Test101a

Asynchronous Transfer Mode (ATM) Fundamentals

What is ATM?

Asynchronous Transfer Mode (ATM) is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length cells to carry different types of traffic (i.e. voice, data and video). It provides an equitable, controllable mechanism to statistically share the bandwidth provided by the underlying infrastructure The demand from the market for a high speed network that can carry a mix of media streams has been the key driver behind the continuously evolving ATM standards. The ATM Forum established in 1991is an international voluntary organization composed of vendors service providers, research organizations and users (i.e.Cisco, Sprint, Nortel, and Network Equipment and Technologies). Its purpose is to accelerate the use of ATM products and services through the rapid convergence of interoperability specifications, promotion of industry cooperation, and other activities.

ATM is also a capability that can be offered as an end-user service by service providers (as a basis for tariffed services) or as a networking infrastructure for these and other services. The most basic service building block is the ATM virtual circuit, which is an end-to-end connection that has defined end points and routes but does not have bandwidth dedicated to it. Bandwidth is allocated on demand by the network as users have traffic to transmit. ATM also defines various classes of service to meet a broad range of application needs (nrtVBR,rtVBR,UBR,ABR,CBR and GFR-Guarantee frame rate).

Benefits of ATM

The benefits of ATM are the following: • high performance via hardware switching (gigabit, terabit switches on the horizon) • dynamic bandwidth for bursty traffic (voice, data and video bursty) • class-of-service support for multimedia (latency requirements) • scalability in speed and network size (DS1 to OC12 (622Mbps)) • international standards compliance (CO and customer premises environments allowing for multivendor operation

ATM Technology & Cell format

In ATM networks, all information is formatted into fixed-length cells consisting of 48 bytes (8 bits per byte) of payload and 5 bytes of cell header . The fixed cell size ensures that time-critical information such as voice or video is not adversely affected by long data frames or packets. The header is organized for efficient switching containing fields to help deal with congestion, maintenance, and error control problems. The header information is generated in the ATM layer, while the payload data is provided in 48 byte chunks by the AAL.

GFC - Generic Flow Control Was intended as a mechanism for flow control between the ATM network and user. No standards were ever developed for the GFC; it is never (or almost never) used.

VPI - Virtual Path Identifier Allows 28 = 256 VPs on the ‘UNI’ and 212 = 4096 VPs on the ‘NNI’

VCI - Virtual Channel Identifier Allows 216 = 65536 VCs

PTI - Payload Type Indicator (3 BIT CELL) Used to identify contents of cell: Bit 1 User data (0) or OAM F5 or reserved (1) Bit 2 Congestion (1) or no congestion (0) Bit 3 Used with AAL5

CLP - Cell Loss Priority Discard priority (equivalent to Frame Relay’s DE)

HEC - Header Error Correction Error check for the header only Can correct 1 bit error, can detect 2 bit errors May not detect 3 or more bit errors, or may interpret 3 or more errors as a single bit error Also used for cell delineation (more later)

ATM is connection oriented. Organizing different streams of traffic in separate calls allows the user to specify the resources required and allows the network to allocate resources based on these needs. Multiplexing multiple streams of traffic on each physical facility (between the end user and the network or between network switches)—combined with the ability to send the streams to many different destinations—enables cost savings through a reduction in the number of interfaces and facilities required to construct a network.

ATM standards defined two types of ATM connections: virtual path connections (VPCs), which contain virtual channel connections (VCCs). A virtual channel connection (or virtual circuit) is the basic unit, which carries a single stream of cells, in order, from user to user. A collection of virtual circuits can be bundled together into a virtual path connection. A virtual path connection can be created from end-to-end across an ATM network. In this case, the ATM network does not route cells belonging to a particular virtual circuit. All cells belonging to a particular virtual path are routed the same way through the ATM network, thus resulting in faster recovery in case of major failures.

An ATM network also uses virtual paths internally for the purpose of bundling virtual circuits together between switches. Two ATM switches may have many different virtual channel connections between them, belonging to different users. These can be bundled by the two ATM switches into a virtual path connection. This can serve the purpose of a virtual trunk between the two switches. This virtual trunk can then be handled as a single entity by, perhaps, multiple intermediate virtual path cross connects between the two virtual circuit switches.

Virtual circuits can be statically configured as permanent virtual circuits (PVCs) or dynamically controlled via signaling as switched virtual circuits (SVCs). They can also be point-to-point (router to router) or point-to-multipoint (ELAN), thus providing a rich set of service capabilities. SVCs are the preferred mode of operation because they can be dynamically established, thus minimizing reconfiguration complexity. We use SPVC’s.

ATM and OSI model:

The protocols defining ATM are based on the 7-layer model for Open Systems Interconnection (OSI). ATM is only concerned with the bottom two OSI layers: the physical layer and the data link layer.

• • • • As per Eric S this is open to interpretation as some argue that it also involves the network layer***

• • • • • • • • • • • • NOTE OF INTEREST************

How is ATM used as the backbone for other networks?

The vast majority (roughly 80 percent) of the world's carriers use ATM in the core of their networks. ATM has been widely adopted because of its unmatched flexibility in supporting the broadest array of technologies, including DSL, IP Ethernet, Frame Relay, TLS, SONET/SDH and wireless platforms. It also acts as a unique bridge between legacy equipment and the new generation of operating systems and platforms. ATM freely and easily communicates with both, allowing carriers to maximize their infrastructure investment.

FRF.8 (frame relay forum) SIWF maps frame relay DLCI’s to ATM VCC’s on a one-to-one basis FRF.5 (frame relay forum) NIWF encapsulates frame relay over ATM and multiplexes many frame relay DLCI’s to one ATM VCC

ATM Classes of Services

ATM is connection oriented and allows the user to specify the resources required on a per-connection basis (per SVC) dynamically. There are the five classes of service defined for ATM (as per ATM Forum UNI 4.0 specification). The QoS parameters for these service classes are summarized in the tables below.

Asynchronous Transfer Mode Adaptation Layers (AALs) support the convergence of different information types into ATM cells. Five AALs have been developed to provide QoS options and offer specific support needed for various traffic types. AALs can be correlated to the traffic types supported by ATM.

The AAL consists of 2 sublayers: CS- Convergence sublayer- assures the necessary error control and sequencing as well as the sizing of information SAR- Segmentation and Reassembly sublayer – divides the CS message into 48-byte payload packets and attaches them to the five-byte header.

Service Class	Quality of Service Parameter
AAL 1	This class is used for emulating circuit switching. The cell rate is constant with time. CBR applications are quite sensitive to cell-delay variation. Examples of applications that can use CBR are voice traffic (i.e., nx64 kbps), videoconferencing, and television. AAL 1 supports traffic that is critically impaired by time delays in transmission
AAL 3,4,5	This class allows users to send traffic at a rate that varies with time depending on the availability of user information. Statistical multiplexing is provided to make optimum use of network resources. Multimedia e-mail is an example of VBR–NRT.
AAL 3	and AAL 4 are combined to provide support for connectionless VBR traffic such as frame relay applications.
AAL 5	traffic requires sequencing and minimum error correction. variable bit rate–real time (VBR–RT)
AAL 2	This class is similar to VBR–NRT but is designed for applications that are sensitive to cell-delay variation. Examples for real-time VBR are voice with speech activity detection (SAD) and interactive compressed video. available bit rate (ABR)
AAL 5	This class of ATM services provides rate-based flow control and is aimed at data traffic such as file transfer and e-mail. Although the standard does not require the cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable for switches to minimize delay and loss as much as possible. Depending upon the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate, which is guaranteed to the connection by the network. unspecified bit rate (UBR)
AAL 5	This class is the catch-all, other class and is widely used today for TCP/IP.

ATM QoS

Quality of Service (QoS) is a measurement of the delay and reliability that a particular connection will support. QoS is used to designate resources at connection setup time and to ensure that a network performance objectives are met. See table below.

ATM QoS Parameters

Technical Parameter	Definition
cell loss ratio (CLR)	CLR is the percentage of cells not delivered at their destination because they were lost in the network due to congestion and buffer overflow. i.e ratio of lost cells to total transmitted cells
cell transfer delay (CTD)	The delay experienced by a cell between network entry and exit points is called the CTD. It includes propagation delays, queuing delays at various intermediate switches, and service times at queuing points.
cell delay variation (CDV)	CDV is a measure of the variance of the cell transfer delay. High variation implies larger buffering for delay-sensitive traffic such as voice and video.
peak cell rate (PCR)	The maximum cell rate at which the user will transmit. PCR is the inverse of the minimum cell inter-arrival time.
sustained cell rate (SCR)	This is the average rate, as measured over a long interval, in the order of the connection lifetime.
burst tolerance (BT MBS)	This parameter determines the maximum burst that can be sent at the peak rate. This is the bucket-size parameter for the enforcement algorithm that is used to control the traffic entering the network.
Cell error ratio (CER)	This ratio of errored cells in a transmission in relation to the total cells sent in a transmission. The measurement is taken over a time interval.
Severly errored cell block ratio (SECBR)	Produces a ratio of the number of severely errored cell blocks divided by the total number of transmitted blocks.

Traffic Management Parameters

Terms and definitions

VC	Virtual Channel	This is an end-to-end virtual connection between two applications or services. All traffic between end applications or services occurs at the Virtual Channel level. The Virtual Channel is a series of Virtual Channel Links (identified by Virtual Channel Identifiers) that, together, form a Virtual Channel Connection (or just a Virtual Channel).
VP	Virtual Path	This is a virtual connection that consists of a group of one or more Virtual Channels with some common characteristic. The Virtual Path is a series of Virtual Path Links (identified by Virtual Path Identifiers) that, together, form a Virtual Path Connection (or just a Virtual Path).
VCI	Virtual Channel Identifier	This identifies a specific instance of a virtual channel within a virtual path on a physical link.
VPI	Virtual Path Identifier	This identifies a specific instance of a virtual path on a physical link.
VCL	Virtual Channel Link	This is a segment of a VC from where a VCI is assigned to where the VC is switched, farther along. This is a segment of the VC as it exists on one physical link; a section of the VC where one VCI value stays the same.
VPL	Virtual Path Link	This is a segment of a VP from where a VPI is assigned to where the VC is switched, farther along. This is a segment of the VP as it exists on one physical link; a part of the VP where one VPI value stays the same.
VCC	Virtual Channel Connection	A concatenation of VCLs to form an end system to end system channel.
VPC	Virtual Path Connection	A concatenation of VPLs between virtual path end points.
SVC	Switched Virtual Channel	This is a virtual channel that is dynamically established between two application end points (typically CPE to CPE). Originating CPE signals the ATM network requesting a VC to a remote end-point (using its address). This requires UNI signaling at the network access points and ATM routing within the ATM network(s).
SVP	Switched Virtual Path	This is a virtual path that is dynamically established between two application end points (typically CPE to CPE). Originating CPE signals the ATM network requesting a VP to a remote end-point (using its address). This requires UNI signaling at the network access points and ATM routing within the ATM network(s). While this is supported by standards and ATM networks, there does not appear to be any CPE that actually does SVPs yet.
SPVC	Soft Permanent Virtual Channel	This is a hybrid; part SVC, part PVC. The CPE provision the connection as a PVC. One ATM network end-point provisions the VC to call a remote address and VPI/VCI. The connection through the ATM network is dynamically setup. The connection will be re-routed if an element within the ATM network fails (provided alternate facilities are available).
SPVP	Soft Permanent Virtual Path	This is a hybrid; part SVP, part PVP. The CPE provision connections as a PVCs. One ATM network end-point provisions the VP to call a remote address and VPI. The connection through the ATM network is dynamically setup. The connection will be re-routed if an element within the ATM network fails (provided alternate facilities are available).
PVC	Permanent Virtual Channel	This is a virtual channel where each connection point along the VC is permanently linked to the next connection point. There are no re-route capabilities in case of link failure since each part of the VC is permanently connected to each resource.
PVP	Permanent Virtual Path	This is a virtual path where each connection point along the VP is permanently linked to the next connection point. There are no re-route capabilities in case of link failure since each part of the VP is permanently connected to each resource.

ATM addressing:

NSAP formats used by Sprint Canada

The ATM address uniquely identifies each piece of ATM equipment in the network. It is 20 bytes in length and contains 40 hexidecimal characters. It is structured so that the bytes towards the left of the address represent more general information, while the bytes to the right represent more specific information.

TORACCW AtmIf/50 Uni Addr/39124F81000122000000005F00 00206850000000,primary
TORACCW AtmIf/50 Uni Addr/39124F81000122000000005F00 0020480D003200,default

d -p mod nodePrefix 39124f81000122000000005f00

39124f81000122000000005f00LLmmmmnnoooo00

LL Reserved ID 8 bits number range 00 mmmm Customer ID 16 bits 0000>9999 nn Customer port ID 16 bits 0000>9999 oooo Reserved ID 8 bits 0000

d fratm/40000 addr/* myAddress 3730361500999999000000000000000000000000

15 Ontario 009 TORACCG 99999 identifies one switch

ATM Networking Standards

Type	Name	Description
P-NNI	Private Network to Network Interface	This is an interswitch protocol that supports dynamic routing and addressing and is intended for use between switches belonging to the same network. Switches in a P-NNI network can be configured into a hierarchy which allows P-NNI networks to efficiently grow to very large sizes. P-NNI provides two types of protocols: -Routing protocol is used for the exchange of network topology information -Signaling protocol is used between switches during connection establishment
B-ICI	Broadband Inter Carrier Interface	A formal specification on an ATM interface between to different ATM networks used to carry multiple services. It specifies responsibility for numerous issues including space, power, provisioning, and addressing, and defines procedures for signaling. Signaling is based on a standard called B-ISUP, which is an extension of SS7 signaling.
UNI	User-Network Interface	This is intended primarily for connecting terminal equipment (CPE) to an ATM network switch.
IISP	Interim Interswitch Signaling Protocol	This is intended primarily between ATM switches within the same network. It supports signaling for the setup of VCs, but addressing is static. This was the only inter-switch protocol available before P-NNI was released.
NNI	Network to Network Interface	There isn’t an NNI interface defined for ATM. The standard for connecting two ATM networks is the B-ICI process document. In practice most carriers connecxt with back to back UNI, with no signalling enabled.

OAM - Operations, Administration, and Maintenance

ATM allows for special OAM cells to flow along with user data through the network. These OAM cells are used by the network to check for faults, report fault conditions, and exchange some basic performance information between network elements.

There are two main types of ATM OAM cells: F4 and F5. F4 cells travel along VPs and F5 cells travel along VCs. Also, there are segment OAM cells and end-to-end OAM cells.

F4 segment OAM cells use VCI=3 of the VPI and F4 end-to-end OAM cells use VCI=4. F5 OAM cells are distinguished from user cells by the PTI

Three types of OAM cells are:

Type	Description
Loopback cells	Generation of these cells is optional. Loopback cells travel across a VC segment or between VC end points and verify the VC has no faults along its path. If loopback cells do not return to the originator, the VC is marked inactive until loopback cell flow is restored.
AIS and RDI cells	Alarm Indication Signal and Remote Defect Indication. These OAM cells signal upstream and downstream equipment of a fault along the VCs path.
Passport Trace cells	These are proprietary Nortel Passport OAM cells that function similarly to TCP/IP’s traceroute command. All reachable connection points within an ATM segment respond back to the originator of the cells; a list of all reachable connection points is generated.