CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
Lecture No. 17
Virtual Paths with ATM
• Two level hierarchy of virtual connection: 8bit
VPI and 16bit VCI
– Switches in the public network use 8bit VPI
– Corporate sites use full 24bit address (VPI + VCI)
– Much less connectionstate info in switches
– Virtual path: fat pipe with bundle of virtual circuits
Public network
Network A
Network B
3
Physical Layers for ATM
• ATM may run over several phy media
• ATM was assumed to run over SONET
but both are entirely separable entities
• ATM frame boundaries to be correctly
identified
– Successive 53byte ATM frames in payload
– SONET overhead byte points to the payload
– Another way is to calculate CRC (5th byte
4
of the cell)
ATM and LANs
• ATM grew out of the telephone community and
later used for computer communication
• Significant advantage of performance and
better scalability of switched over shared media
• No distance limitation in ATM making it a good
choice for highperformance LAN backbone
• Pointtopoint, long distance Gigabit Ethernet is
a competing technology with ATM
5
ATM as a LAN Backbone
H4
H5
Ethernet links
ATM links
H6
H3
E2
E1
Ethernet switch
ATM-attached
host
H7
ATM switch
E3
H1
H2
• Different from traditional LANs; no native
support for broadcast or multicast
6
ATM in a LAN
• How to broadcast to all
nodes on an ATM LAN ?
–Without knowing all the
addresses
–Without setting up VC to all
of them
7
ATM in a LAN
• Two solutions
– Redesign protocols that consider LAN
different from what ATM can provide
(e.g. ATMARP)
– Make ATM behave like shared media,
without loosing performance advantage
of switched media (e.g. LANE)
• ATM address is different from a unique
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48bit MAC address
Shared Ethernet Emulation with LANE
• All hosts think they are on the same Ethernet
ATM Switch
ATM Switch
LANE / Ethernet
LANE / Ethernet
Adaptor Card
Adaptor Card
LANE / Ethernet
LANE / Ethernet
Adaptor Card
Adaptor Card
Ethernet
Switch
Ethernet
Switch
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
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LAN Emulation (LANE) with ATM
• Transparent shared media
emulation of ATM
• Adds (not changes) functionality to
ATM switches
• Each device needs a global MAC
address, as well as an ATM
address to establish a VC
10
LAN Emulation (LANE) with ATM
• Devices connect as LAN Emulation
Clients (LEC)
• LANE provides Ethernetlike
interface to LECs
• Similar solutions for other networks:
VPNs on WANs, VLANs on large,
switched Ethernets
11
ATM / LANE Protocol Layers
Higher-layer
protocols
(IP, ARP, . . .)
Signalling
+ LANE
Ethernet-like
interface
AAL5
ATM
PHY
Host
Higher-layer
protocols
(IP, ARP, . . .)
Signalling
+ LANE
AAL5
ATM
PHY
PHY
Switch
ATM
PHY
Host
12
Clients and Servers in LANE
• LAN Emulation Client (LEC)
– Host, bridge, router or switch
• LAN Emulation Server (LES)
– Maintains client’s MAC and
ATM addresses
– Maintains ATM address of BUS
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Clients and Servers in LANE
• LAN Emulation Configuration
Server (LECS)
– Highlevel network management
when LEC starts up
– Reachable by preset VC (recall
known server port#)
– Maintains mapping of ATM address
to LANE type
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Clients and Servers in LANE
• Broadcast and Unknown Server (BUS)
– Emulates broadcast and multicast; critical to LANE
– Uses pointtomultipoint VC with all clients
• Servers physically located in one or more devices
LES
ATM network
BUS
Point-to-point VC
Point-to-multipoint VC
LECS
H1
H2
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LANE Registration
1. Client contacts LECS on predefined
VC, and sends ATM address to it
2. LECS returns LAN type, MTU and
ATM address of LES
3. Client signals connection to LES, and
registers MAC and ATM addresses
with LES
4. LES returns ATM address of BUS
5. Client signals connection to BUS
6. Bus adds client to pointtomultipoint
VC
LES
H3
BUS
ATM Network
H1
LECS
H2
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LANE Circuit Setup
1. Client (H1) knows destination
MAC address of receiver (H2)
2. Client (H1) sends 1st packet to
BUS
3. BUS sends address resolution
request to LES
4. LES returns ATM address to
client (H1)
5. Client (H1) signals connection
to H2 for subsequent packets
LES
H3
BUS
ATM Network
H1
LECS
H2
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Switches: The Intersections
The Intersections
Pir Wadhai
Islamabad
Zero Point
Rawalpindi
Saddar
Rawal
Dam
Faizabad
Ayub
Park
Airport
Faizabad
Flyover
Design intersection to accommodate traffic flows
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Contention in Switches
• Some packets destined for same output
– One goes first
– Others delayed or dropped
• Delaying packets requires buffering
– Finite capacity, some packets must still drop
– At inputs
• Increases/adds false contention
• Sometimes necessary
– At outputs
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– Can also exert “backpressure”
Output Buffering
Customer
service
Standard checkin lines
x
you
trying to checkin
1x6 Switch
Mr. A
waiting to
claim refund
of Rs.100
a
Mr. X
writing
complaint
letter
21
Input Buffering: Headofline
Blocking
Customer
Standard checkin lines
service
agents are standing by !
x
1x6 Switch
a
trying to checkin
Mr. X
Mr. A
writing
waiting to complaint
claim refund letter
of Rs.100
you
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Backpressure
Standard checkin lines
Customer
service
x
you
“no more,
please”
trying to checkin
a
i
1x6 Switch
propagation delay requires that switch exerts
backpressure before buffer is full; thus used
in networks with small propagation delay
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Backpressure
Switch 2
Switch 1
“no more, please”
• Propagation delay requires that switch 2 exert
backpressure at highwater mark rather than
when buffer completely full
• It is thus typically only used in networks with
small propagation delays (e.g., switch fabrics)
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Switching Hardware
• Multiinput multioutput device, getting packets
from inputs to the outputs as fast as possible
• Performance of a switch is limited by I/O bus
bandwidth (each packet traverse twice)
– 1Gbps I/O bus can support ten T3 (45 Mbps) links,
three STS3 (155 Mbps) links, and not even one
STS12 (625 Mbps) link
• Success or failure of a new protocol depend on
whether it takes advantage of switch’s
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capabilities