PRACTICES GUIDE TO ATM CROSS PRODUCT DOCUMENTATION ISSUE 1, JANUARY 2002 FUJITSU NETWORK COMMUNICATIONS, INC.
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Table of Contents TABLE OF CONTENTS CHAPTER 1 What is ATM? 1.1 Overview................................................ 1-2 1.2 Benefits of ATM.......................................... 1-2 1.3 ATM Standards........................................... 1-3 1.4 ISO OSI Protocol Model.................................... 1-3 1.5 ATM Protocol Model....................................... 1-4 1.6 ATM Cell Size............................................ 1-4 1.7 ATM Cell Header.......................................... 1-5 CHAPTER 2 ATM SONET Interface 2.1 Overview................................................ 2-2 2.2 Network Interfaces........................................ 2-2 2.3 Connections by Interface.................................. 2-3 CHAPTER 3 ATM Connections and Topologies 3.1 Overview................................................ 3-2 i
Table of Contents 3.2 Connection Topologies.................................... 3-2 3.3 Virtual Paths and Virtual Channels........................... 3-3 3.4 Links................................................... 3-3 3.5 Termination Points........................................ 3-3 3.6 Connection Identifiers..................................... 3-3 3.7 Signaling................................................ 3-4 3.8 Services................................................. 3-4 ii
CHAPTERS 1 What is ATM? 2 ATM SONET Interface 3 ATM Connections and Topologies WHAT IS ATM? This chapter includes the following sections: 1 NO. SECTION PAGE 1.1 Overview.................................... 1-2 1.2 Benefits of ATM.............................. 1-2 1.3 ATM Standards............................... 1-3 1.4 ISO OSI Protocol Model........................ 1-3 1.5 ATM Protocol Model........................... 1-4 1.6 ATM Cell Size................................ 1-4 1.7 ATM Cell Header.............................. 1-5
CHAPTER 1 What is ATM? Overview 1.1 Overview Asynchronous Transfer Mode (ATM) is a multiplexed packet switching technique that uses packets (cells) with a fixed 53-octet length. Each cell contains a 48-octet information field and a 5-octet header that identifies the cell as belonging to a certain virtual path or channel. The 53-octet cell size is the result of a compromise between countries. The U.S. and Japan wanted a 64-octet payload size to minimize overhead issues. The European countries wanted a 32-octet payload size to minimize packetization delay and ensure that it would accommodate all traffic types. The 53-octet size became the compromise because it splits the difference down the middle. 1.2 Benefits of ATM ATM provides several advantages: Fixed length cells require lower processing overhead and produce higher transmission speeds than traditional packet switching methods. ATM transmits asynchronous data in a synchronous network. It can also prioritize time-sensitive traffic ahead of non time-sensitive traffic to ensure that quality of service is maintained. ATM allocates bandwidth on demand, making it suitable for high-speed connection of voice, data, and video services. It also employs statistical multiplexing techniques to efficiently utilize resources. ATM is application-independent; it can be used as a common infrastructure for many types of networks, including public, private, LAN, and campus backbone. ATM is designed for high-performance multimedia networking. It has been implemented in a very broad range of networking devices: PC, workstation, and server network interface cards Switched-Ethernet and token ring work group hubs Work group and campus ATM switches Enterprise network switches, multiplexers, edge, and backbone switches International standards compliance in central office and customer premise environments allows multi-vendor operation and interoperability. 1-2
CHAPTER 1 What is ATM? ATM Standards 1.3 ATM Standards The ATM standards have been established for the industry through several standards organizations: The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is the leader in defining Integrated Services Digital Network (ISDN), Broadband ISDN (B-ISDN), and ATM specifications on an international level. The organization formerly was known as the Consultative Committee on International Telegraphy and Telephony (CCITT). The American National Standards Institute (ANSI) is the formal standards body guiding the development of ATM in the U.S. The purpose of the ATM Forum is to quickly build industry consensus and produce implementation agreements at a quicker rate than normal standards organizations. The ATM Forum is made up of equipment vendors, service providers, and users. The focus of Bellcore is on networking products and services as they affect the local exchange service of the Regional Bell Operating Companies (RBOCs). Bellcore is now known as Telcordia Technologies. The Internet Engineering Task Force (IETF) is responsible for the operation and management of the Internet, including the definition of protocols to be used over the Internet. 1.4 ISO OSI Protocol Model The International Standards Organization Open System Interconnection (ISO OSI) protocol model is a commonly accepted standardization of communication interfaces and protocols. The functions and features of the model include a seven-layer protocol stack. Layers build upon each other, and the model allows modification of one layer without changing the entire architecture of a system. Layers 1, 2, and 3 are the essential components for most networks, including ATM networks. 1-3
CHAPTER 1 What is ATM? ATM Protocol Model 1.5 ATM Protocol Model ATM is a Layer 2 protocol. The ATM layer adds five octets of header to each cell. The header carries sufficient information to route the cells. The OSI, Layer 1 Physical layer transmits and receives cells. Layer 1 also encodes and decodes bits into electrical/optical systems. The OSI, Layer 2 Data link layer, subdivided into two ATM layers, contains an ATM layer and an ATM adaptation layer (AAL). The ATM layer: Detects and corrects header errors Generates and extracts the cell header Translates virtual channel identifiers (VCIs) and virtual path identifiers (VPIs) and places them into the header Multiplexes and demultiplexes the cell to and from facilities Inserts and removes idle cells used to pad the transmission rate Routes and switches cells Controls ATM traffic flow The AAL handles the segmentation and reassembly (SAR), the payload error checking and correction, and end-to-end timing responsibilities. This layer contains three fully-defined AALs. The OSI, Layer 3 Network layer identifies end-point, selects service, and signals. 1.6 ATM Cell Size The cell size consists of a 5-octet header that contains information for switching cells. The header is examined and updated on a node-by-node basis and is service-independent. The cell size also consists of information or data that is 48 octets long. This part of the cell contains payload or information relevant to a specific service. This information is relevant only to end-user systems and is service-specific. ATM encapsulates all services into cells, places them into a synchronous cell stream, and routes them across an ATM network. ATM cells interleave when there is an available slot. If a slot is not available, the cell is queued; therefore, cells are said to be transferred asynchronously with guaranteed cell order. 1-4
CHAPTER 1 What is ATM? ATM Cell Header 1.7 ATM Cell Header The ATM cell header consists of: Generic flow control (GFC) Has local significance only and can be used to provide standardized local functions (e.g., flow control) on the customer site. The value encoded is not carried end-to-end and is overwritten in the public network. VPI Distinguishes data of one virtual path from data of another. VCI Distinguishes data of one virtual channel from data of another. Payload type indicator (PTI) Indicates whether the cell contains layer management information. PTI is also used for indicating a network congestion state or for network resource management. Cell loss priority (CLP) Indicates the loss priority of the cell to assist the cell discarding process and to minimize the quality of service (QOS) degradation. Header error control (HEC) Is used by the physical layer for detection/correction of bit errors in the cell header. HEC may also be used for cell delineation. 1-5
CHAPTER 1 What is ATM? ATM Cell Header 1-6
CHAPTERS 1 What is ATM? 2 ATM SONET Interface 3 ATM Connections and Topologies ATM SONET INTERFACE This chapter includes the following sections: 2 NO. SECTION PAGE 2.1 Overview.................................... 2-2 2.2 Network Interfaces............................ 2-2 2.3 Connections by Interface...................... 2-3
CHAPTER 2 ATM SONET Interface Overview 2.1 Overview Synchronous Optical Network (SONET) is the transmission methodology that has become the standard for high-speed optical communications for many transmission types, including broadband. A SONET connection is divided into two sections: overhead and payload. ATM multiplexing occurs at the cell level, and the SONET payload is filled with cells. Digital signals are also supported by ATM. Digital signal, level 1 (DS1) and digital signal, level 3 (DS3) are the typical levels supported. 2.2 Network Interfaces Interfaces in the network include the user network interfaces (UNIs) from subscribers or the network node interfaces (NNIs) from other switches in the network. A UNI is a subscriber interface because it directly connects to subscriber terminal equipment. The following electrical UNI facilities are supported: Electrical DS1 Japan, level 2 (J2) UNI for cell relay service (CRS) DS3 for circuit emulation service (CES) and CRS ATM E3 (AE-3) for CRS Optical Optical Carrier, level 3 signal concatenated (OC-3c) for CRS Synchronous Transport Mode, level 1 (STM-1) for CRS Optical Carrier, level 12 signal concatenated (OC-12c) for CRS Synchronous Transport Mode, level 4 (STM-4) for CRS The makeup of a UNI varies depending upon whether the service type is CES or CRS. For these services, the UNI denotes the entire transmission facility meaning there is a one-to-one correlation between the package and the UNI. An NNI is considered to be a network interface because it provides a connection between the network elements. 2-2
CHAPTER 2 ATM SONET Interface Connections by Interface For CRS, the NNI denotes the entire transmission facility, which means there is a one-to-one correlation between the port location and the NNI. The UNIs and the NNIs can be connected in the following arrangements: UNI to UNI Analogous to a subscriber-to-subscriber connection UNI to NNI Analogous to a subscriber-to-network connection NNI to UNI Analogous to a network-to-subscriber connection NNI to NNI Analogous to a network-to-network connection 2.3 Connections by Interface Each interface permits a maximum number of virtual path (VP) and virtual channel (VC) connections for user data. These are the physical limits due to the hardware limits, not the software. The total for each physical limit is the combined totals for VPs and VCs, as listed in Table 2-1. Table 2-1: Connection Interfaces Interface(s) Connections DS1 254 J2 254 DS3 (for single-line card) 1022 STM-1 (for single-line card) 1022 OC-3c (for single-line card) 1022 OC-12c (for single-line card) 4094 DS3 (for multiline card) 8190 STM-1 (for multiline card) 8190 OC-3c (for multiline card) 8190 0C-12c (for multiline card) 8190 STM-4 (for multiline card) 8190 AE-3 (for multiline card) 8190 2-3
CHAPTER 2 ATM SONET Interface Connections by Interface 2-4
CHAPTERS 1 What is ATM? 2 ATM SONET Interface 3 ATM Connections and Topologies ATM CONNECTIONS AND TOPOLOGIES This chapter includes the following sections: 3 NO. SECTION PAGE 3.1 Overview.................................... 3-2 3.2 Connection Topologies........................ 3-2 3.3 Virtual Paths and Virtual Channels............... 3-3 3.4 Links....................................... 3-3 3.5 Termination Points............................ 3-3 3.6 Connection Identifiers......................... 3-3 3.7 Signaling.................................... 3-4 3.8 Services.................................... 3-4
CHAPTER 3 ATM Connections and Topologies Overview 3.1 Overview A permanent virtual connection (PVC) is a logical connection between two end users established by administrative procedures. This type of connection is generally created well in advance of its use, and remains in place until the connection is deprovisioned. A PVC can be a virtual path connection (VPC) or a virtual channel connection (VCC). Bandwidth is allocated whether or not the connection is being used. A switched virtual connection (SVC) is a logical connection established and disestablished with access and network signaling procedures. Standard signaling protocols have been established to support interoperability. SVCs are also logical connections between two subscribers. Cell transfer instructions for user traffic are established as each SVC is created. Bandwidth is allocated on an as-needed basis, providing for dynamic bandwidth allocation. A soft permanent virtual channel (SPVC) combines PVC and SVC functionality. Within the network, SVCs are provisioned. Outside the network, PVCs are provisioned to prevent vendor incompatibility. The signaling protocol on a UNI is digital subscriber signaling 2 (DSS2) or Q.2931. The signaling protocol on an NNI is B-ISDN user part (B-ISUP). B-ISUP is based on signaling system 7 (SS7) network protocol. The SVCs also require provisioning, but the cell transfer instructions for the end-user traffic are established as each call is initiated. For SVCs, virtual channel connections are created by the signaling protocol. 3.2 Connection Topologies The simplest connection involves two endpoints and is called a point-to-point connection. Point-to-point connections can be unidirectional or bidirectional communications, meaning data can flow in one or both directions. A point-to-point arrangement supports SVCs and PVCs. Unidirectional point-to-multipoint PVCs are also supported. In the point-to-multipoint configuration, a single user can send the same data to more than one receiver. The source of the information is the primary or root and the multiple endpoints that receive copies of this information are the commons or leaves. The primary source can be from a UNI only. The commons can be UNI or NNI. The point-to-multipoint makes use of the multicasting. Multicasting involves the replication of data traffic and its distribution to multiple subscribers. There are two types of multicasting: logical and spatial. In logical multicasting, multiple leaves of the multicast group appear on the same physical interface, so multiple copies of the user data must be sent over the single interface. In spatial multicasting, the leaves are on separate physical interfaces. 3-2
CHAPTER 3 ATM Connections and Topologies Virtual Paths and Virtual Channels 3.3 Virtual Paths and Virtual Channels 3.4 Links ATM is based on a connection-oriented transport infrastructure. Addressing information is maintained in the cell header. Cells do not contain explicit host addressing. VPCs are logical connections between two end points for the transfer of ATM cells. VPCs can also be a logical combination of VCCs. VPCs are identified with a VPI. VPCs are primarily for network-to-network applications. VCCs are logical connections between two end points for the transfer of ATM cells. They are identified with a VPI and a VCI. VCCs are primarily for user-to-network applications. A virtual channel link (VCL) is a portion of a VCC. A VCC can be composed of many VCLs, depending on the number of NEs in the connection. A virtual path link (VPL) is a portion of a VPC. Like a VCC, a VPC can be composed of many VPLs, depending on the number of NEs in the connection. A VPC is a concatenation of VPLs, and a VCC is a concatenation of VCLs. For processing within a network, virtual paths and virtual channels can be identified by VPIs and VCIs. 3.5 Termination Points Termination points are the endpoints of a connection. The type of termination point is determined by the connection. A connection termination point is the endpoint in a VCL or VPL. Since a connection can encompass links between several NEs, a connection termination point identifies the endpoints in each of those links. A trail termination point is the overall connection endpoint in a VPC or VCC. 3.6 Connection Identifiers A virtual path connection identifier (VPCI) is an identifier for a VPC and is used by the signaling system 7 (SS7) B-ISUP protocol to specify the particular VPC to be used for an SVC call/connection. 3-3
CHAPTER 3 ATM Connections and Topologies Signaling 3.7 Signaling Signaling is the process of exchanging information pertaining to a call. The signaling process transfers information over a facility to control setup, holding, charging, and releasing connections in a communications network. Signaling alerts a switch to be ready to: Collect information Alert the subscriber to send information Pass information to verify subscriber authenticity (originating party) Route the call to the proper destination Enable charging of the proper party for the call Alert a called party and relay the called party status Be involved at the time of hang-up when the call is to be cleared The signaling protocol defined by the standards bodies to address the subscriber side (UNI) is Q.2931. B-ISUP is defined as the network side (NNI) protocol. 3.8 Services Services provide applications. Services are different from technologies; services have a price and quality of service parameters and are more complicated. The following services are supported by ATM: Cell relay service (CRS), which is an ATM cell-based information transfer service. CRS offers users direct access to the ATM layer at virtually any rate up to the access line rate. It also supports variable bit rate (VBR), constant bit rate (CBR), unspecified bit rate (UBR), and available bit rate (ABR). Service adaptation is performed in the CPE. Circuit emulation service (CES), which transports CBR signals (or circuits) using ATM. CES requires adaptation through the ATM adaptation layer (AAL 1). Switched multimegabit data service (SMDS), which requires adaptation through the AAL 3/4. This adaptation layer provides support for connectionless VBR traffic. ATM supports SMDS packets. Frame relay service (FRS), which requires adaptation through the AAL 5. This adaptation layer provides support for connection-oriented VBR traffic. ATM supports frame relay frames. 3-4