Published on February 19, 2014
Lecture 7 ATM Introduction Instructor: Musfara Farooqui
Lecture Objectives Introduction to ATM technology Design Goals of ATM Significance of small and fixed sized cells ATM network interfaces. Cell structure of UNI and NNI ATM Headers. ATM Virtual Connections Identify the layers in ATM reference model.
Asynchronous Transfer Mode (ATM) Asynchronous Transfer Mode (ATM) is an International Telecommunication UnionTelecommunications Standards Section (ITU-T) standard Also called cell relay In ATM, information for multiple service types, such as voice, video, or data, is conveyed in small, fixed-size packets called cells. ATM Forum (www.atmforum.org) has designed ATM specifications
Basic Concepts Idea is to integrate heterogeneous sources (different rates, QoS requirements) and provide differentiated services (“to each application according to its needs”) Asynchronous transmission ◦ no exclusive reservation of resources (e.g. preassigned slots) to calls ◦ allocation of resources is possible to provide minimum quality of service (QoS) guarantees statistical multiplexing
Basic Concepts … Connection-oriented service All information is transmitted in 53-byte cells Uniform cell format simplifies broadband switch architectures Streamlined functionality to support very high data rates
ATM vs. Telephone Networks Current phone networks are synchronous (periodic) while ATM = Asynchronous Transfer Mode Phone networks use circuit switching and ATM networks use “Packet” Switching In phone networks, all rates are multiple of 8 kbps. With ATM service, you can get any rate. You can vary your rate with time. With current phone networks, all high speed circuits are manually setup. ATM allows dialing any speed.
Goals One objective is to move as many of the functions to hardware as possible (for speed) and eliminate as many software functions as possible (again for speed) Optimize use of high data rate transmission media System must interface with existing system Design must be inexpensive New system must be connection oriented System should be able to work with existing telecommunication hierarchies.
ATM ATM is based on ITU-T Broadband ISDN standard (BISDN) It was originally conceived as a high-speed transfer technology for voice, video, and data over public networks. It provides scalable bandwidth (Mbps to Gbps) The ATM Forum extended the ITU-T's vision of ATM for use over public and private networks ATM networks are connection-oriented. Asynchronous Transfer Mode (ATM) is the world's most widely deployed backbone technology
ATM Network Environment ATM is a cell-switching and multiplexing technology Efficient than TDM (synchronous). It combines the benefits of both: ◦ circuit switching guaranteed capacity and constant transmission delay ◦ packet switching flexibility and efficiency for intermittent traffic
ATM; Efficient than TDM TDM (synchronous) ◦ Time slots are pre-allocated ◦ If a station has much data to send, it can send only when its time slot comes up ◦ If a station has nothing to transmit its time slot is sent empty and is wasted. ATM is asynchronous, time slots are available on demand with information identifying the source of the transmission contained in the header of each ATM cell.
ATM Devices An ATM network is made up of two general devices: ◦ ATM switch ◦ ATM endpoints. ATM switch ◦ responsible for cell transit through an ATM network. ATM endpoint (or end system) ◦ contains an ATM network interface adapter
ATM Network Architecture • An ATM Network Comprises: • ATM Switches • Endpoints • An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or interfaces.
ATM Network Interfaces ATM switches support two primary types of interfaces: ◦ UNI The UNI connects ATM end systems (such as hosts and routers) to an ATM switch. ◦ NNI The NNI connects two ATM switches. ◦ These interfaces are further subdivided into private and public UNIs and NNIs.
UNI and NNI C o m p u te r U N I N N I A T M S W IT C H A T M S W IT C H N N I U N I A T M S W IT C H N N I N N I LA N A T M S W IT C H C om p C om p
ATM Interface Specifications ATM Interface Specifications Differ for Private and Public Networks
ATM Interfaces User to Network Interface (UNI): ◦ Public UNI, Private UNI Network to Node Interface (NNI): ◦ Private NNI (P-NNI) ◦ Public NNI =Inter-Switching System Interface (ISSI) Broadband Inter-Carrier Interface (B-ICI) ◦ Connects two public ATM switches from different service providers Data Exchange Interface (DXI) ◦ Between routers and ATM Digital Service Units (DSU)
ATM Cell Basic Format ATM transfers information in fixed-size units called cells. Each cell consists of 53 octets, or bytes. ◦ 5 bytes Header ◦ 48 bytes payload (user data) Small, fixed-length cells are well suited to transferring voice and video traffic at high data rates because such traffic is intolerant of delays that result from having to wait for a large data packet to download, among other things.
Significance of small size ATM Header To guarantee a fast processing in the network, the ATM header has very limited function. Its main function is the identification of the virtual connection by an identifier which is selected at call set up and guarantees a proper routing of each packet. In addition, it allows an easy multiplexing of different virtual connections over a single link
Significance of small size ATM Cell The Payload/Data field (48 bytes) length is relatively small: ◦ to reduce the internal buffers in the switching node ◦ to limit the queuing delays in those buffers small buffers guarantee a small delay and a small delay jitter as required in real time systems. The data field of ATM cells is carried transparently through the network. No processing is performed on it inside the network. ◦ All services (voice, video, data) can be transported via ATM , including connectionless services.
ATM Virtual Connections ATM networks are fundamentally connection oriented ◦ A virtual circuit is established prior to any data transfer Two types of ATM connections exist: ◦ virtual paths connections (VPC) identified by virtual path identifiers (VPI) ◦ virtual circuit connections (VCC) identified by the combination of a VPI and a virtual channel identifier (VCI).
Why two VC identifiers are used in ATM The whole idea behind dividing a virtual connection identifier into two parts is to allow hierarchical routing Most switches in a typical ATM network do switching using VPIs. The switches at the boundaries of the network, those that interconnect directly with the endpoints, use both VPIs and VCIs. An ATM switch route the cell using both the VPIs and VCIs
Virtual Paths & Virtual Circuits A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM network based on the common VPI. VPIs and VCIs have only local significance across a particular link and are remapped, as appropriate, at each switch. ◦ implemented with translation tables ◦ allows identifiers to be re-used (a necessity due to their limited size) ◦ Switching can be done based on VP only (VP switching) or both VP and VC (VC switching)
ATM Connection Relationships
Virtual Connections Switches transfer cells from input port to output port based on Virtual Circuit and Virtual Path identifiers ◦ VPI’s and VCI’s are re-usable ◦ VPI and VCI are replaced by switches according to translation table
Advantages of VP and VC Simplified Network Architecture ◦ Network Transmission functions can be separated into those related to an individual logical connection (VC) and those related to a group of logical connections (VP) Increased Performance and Reliability ◦ The network deals with fewer aggregated entities Reduced Processing and Short Connection Setup Time ◦ Much of the work is done, when VP is set up ◦ New VC can be established by executing simple control functions at the end points of VP connection. ◦ No call processing is required at the transit nodes ◦ The addition of new virtual circuits to an existing virtual path involves minimal processing
Routing with a switch
ATM Virtual Circuits
ATM Cell Header Format An ATM cell header can be one of two formats: ◦ UNI The UNI header is used for communication between ATM endpoints and ATM switches in private ATM networks. ◦ NNI The NNI header is used for communication between ATM switches.
An ATM cell
Generic ATM Cell ATM UNI Cell ATM NNI Cell
Virtual connection identifiers in UNIs and NNIs
ATM Cell Header Fields ◦ ◦ ◦ ◦ ◦ ◦ Generic Flow Control (GFC) Virtual Path Identifier (VPI) Virtual Circuit Identifier (VCI) Payload Type (PT) Cell loss Priority (CLP) Header Error Correction (HEC)
ATM Cell Header Fields Generic Flow Control (GFC) ◦ Provides local functions, such as identifying multiple stations that share a single ATM interface. This field is typically not used and is set to its default value of 0 (binary 0000). Virtual Path Identifier (VPI) Virtual Channel Identifier (VCI) ◦ In conjunction with the VCI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination. ◦ In conjunction with the VPI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination.
ATM Cell Header Fields Payload Type (PT) ◦ 1st Bit data type Indicates in the first bit whether the cell contains user data or control data. If the cell contains user data, the bit is set to 0. If it contains control data, it is set to 1. ◦ 2nd Bit Congestion Identifier (CI) The second bit indicates congestion (0 = no congestion, 1 = congestion) rd ◦ 3 Bit AAU The third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame (1 = last cell for the frame). Cell Loss Priority (CLP) ◦ Indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. If the CLP bit equals 1, cell should be discarded in preference to cells with the CLP bit equal to 0. Header Error Control (HEC) ◦ Calculates checksum only on the first 4 bytes of the header. HEC can correct a single bit error in these bytes, thereby preserving the cell rather than discarding it.
ATM Services Main types of ATM services: ◦ permanent virtual circuits (PVC) ◦ switched virtual circuits (SVC)
PVC permanent virtual circuits (PVC) ◦ Similar to a leased line ◦ Guarantees availability of a connection ◦ Does not require call setup procedure between switches ◦ Disadvantage: static connectivity Manual setup Non-flexibility
SVC switched ◦ ◦ ◦ ◦ ◦ virtual circuits (SVC) Similar to dial-up service/telephone call SVC is created and released dynamically Flexible Automatic call setup handling Disadvantage: Extra time and overhead required to set up the connection
ATM Reference Model ATM Planes ◦ Control responsible for generating & managing signaling requests. traffic policing, call control, congestion control, etc. ◦ User responsible for managing the transfer of data. ◦ Management Layer management manages layer specific functions (manages resources and parameters residing in protocol entities e.g. provisioning for QoS) Plane management manages functions for system as a whole, coordination between planes
The ATM Reference Model
The ATM Reference Model
Layers in ATM Model Three layers are there in ATM Reference Model: ◦ Physical Layer ◦ ATM Layer ◦ ATM Adaptation Layer
Physical layer Physical layer ◦ Analogous to the physical layer of the OSI model, it manages the medium-dependent transmission ATM also support wireless technology Original design of ATM was based on SONET ◦ 2 Reasons of using SONET High data rates Boundaries of cells are clearly defined. ATM support any technology of physical layer but the boundaries of the cell should be clearly defined.
ATM Layer Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the OSI model. The ATM layer is responsible for ◦ cell multiplexing the simultaneous sharing of virtual circuits over a physical link ◦ cell relay passing cells through the ATM network ◦ To do this, it uses the VPI and VCI information in the header of each ATM cell.
ATM Layer Provides routing, switching, multiplexing and traffic management. Transmission/Reception Cell header generation/removal at source/destination ◦ Header format is different for UNI and NNI Cell address translation Congestion Control/Buffer management Sequential delivery
ATM Adaptation Layer ATM ◦ ◦ ◦ ◦ adaptation layer (AAL) Breaks messages to cells Combined with the ATM layer, the AAL is roughly analogous to the data link layer of the OSI model. The AAL is responsible for isolating higher-layer protocols from the details of the ATM processes. The adaptation layer prepares user data for conversion into cells and segments the data into 48byte cell payloads
ATM layers in endpoint devices and switches
ATM Adaptation Layer Services: ◦ handling of transmission errors ◦ segmentation and reassembly (enables large blocks of data to be carried in payload field of ATM cells) ◦ handling of lost cells ◦ flow control Several AAL protocols have been specified to meet the needs of different types of application
ATM Adaptation Layer Subdivided into two logical layers: ◦ Convergence Sublayer (CS) upper-layer frames are basic data units concerned with flow and error control for connection-oriented traffic ◦ Segmentation and Reassembly Sublayer (SAR) segments of upper-layer frames are basic data units concerned with segmenting frames at the source and reconstructing frames at the destination
ATM Adaptation Layer Four ◦ ◦ ◦ ◦ versions AAL1 (for constant-bit-rate stream) AAL2 (for short packets) AAL3/4 (3 for connection oriented services and 4 for connection less services) AAL5 (for packets requiring no sequence and no error control mechanism)
ATM Adaptation Layers (AAL) AAL1 ◦ Connection-oriented service ◦ suitable for handling constant bit rate sources (CBR) ◦ Such as voice and videoconferencing ◦ AAL1 requires timing synchronization between the source and the destination ◦ Thus, AAL1 depends on a medium, such as SONET, that supports clocking.
AAL2 ◦ Connection-oriented service ◦ suitable for handling bursty traffic with variable bit rate sources (VBR) ◦ Also has timing requirements like AAL1 ◦ Traffic such as packetized voice or video That do not have a constant data transfer speed But require service similar to CBR
AAL3/4 ◦ AAL3/4 supports both connection-oriented and connectionless data. ◦ It was designed for network service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). ◦ AAL3/4 is used to transmit SMDS packets over an ATM network.
AAL5 ◦ AAL5 is the primary AAL for data and supports both connection-oriented and connectionless data. ◦ It is used to transfer most non-SMDS data, such as classical IP over ATM and LAN Emulation (LANE). ◦ AAL5 also is known as the simple and efficient adaptation layer (SEAL)
The ATM Reference Model (2)
References Data and Computer Communications ◦ 7th Edition, William Stallings Data Communications and Networking ◦ 3rd Edition, Behrouz A. Forouzan Internetworking Technologies Handbook ◦ 3rd Edition, Cisco Systems Data Communications and Computer Networks ◦ Curt M. White www.atmforum.org
Question Bank Group Activity 5 ◦ Submit questions related to the lectures and topics discussed in class per week. ◦ Questions format: (use both) 4 MCQs with 4 options and correct answer MCQs options should be written horizontally like: (a) 10 (b) 100 (c) 1000 (d) 10000 1 Short conceptual or logical question ◦ Submission will be done in soft form via group email to CR
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