Asynchronous Transfer Mode (ATM) NETE0514 Presented by Dr.Apichan Kanjanavapastit
Definition • Asynchronous transfer mode (ATM) is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic • ATM was designed by the ATM Forum and adopted by the ITU-T
Packet Networks • Data communications are based on packet switching and packet networks • A packet is a combination of data and overhead bits that can be passed through the network as a self-contained unit • The overhead provides identification and addressing information as well as the data required for routing, flow control, error control, and so on • Different protocols use packets of varying size and intricacy • As networks become more complex, the information carried in the header becomes more extensive
Packet Networks (con’t) • The result is larger overhead relativeto the size of data unit • Some protocols have enlarged the size of data unit to make header use more efficient • Thus, packets can be as long as 60,000 bytes sharing long-haul links with packets of fewer than 200 bytes
Mixed Network Traffic • Since packet networks have unpredictable packet sizes, switches, multiplexers, and router must incorporate elaborate software systems to manage the various sizes of packets • A grate deal of header information must be read and each bit counted and evaluated to ensure the integrity of every packet • Another problem is that of providing consistent data-rate delivery when packet sizes are unpredictable and can vary so dramatically • To get the most out of broadband technology, traffic must be time-division multiplexed onto shared paths
Mixed Network Traffic (con’t) • Because audio and video packets ordinarily are small, mixing them with conventional data traffic often creates unacceptable delays of this type and makes shared packet links unusable for audio and video information router
Cell Networks • Many of the problems associated with the packet internetworking are solved by adoption a concept called cell networking • A cell is a small data unit of fixed size; thus all data loaded into identical cells can be transmitted with complete predictability and uniformity • As packets of different sizes and formats reach the cell network, they are split into multiple small data units of equal length and loaded into cells • The cells are then multiplexed with other cells and routed through the cell network • Since each cell is the same size and all are small, the problem associated with multiplexing different-sized packets are avoided
Cell Networks (con’t) • In this way, a cell network can handle real-time transmission, such as phone call, without the parties aware of the segmentation or multiplexing at all MUX
Asynchronous TDM • ATM uses asynchronous time-division-multiplexing to multiplex cells coming from different channels. It uses fixed-size slots the size of a cell • ATM multiplexers fill a slot with a cell from any input channel that has a cell; the slot is empty if none of the channels has a cell to send • ATM uses fixed-size slots (total 53 bytes: 48 bytes for payload and 5 bytes for overhead)
WHY USE A 48-BYTE PAYLOAD? 48 bytes corresponds to approximately 6 milliseconds of voice • Losing one 48-byte payload wouldn’t be disruptive to a listener (a speech phoneme is about 32 milliseconds long) The U.S. preferred a 64-byte payload • Studies indicated that data communication efficiency would be improved with somewhat larger cells (i.e., less overhead per PDU) Europe preferred a 32-byte payload • Echo cancellers for audio wouldn’t be needed in smaller countries if PDU sizes were kept small enough Everyone wanted the payload size to be a power of two • Memory transfer and switching would all be simplified The Solution?
ATM Architecture End points are user access devices
ATM Architecture (cont.) • Virtual Connection • Connection between two end points is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs) • A transmission path (TP) is the physical connection (wire, cable, satellite, and so on) between an end point and a switch or between two switches • A transmission path is divided into several virtual paths. A virtual path provides a connection or a set of connections between two switches • Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual channel and remain in their original order until they reach their destination
ATM Architecture (cont.) Since the virtual connections need to be identified, there are two levels of identifier: a virtual path identifier (VPI) and a virtual circuit identifier (VCI).
ATM Layers • The ATM standard defines three layers, from the top to bottom, the application layer, the ATM layer, and the physical layer. The physical and ATM layer are used in both switches and end points. The AAL is used only by the end points.
Application Adaptation Layer (AAL) • The AAL allows existing networks (such as packet networks) to connect to ATM facilities • AAL protocols accept transmissions from upper-layer services (e.g., packet data) and map them into fixed-sized ATM cells • These transmissions can be of any type (voice, data, audio, and video) and can be of variable or fixed rates • At the receiver, this process is reversed– segments are reassembled into their original formats and passed to the receiving service
AAL (con’t) Upper Layers • Convergence Sublayer • Provide application-specific interface • Handle lost and delayed cells • Error detection and handling A A L • Segmentation and Reassembly Sublayer • Pack Convergence Sublayer information into 48-byte • blocks for transfer down to the ATM Layer. • Unpack ATM Layer cells for transfer up to the • Convergence Sublayer. ATM Layer Physical Layer
Application Layer Message Convergence Sublayer CS Trailer CS Header Segmentation and Reassembly Sublayer Pad Segmentation and Reassembly Sublayer (continued) SAR Hdr SAR Trlr SAR Hdr SAR Trlr SAR Hdr SAR Trlr SAR Hdr SAR Trlr SAR Hdr SAR Trlr ATM Layer ATM Hdr ATM Hdr ATM Hdr ATM Hdr ATM Hdr TRAVERSING THE AAL
AAL (cont.) • AAL Type 1 supports constant bit rate (CBR), synchronous, connection oriented traffic. Examples include T1 (DS1), E1, and x64 kbit/s emulation. • AAL Type 2 supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented, synchronous traffic. Examples include Voice over ATM. AAL2 is also widely used in wireless applications due to the capability of multiplexing voice packets from different users on a single ATM connection. • AAL Type 3/4 supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25.
AAL (cont.) • AAL Type 5 is similar to AAL 3/4 with a simplified information header scheme. This AAL assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services that use AAL 5 are IP over ATM, Ethernet Over ATM
AAL5 • AAL 5 is sometimes called the simple and efficient adaptation layer (SEAL), assumes that all cells belonging to a single message travel sequentially and that control functions are included in the upper layers of the sending application (addressing, sequencing, or other header information) • AAL5 accepts an IP packet of no more than 65,535 bytes and adds an 8-byte trailer as well as any padding required to ensure that the position of the trailer falls where the receiving equipment expects it (at the last 8 bytes of the last cell)
ATM Layer • The ATM layer provides routing, traffic management, switching, and multiplexing services • It processes outgoing traffic by accepting 48-byte segments from the AAL and transforming them into 53-byte cells by the addition of a 5-byte header • Most of the header is occupied by the VPI and VCI. The combination of VPI and VCI can be thought of as a label that defines a particular virtual connections
Physical Layer • The physical layer defines the transmission medium, bit transmission, encoding, and electrical to optical transformation • It provides convergence with physical transport protocol such as SONET/SDH as well as the mechanisms for transforming the flow of cells into a flow of bits • The ATM Forum has left most of the specifications for this level to the implementer
QoS, PVC, and SVC • Quality of Service (QoS) requirements are handled at connection time and viewed as part of signaling. • ATM provides permanent virtual connections and switched virtual connections. • Permanent Virtual Connections (PVC) permanent connections set up manually by network provider. The VPIs and VCIs are defined for the permanent connections and the values are entered in a table for each switch • Switched Virtual Connections (SVC) set up and released on demand by the end user via signaling procedures.
ATM Signaling Protocol • Signaling protocol consists of two parts • User-Network Interface (UNI) • defines how end points talk to switches • Network-Network Interface (NNI) • defines how switches talk to other switches • Cell formats of the two protocols are slightly different
UNI Signaling • UNI signaling is performed between an end station and a private ATM switch, or between a private ATM switch and the public ATM network • The UNI signaling is simpler because it does not involve routing. The standards are produced by the ATM Forum and are called UNI 3.1 (1994) and UNI 4.0 (1996) • UNI 4.0 is an addition to UNI 3.1, UNI 3.1 is derived from the Public Network Signaling protocol Q.2931 brought by the ITU-T which is further derived from Q.931 used in ISDN and Frame Relay
UNI Header Format • GFC---4 bits of generic flow control that are used to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value. • VPI---8 bits of virtual path identifier that is used, in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination. • VCI---16 bits of virtual channel identifier that is used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.
UNI Header Format (cont.) • PT---3 bits of payload type. The first bit indicates whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame. • CLP---1 bit of congestion loss priority that indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. • HEC---8 bits of header error control that are a checksum calculated only on the header itself.
NNI Signaling • NNI signaling is performed between the switches of a public ATM network. Since a public network generally involves several (or many) switches the routing becomes very important component of the NNI signaling • NNI signaling has two major standards: IISP (Interim Inter-switch Signaling Protocol) and PNNI (Private Network-to-Network Interface) • IISP is a simple signaling protocol which uses static routing which have to be manually created and maintained and is designed for small private ATM networks • PNNI is a signaling protocol that uses very elaborate dynamic routing algorithms which can easily handle small to large ATM networks which can have hundreds, thousands and even tens of thousands of ATM switches
NNI Header Format • The GFC field is not present in the format of the NNI header. Instead, the VPI field occupies the first 12 bits, which allows ATM switch routers to assign larger VPI values. With that exception, the format of the NNI header is identical to the format of the UNI header.
ATM End System Addressing (AESA) • All ATM switches and end stations in an ATM network must have a unique ATM address • The address is a crucial part of ATM signaling. This address must be long enough to accommodate a potentially huge number of ATM devices.
Automatic Address Registration in UNI • The ATM addresses (prefix only) of switches must be entered manually by the network manager • Once the address is in place, each work station (edge device) attached to that switch can now be configured automatically • The configuration is dynamic, it happens each time a device is attached to the switch, or when the device is moved from one switch to another • Automatic address registration is performed through the Integrated Local Management Interface (ILMI)
Integrated Local Management Interface (ILMI) • ILMI is based on IP's SNMP and uses a similar MIB and access procedures like Get, Set and Trap requests and responses • All ILMI communications go over a dedicated (default) VC (VPI = 0, VCI = 16) • Each ATM device (edge device or switch) that implements UNI (private or public) has ILMI and a component called Interface Management Entity (IME) • This entity acts as a symmetric component that can both send requests and respond to a peer IME • IME is responsible to maintain MIB and interpret/respond to SNMP messages. There are four types of SNMP messages used in automatic address registration: trap, get, getnext, set
Automatic Address Registration in UNI (cont.) Edge Device ATM Switch
UNI Signaling • Once an AESA address is established the user can place a call across an ATM network • The calls are accomplished by a set of signaling frames • connection setup frames • maintenance frames • connection teardown frames • All frames use dedicated VC, VPI = 0, VCI = 5
NNI Signaling: IISP • IISP (Interim Interswitch Signalling Protocol) is an extension of UNI 3.1/4.0 (approved in 1994) which includes simple hop-to-hop routing based on AESA addresses • Usually, the routing table has two additional fields for output ports: the second and the third routing choice in case the link for the first choice fails. • For routing are used only the first n octets of the address (n is the column indicated by "Octets to use"). An IISP routing table must be configured by the network administrator.
ATM Classes of Services • Constant Bit Rate (CBR) • 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 telephone traffic (i.e., nx64 kbps), videoconferencing, and television • Variable Bit Rate–Non-Real Time (VBR–NRT) • 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
ATM Classes of Services (con’t) • Variable Bit Rate–Real Time (VBR–RT) • 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) • This class 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
ATM Classes of Services (con’t) • Unspecified Bit Rate (UBR) • The bandwidth allocation service of this class does not guarantee any throughput levels and uses only available bandwidth. UBR is often used when transmitting data that can tolerate delays.The most widely use today is the TCP/IPdata
ATM Technical Parameters • 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 • 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
ATM Technical Parameters (con’t) • 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) • 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