
Video
Video
• ATM network moves cells (fixed length packets) with low delay
and low delay variation at high speeds
• Devices at ends translate (e.g., segment and reassemble)
between cells and original traffic
ATM Basic Concepts
• Negotiated Service Connection
– End-to-end connections,
called virtual circuits
– Traffic contract
• Virtual circuit based switching
– Dedicated capacity
• Cell Based
– Small, fixed length
A
Negotiated Service
Connection
Traffic Contract
 Parameters
 Traffic Characteristics
 Peak Cell Rate
 Sustainable
Cell Rate
 Quality of Service
 Delay
 Cell Loss
Virtual Connection 1-QOS A
Virtual Connection 1-QOS B
Virtual Connection 1-QOS b
ATM System Architecture
Voice
Cell
Data
Cell
Video
Cell
A
The ATM Cell
Header
Payload
5 Bytes
48 Bytes
• Small Size (low delay, but high overhead)
– 5 Byte Header
– 48 Byte Payload
• Fixed Size (easy switch implementation, but
padding overhead)
• Header contains virtual circuit information
• Payload can be voice, video or other data types
A
ATM Adaptation Layer (AAL)
AAL
Types
1
Circuit Emulation
-Constant Bit Rate (CBR)
2
Low Bit Rate Voice (Real Time)
48 Bytes
-Variable Bit Rate (VBR)
3/4
5
Time Invariant Data
“Simple” Data
• Only at edge of ATM network (end system)
• Roughly analogous to Internet transport layer
• Provides mapping Of applications (IP or native ATM
applications) to ATM service of the same type
• Segments/Reassembles into 48 Payloads
• Hands 48 Byte Payloads To ATM Layer
A
ATM Layer
48-Byte
Payloads
From AAL
5-Byte Header
}
53-Byte Cell
To Physical Layer
Header Contains Virtual
Path
and Channel Identifiers
• Adds/Removes Header To 48 Byte Payload
– Header Contains Connection Identifier, multiplexes 53 Byte
cells into virtual connections,
• ATM’s “Network” layer
– Transport cells across ATM network (analogous to IP network
layer, but very different strategy and services than IP
network layer)
– Signaling, cell switching, routing
A
Physical Layer
Speed Matching
and Framing
Cable
Plants
Uses Existing Media
Wide Range of
Speeds
Twisted Pair
Coax
Transmission
Frame
Fiber
LAN, MAN, WAN
-Multimode
Compatibility
-Single Mode
A
ATM PHY: Two Sublayers
Transmission Convergence
Sublayer
Physical Layer Medium Dependent
Sublayer
•
•
Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD
sublayer below
– Specific to the PMD
– Cell delineation
– Cell rate decoupling, inserting idle (empty) cells when no data cells to send
(with “unstructured” PMD sublayer)
Physical Layer Medium Dependent Sublayer (PMD): depends on physical
medium being used
– Probably use existing standards and technology
– Medium, line code, connectors
ATM Physical Layer (Cont.)
Physical Medium Dependent (PMD) sublayer
• SONET/SDH: transmission frame structure
(like a container carrying bits);
– bit synchronization;
– bandwidth partitions (TDM);
– several speeds: OC3 = 155.52 Mbps; OC12 =
622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6
Gbps
• TI/T3:transmission frame structure (old
telephone hierarchy): 1.5 Mbps/ 45 Mbps
• unstructured: just cells (busy/idle)
155 Mbps, SONET STS3c/SDH
STM-1
270 columns
9
R
o
w
s
...
Maintenance
and
operations
1 Synchronous
Payload Envelope
(1 column of overhead)
125 msec
9 bytes
• 9 ´ 260 ´ 8/125 msec = 149.76 Mbps
payload
A
ATM: network or link layer?
• Vision: provide the end-to-end transport:
“ATM from desktop to desktop”
– ATM is network technology
• Reality: used to connect IP backbone routers
– “IP over ATM”
– ATM as switched link layer, connecting IP routers
ATM Interfaces
Private
NNI
Private
UNI
Public
NNI
Metropolis Data
Services Inc.
ATM
DXI
Public
UNI
FUNI
FUNI
UNI
NNI
B-ICI
ATM DXI
FUNI
User Network Interface
Network Node Interface
BISDN Inter-Carrier Interface
Data eXchange Interface
ATM Frame Based UNI Interface
B-ICI
Country Wide Carrier
Services
D
ATM UNI Cell
7
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path Identifier
Virtual Path Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
5 Byte Header
CLP
Header Error
Check
Payload
(48 bytes)
CLP = Cell Loss Priority
48 Byte Payload
ATM NNI Cell
7
6
5
4
3
2
1
0
Virtual Path Identifier
Virtual Path Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
5 Byte Header
CLP
Header Error
Check
Payload
(48 bytes)
CLP = Cell Loss Priority
48 Byte Payload
7
Generic Flow Control
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
CLP
Header Error
Check
Payload
(48 bytes)
•
•
•
•
Used for UNI only - Not NNI
Currently undefined
Set to 0000B
Proposed future uses
– Flow control
– Shared media multiple access
B
7
Payload Type Identifier (PTI)
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
• Bit 3: Used to discriminate data cells
from operation, administration,
maintenance cells.
• Bit 2: Used to indicate congestion in
data cells (Bit 3 = 0)
Payload Type
Identifier
CLP
Header Error
Check
Payload
(48 bytes)
– Set by Switches
– Source and Destination Behavior Defined for
Available Bit Rate Flow Control VCC’s
• Bit 1: Carried transparently end-to-end
in data cells
– Used by AAL5
C
7
Cell Loss Priority
• Cells with bit set (CLP =1) should be discarded
before those with bit not set (low priority)
• Can be set by the terminal
• Can be set by ATM switches for internal network
control
– Virtual channels/paths with low quality of
service
– Cells that violate traffic management contract
• Key to ATM Traffic Management
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
Header Error
Check
Payload
(48 bytes)
CLP
7
Header Error Check
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
• Header error control
– Detection mode:
• Protects header only (all five bytes)
• Discards cell when header error
– Correction mode (optional): Correct 1 bit errors else
discard when error detected
• Reduced cell loss in face of single bit errors
• Reduced error detection for multiple bit errors
• Cell delineation for SONET, SDH, etc...
• Recalculated link-by-link because of VPI/VCI value changes
Payload Type
Identifier
CLP
Header Error
Check
Payload
(48 bytes)
B
Why 53 Bytes?
64 + 5
32 + 4
48 + 5
• Compromise reached in ITU-TS
Study Group XVIII in June 1989
Queuing Delay Advantage of Small Cells
100 byte message
100 other active
connections
45 Mbps
•
•
•
Max
Delay
(ms)
12
10
8
6
4
2
0
High overhead
Wait for other cells
Just fits in one cell
1
50
100
150
200
Payload (bytes)
250
300
Delay and delay
variation are
small for small
messages e.g.,
a digitized voice
sample
But high header
overhead
A
Packetization Delay
Advantage of Small Cells
Percent Overhead and Packetization Delay
for 64 Kbps Voice
Delay
80
10
8
Overhead
60
6
40
4
20
2
0
0
80
0
20
40
Payload (Bytes)
60
Delay (ms)
% Overhead
100
Virtual Circuit Switching
•
Establish connection (virtual circuit) before any data is sent
– Permanent Virtual Circuit (PVC), manually or setup signaling
initiated by the network administrator,
• Long lasting connections, e.g. “permanent” coonections for two IP
routers
– Switched Virtual Circuit (SVC), setup using signaling by one of
the hosts
–
• Dynamically set up on per-call basis
Negotiate QoS (bandwidth, delay, etc)
– link,switch resources (bandwidth, buffers) may be allocated to VC: to get
circuit-like performance
•
•
•
Each switch on source-destination path maintains “connection state”
for each passing connection
– Incoming interface, incoming virtual circuit identifier (VCI),
outgoing interface, outgoing VCI, reserved bandwidth, buffer,
delay…
Tear down
Forwarding: each cell/packet carries VC identifier (not destination ID)
Virtual Paths and Virtual
Channels
Physical Link
7
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
Header Error
Check
Payload
(48 bytes)
Virtual Path
Virtual Channel
• Bundles of Virtual Channels are switched via Virtual Paths
– Better scalability (i.e. more capable of growing to large
numbers of circuits)
CLP
ATM VCs
• Advantages of ATM VC approach:
– QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
• Drawbacks of ATM VC approach:
– Inefficient support of datagram traffic
– one PVC between each source/dest pair) does not
scale (N*2 connections needed)
– SVC introduces call setup latency, processing
overhead for short lived connections
Switched Virtual Circuits
Signalling Channel
(VPI/VCI = 0/5)
Signalling Channel
(VPI/VCI = 0/5)
Call
Processing
ATM Switch
• Switch and terminal exchange signalling messages using
the predefined signalling channel, VPI/VCI = 0/5
B
Permanent Virtual Circuits
VPI/VCI
VPI/VCI
VPI/VCI
VPI/VCI
Network
Management
System
• Long setup time (especially with human intervention)
means that connections are left active for long periods of
time e.g., days, weeks
• VPI/VCI tables setup in terminals and switches
B
7
Virtual Connections
76
Video
Video
42
4
Data
88
Voice
52
Data
22
Video
1
37
Video
78
Voice
5
2
6
Connection Table
Video
Data
Video
Voice
Port
1
1
2
2
VPI/VCI
0/37
0/42
0/37
0/78
Port
3
5
6
4
5
4
3
VPI/VCI
0/76
0/52
0/22
0/88
2
1
0
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
Header Error
Check
Payload
(48 bytes)
3
37
6
Generic Flow
Control
CLP
Call Control Signalling
Call control protocol is used to establish,
maintain, and clear virtual channel
connections between a user and network
User
Call Control
Signalling
Network
UNI
UNI or NNI
Call Control
Signalling
Virtual
Channel
Connections
Interface
UNI or NNI
Setting Up a Call - 1
A wants to communicate with B
A
B
Setup
Call
Proceeding
• Setup message
– Call reference
– Called party address
– Calling party address
– Traffic characteristics
– Quality of service
• Call proceeding message
– Call reference
– VPI/VCI
B
Setting Up a Call - 2
Setup
Call
Proceeding
• Internal network
processing
– Resource availability
checking
– Virtual channel or path
routing
– Function of the
Network Node
Interface (NNI)
B
Setting Up a Call - 3
Setup
Setup
Call
Proceeding
Call
Proceeding
• Setup message
– Call reference
– Called party address
– Calling party address
– Traffic characteristics
– Quality of service
– VPI/VCI
Call Proceeding
Call reference
Called user deciding
to accept call
B
Setting Up a Call - 4
Setup
Setup
Call
Proceeding
Call
Proceeding
• Connect message
– Call reference
– Indicates call acceptance
• Connect Acknowledge
– Call reference
Connect
Connect Ack
B
Setting Up a Call - 5
Setup
Setup
Call
Proceeding
Call Proceeding
Connect
Connect
Connect Ack
Connect Ack
• Calling party informed that call is available for user
information exchange
B
Bandwidth Negotiation
UNI
Setup
(20 Mb/s)
Connect
(10 Mb/s)
NNI
Setup
(15 Mb/s)
Connect
(10 Mb/s)
UNI
Setup
(10 Mb/s)
Connect
(10 Mb/s)
NNI
Cell Format
7
6
5
4
3
2
1
0
Virtual Path Identifier
Virtual Path Identifier
Virtual Path Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
Header Error
Check
Payload
(48 bytes)
CLP = Cell Loss Priority
CLP
• Supports 212 Virtual Paths
• Supports virtual connection
routing
– Distribution of topology
information
– Distribution of resource
availability information
• Public version being
standardized by ITU TS
• Private version specified by
ATM Forum Technical
Working Group
C
Address to End Station
ATM End
System
Address
(AESA) Format
Native
E.164 or
AESA
Public
ATM
Network
Private ATM Switch
Private
UNI
Public
UNI
ATM Service Categories
•
•
•
•
•
Constant Bit Rate (CBR)
– Continuous flow of data with tight bounds on delay and delay variation
Real-Time Variable Bit Rate (rt-VBR)
– Variable bandwidth with tight bounds on delay and delay variation
Non-Real-Time Variable Bit Rate (nrt-VBR)
– Variable bandwidth with tight bound on cell loss
Available Bit Rate (ABR)
– guarantee minimum
– Flow control on source with tight bound on cell loss
Unspecified Bit Rate (UBR)
– No guarantees (i.e., best effort delivery)
Service
Model
Guarantees?
Congestion
feedback
Bandwidth
Loss
Order
Timing
CBR
Constant rate
yes
yes
yes
No congestion
VBR
Guaranteed
rate
yes
yes
yes
No congestion
ABR
Guaranteed
minimum rate
no
yes
no
yes
UBR
none
no
yes
no
no
AAL1: Adaptive Clock Method
Received Cells
Reconstructing the bit stream
Continuous Bit
Stream
Speed up bit
Substitute Cells clock
Slow down bit
clock
Water Mark
• AAL1: for constant bit rate (CBR) services, e.g. circuit
emulation
• Bit stream rate is independent of ATM network and
(theoretically) can be any value
• Cell delay variation is critical to buffer sizing and bit clock jitter
B
AAL2
• AAL2: variable bit rate (VBR) services, e.g. MPEG video
• Emulation small payload to reduce packetization delay
– One cell can carry data from multiple users
User 1
User 2
User 3
User 1
User 3
ATM Cell
Cell Header
(5 Octets)
AAL2 Header
(3 Octets)
AAL2 Payload
(variable)
B
AAL3/4
0 - 65535
Protocol Data Unit (PDU)
Bytes
Data
Common Part Convergence Sublayer (CPCS)
4
Protocol Data Unit (CS-PDU)
CPCS
Header
CPCS
Trailer
2
2
Bytes
Bytes
4
44
Segmentation and
Reassembly (SAR)
sublayer
•
T
Y
P
E
S
E
Q
M
I
D
L
E
N
User Data
AAL
Header
AAL3/4 for data (e.g. IP datagrams)
C
R
C
2
T
Y
P
E
S
E
Q
M
I
D
AAL
Trailer
44
2
L
E
N
User Data
C
R
C
2
2
•
•
•
•
•
•
44 Bytes of Data per Cell
Type: the first, last, middle or single cell
SEQ: sequence # of the cell (to detect the cell loss)
Message Identifier (MID) multiplex several PDUs onto a
single virtual connection
Len: length = # of bytes of PDU in the cell
CRC-10: Checking per Cell
44
T
Y
P
E
S
E
Q
M
I
D
Padding
L
E
N
C
R
C
data
D
7
6
5
4
3
2
1
0
Generic Flow
Control
Virtual Path
Identifier
Virtual Path
Identifier
Virtual Channel
Identifier
AAL 5
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload Type
Identifier
CLP
Header Error
Check
0 - 65535
Payload
(48 bytes)
Bytes
Data
PDU
Error detection fields
0-47 2 2 4
CS-PDU
Pad 0
L
e
n
Bytes
C
R
C
48
• 48 Bytes of Data per Cell
• Uses a paidload type
identifier (PTI) bit in the
ATM header to Indicate
Last Cell
• Only One PDU at a Time
on a Virtual Connection
• CRC-32: Per PDU CRC
for error checking
0
...
48
1
Last cell flag
Not drawn to scale
C
ATM Addressing
• Public networks
– E.164 numbers (telephone numbers)
– Up to 15 digits
• Private networks
– 20 byte address
– Format modeled after OSI NSAP (Network Service
Access Point)
– Mechanisms for administration exist
– Hierarchical structure will facilitate virtual
connection routing in large ATM networks
– MAC address will be encapsulated within NSAP
B
ATM End System Address (AESA)
AESA
Format
Network-Supplied
Data Country Code (DCC)
39 DCC
HO-DSP
End
SystemSupplied
End System ID
SEL
SEL
End System ID
SEL
SEL
End System ID
SEL
International Code Designator (ICD)
47
IDC
HO-DSP
E.164 Private Address
45
Private
UNI
E.164 Number
HO-DSP
Selector (Not used by Network for Routing)
Higher Order Domain- Specific Part (HO-DSP)-routing field
•
IP over ATM
Classic IP
•
– Layer 3 “networks”
– connect LANs
– MAC (802.3) and IP addresses
IP over ATM
– Replace LAN segments with
ATM network
– ATM addresses, IP addresses
Appl
Transport
IP
AAL
ATM
Phy
IP
Eth
Phy
AAL
ATM
Phy
ATM
Phy
IP
AAL
ATM
Appl
Transport
IP
Eth
Phy
ATM
Phy
Phy
Eth
Phy
Appl
Transport
IP
Eth
Phy
IP-over-ATM (Cont)
• Packet journey in IP-over-ATM network
– at Source Host (IP-over-ATM router):
• IP layer maps between IP and ATM dest address
– IP packet into ATM AAL5 PDUs
– from IP addresses to ATM addresses just like IP
addresses to 802.3 MAC addresses (ARP)
• passes datagram to AAL5
• AAL5 encapsulates data, segments cells, passes to ATM
layer
– ATM network: moves cell along VC to destination
– at Destination Host (IP-over-ATM router): :
• AAL5 reassembles cells into original datagram
• if CRC OK, packet is passed to IP
Ethernet Switching vs. Virtual Circuit Switching
•
•
•
•
•
•
No connection setup
(connection less)
Packet carries dest. addr.
Switching based on globally
unique MAC address
a host does not know whether
the network is capable of
delivering the packet when it
sends the packet
Each packet is forwarded
independently and may be out
of order
A switch and link failure might
not have any serious effect if it
is possible to find an alternate
route
•
•
•
•
•
•
Establish connection state before
sending any data (connection
oriented)
– Setup latency, processing
overhead, scalability (capability
to grow to a large network)
Packet/cell carries VCI
Switching based on incoming port +
VCI (unique per port)
– VCI changed at the output port
Negotiate the QoS parameters and
allocate resources (buffer,
bandwidth) to VC
– If not enough resource, reject
the connection request
– QoS performance guranteed for
connection (bandwidth, delay,
delay jitter)
Each cell is routed along the
established connection in order
If a switch or a link fail, tear down
the old connection and establish a
new connection
Many ATM ideas adopted in IP networks called MPLS (more later)
Today’s Homework
• Peterson & Davie, Chap 3, 4th ed
-3.1
-3.5
-3.7
-3.8
-3.13
-3.26
Vol 5: 3.1,5,7,8,13, (ATM AAL5 problem on next
page)
• Download and browse ATM UNI4.0 spec and relate
contents to today’s lecture
70
Today’s Homework
• Q26 from vol. 4
The IP datagram for a TCP ACK message is 40B long.
It contains 20B TCP header and 20B IP header.
Assume that this ACK is traversing an ATM network that
uses AAL5. How many ATM cells will be needed to
carry the ACK. What if AAL3/4 is used instead?
71