Intro to ATM

• Asynchronous Transfer Mode

• Text References– 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?

AAL

ATM

PLPhysical

Data/Link

Network

Transport

Session

Presentation

Application Note: Tanenbaum considersthis more a network layertechnology.

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 Switch

Edge Device

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.

(8 bits wide)VPI

VPI VCI

VCI

VCI

HEC

Type Res CLP

VPI - Virtual Path IDVCI - Virtual Channel IDType - Payload type (internal)Res - reservedCLP- Cell loss priorityHEC- 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

A117

A13

A12

A11

B25

A1BA2

B3

B2B1A23

A22

A21

NSAP Domain

A11

A12

A13

A2 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 ConnectionsVirtual Channel Connections

LANE Client (LEC) LANE Client

(LEC)

LANE Server (LES)

LANE Configuration Server (LECS)

ATM Host

LAN Switch

Configuration Direct VCC

Configuration Direct VCC

Control Direct VCC

Control Direct VCC

Control Distribute VCC

LAN Switch

ATM Host

LANE Client (LEC) LANE Client

(LEC)

Multicast Forward VCC

Data Direct VCC

Multicast Send VCC

Multicast Send VCC

Broadcast and Unknown Server (BUS)

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