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Intro to ATM
• Asynchronous Transfer Mode
• Text References
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Sect 2.6
Sect 3.6.3
Sect 5.6
Sect 6.5
ATM Background
• Outgrowth of TELCO transition to
integrated services
• Only “real” >100Mbit standard
• Offers multiservice (voice video data)
potential
• Switched architecture familiar to TELCOs,
not to high speed data networks
What is ATM?
Application
Presentation
Note: Tanenbaum considers
this more a network layer
technology.
Session
Transport
Network
AA L
Data/Link
ATM
Physical
PL
ATM - A layered standard
AAL - ATM Adaptation Layer
• Assembles and disassembles broadband servicesinto
a stream of cells
• Each cell has a header that contains routing information
ATM - Asynchronous Transfer Mode
• Switches the cells around the network based on the routing
information in the header
Physical Layer
• Provides the physical transportation of cells across the
network
(Note: CCITT reference model, p. 63)
ATM - A Switched Architecture
• Cells (small, fixed length packets) are
switched in a connection-oriented manner
but not using circuits like today’s voice.
Switch
Edge
Device
Switch
Edge
Device
What is ATM Switching?
• Why small cells?
– (32+64)/2=48 + 5 header bytes
– Mixed Traffic
• Packet (random)vs Circuit (TDM)
Switching
• Q.2931
– SVC, PVC
Physical Layer Options
• SONET (US)/ SDH (Europe)
• SMDS
• DQDB
• Speeds from DS3 on up! (45Mbs to Gbps)
• OC-3c => 155.52Mbps => 149.76Mbps
^ optical carrier
^ 3rd level in heirarchy
^ full duplex (two strands of fiber)
• Also OC-12c (622Mbps), OC-48c (2048Mbps)
[Look at the interesting way to frame cells]
ATM Adaptation Layer
(AAL)
• Classes of Service: 1, 2, 3/4, 5
1: circuit emulation
2: variable bit rate service
3/4: connection oriented data service
5: connectionless data service
• SAR - Segmentation and Reassembly
• Convergence Sublayer
the miscellaneous category
ATM Cell
• ATM cells are constant size packets of 53 bytes size.
-- 48 bytes payload, 5 bytes header/overhead.
VPI
VPI
(8 bits wide)
VCI
VCI
VCI
Type Res CLP
HEC
VPI - Virtual Path ID
VCI - Virtual Channel ID
Type - Payload type
(internal)
Res - reserved
CLP- Cell loss priority
HEC- Header Error
Control
VCI/VPI Operation
• A Virtual Channel exists between two switching points
VCI = a
VCI = b
• A Virtual Path contains 'bundles' of VCs
ATM Switch Architecture
•
•
•
•
Crossbar
Banyan
TDM busses
Buffering
– Input
– Output
– Both?
ATM Protocols
• UNI, NNI
• Services
• “LAN” Stuff
ATM Services
•
•
•
•
CBR
VBR (RT, NRT)
UBR
ABR
ATM Quality of Service
•
•
•
•
•
QoS: A contract
Traffic Descriptors
Cell Rate Options (pg 462)
Traffic Shaping
Traffic Policing
ATM Congestion Control
•
•
•
•
Admission Policy
Reservation System
Rate Based Control
Other
ATM Flow Control
• The leaky bucket algorithm
• CLP in ATM header
• Frame Relay comparisons
Routing
•IISP (Interim Inter-switch Signaling Protocol)
•PNNI (Private Network-Network Interface)
•Phase 1
•Phase 2
IISP
• Interim
– Allowed multi-vendor interoperability before
completion of NNI
• Signaling
• Routing via manually configured NSAP
prefixes
PNNI
• Topology abstraction
• Peer group(group of nodes)
– One switch elected Peer Group Leader
– All nodes in group have identical view of
group
• Hierarchy of logical groups
– Up to 105 levels of hierarchy
PNNI Routing
A12
A11
A22
A21
A13
B3
A23
B1
A2
A1
NSAP Domain
B2
B
B25
A12
A117
A2
A11
A13
B
View from A117 at A11
Sequence of Events
• A117 -> B25
• Forward to switch (A11)
– Switch knows topology of A1 group
– B reachable by A2 - A2 reachable by either
A12 or A13
• DTL (Designated Transit List)
– [A12][A2][B]
– [A22][A23][B]
– [B2]
ATM “LAN” Stuff
• LAN == Link Layer Domain
• ELANs & VLANs
• LANE & MPOA
– LECS, LES, BUS
LANE v1
• LAN Emulation
• No QoS (Quality of Service) Support
• Uses AAL5 signaling
optimized for data transport
entire cell payload available for user data
• LEC - LAN Emulation Client
• LAN Emulation Service
LECS - LAN Emulation Configuration Server
LES - LAN Emulation Server
BUS - Broadcast and Unknown Server
• STP (Spanning Tree Protocol) supported
LEC - LAN Emulation Client
• Software process on any ATM-connected LAN
switch, router, PC, or workstation
• Layer 2 process
• Prior knowledge of certain parameters
LEC’s ATM address
LAN type to be emulated
maximum data frame size
any route descriptors (for SR bridging)
whether it is willing to proxy (respond to LE-ARP)
LAN name - SNMPv2 display string
LECS - LAN Emulation
Configuration Server
• One per administrative domain
• Gives identity of ELAN (Emulated LAN)
• Returns ATM address of LES, type of LAN
emulated, and maximum PDU size of ELAN
• Controls which physical LANs are combined to
form VLANs (Virtual LAN)
• LECS address known via ILMI or its well-known
NSAP address
LES- LAN Emulation Server
• Adds LEC’s to ELAN
• Assigns LECID to joining LEC
• Table of address information of LEC
MAC address
proxy for MAC address
Token Ring route descriptors
• LECs can communicate directly with each other
only when they are connected to the same LES
• Multiple LESs on the same physical ATM LAN
• Answers LE-ARP requests from LECs
BUS- Broadcast and Unknown
Server
• During address resolution LEC forwards all frames
to the BUS
floods frames to all LECs
after address resolved flush protocol used to guarantee
order of cells
• All multicast and broadcast traffic sent through BUS
• Traffic limited to 10 frames/second
• Intelligent BUS
resolve destinations
CLS- connectionless server
LANE Setup
Connections
• All SVC (switched virtual circuits)
• SVCs required:
LECs and LECS
LES and LECS
Control Direct - LECs and LES
pt-mpt Control Distribute - LES to LECs
Multicast Send - LECs and BUS
pt-mpt Multicast Forward - BUS to LECs
Data Direct - LEC and LEC
• PVC (permanent virtual circuit) possible to
connect LEC and LECS
Virtual Channel Connections
LANE Server
(LES)
Control
Direct
VCC
Broadcast and
Unknown Server (BUS)
Control
Direct
VCC
LANE Client
(LEC)
Multicast
Send VCC
LANE Client
(LEC)
Multicast
Send VCC
LANE Client
(LEC)
Control
Distribute VCC
LANE Client
(LEC)
Multicast
Forward VCC
LAN Switch
Data Direct VCC
ATM Host
Configuration
Direct VCC
Configuration
Direct VCC
LANE
Configuration
Server (LECS)
ATM Host
LAN Switch
NHRP
•
•
•
•
Next Hop Resolution Protocol
Grew out of ATMARP
Only IP
Allows shortcut routes (pt-pt)
direct VCCs across ATM network
• Address resolution across multiple IP networks
• If network unknown, request forwarded to other
NHSs (Next-hop Server)
NHS with knowledge will forward response to source
router
• Router must have ability to bypass default route
RSVP
• Resource Reservation Protocol
• Provides QoS (Quality of Service) guarantees
• Operates in simplex
each direction has separate reservation
maps well to ATM (two individual VCCs)
• Built on IP, but no data transport built-in
• Only if resources available and does not conflict
with policy
• Flowspec (bandwidth and delay) and filterspec
(type of packets) transmitted downstream
hop by hop
MPOA
• Multiprotocol over ATM
• EDFG (Edge Device Functional Groups)
existing LAN segments via LAN switches
• AHFG (ATM-attached Host Functional Groups)
ATM-connected host
• Layer 3
• Only supports IP for now
• Uses LANE for Layer 2 forwarding within a
single Layer 3 subnet
• Adaptation of NHRP to provide connectivity
between hosts in different subnets
MPOA Operation
Competing Technologies
• “Fast Ethernet”
– 100BASE-TX, 100BASE-FX,100BASE-T4,
100BASE-VG
• FDDI, FDDI- II
• HPPI
• Gigabit Ethernet (IEEE 802.3z)
ATM Issues
• SONET/SDH duplication of services
• ATM overhead
• ATM granularity and bandwidth management
• ATM & connectionless service
• End point synchronization
• Flow Control !!! (bandwidth allocation, correlated traffic)
• ATM Forum