Cabletron Systems FDDI TECHNOLOGY GUIDE
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Contents Overview PURPOSE OF THIS MANUAL...................................................................................... v WHO SHOULD USE THIS MANUAL ......................................................................... v STRUCTURE OF THIS MANUAL ................................................................................ v RELATED DOCUMENTS.............................................................................................. vi Chapter 1 INTRODUCTION FDDI OVERVIEW .................
Chapter 5 FDDI RING TOPOLOGY DUAL RING WITHOUT TREES................................................................................. 5-1 DUAL RING WITH TREES ......................................................................................... 5-2 WRAPPED RING .......................................................................................................... 5-3 SINGLE TREE ................................................................................................................
Appendix A ANSI STANDARDS FOR FDDI THE OSI NETWORK MODEL................................................................................... A-2 STATION MANAGEMENT (SMT) ........................................................................... A-3 SMT Frame Services ............................................................................................. A-3 Connection Management..................................................................................... A-4 Ring Management (RMT) .............
PREFACE PURPOSE OF THIS MANUAL Welcome to the FDDI Technology Manual. This manual provides a basic overview of Fiber Distributed Data Interface (FDDI) technology. The objective of this manual is to help Cabletron’s customers better understand FDDI concepts and network operation. WHO SHOULD USE THIS MANUAL This manual is intended for users of Cabletron’s FDDI products and should be used as a supplement to Cabletron’s FDDI User Manuals.
PREFACE RELATED DOCUMENTS The American National Standards Institute (ANSI) Accredited Standards Committee (ASC) Task Group X3T9.5 writes all FDDI standards. The ANSI committee consists of representatives from various networking companies. Cabletron is an active member of the ANSI committee and strictly adheres to these standards while designing new products. For additional FDDI information, refer to the following ANSI documents: • Station Management (SMT) - ANSI X3.
Chapter 1 INTRODUCTION This chapter introduces Fiber Distributed Data Interface (FDDI) features and describes characteristics that distinguish FDDI from other Local Area Network (LAN) technologies such as Ethernet and Token Ring. FDDI OVERVIEW FDDI is a 100 megabit per second LAN technology that transmits data frames over dual counter-rotating rings.
INTRODUCTION FDDI FEATURES FDDI’s most distinguishing features are directly related to the fiber optic medium. Although twisted pair cable is a valid FDDI transmission medium, it does not match the performance features of fiber and is used primarily as a low cost solution for desktop connections. The fiber optic medium provides a number of advantages over twisted pair including greater transmission distances, fault recovery, and security.
FDDI FEATURES Fault Recovery FDDI has a dual counter-rotating ring topology that provides a primary path for normal operation and a secondary path for fault recovery. If the primary ring fails, FDDI changes the data path to the secondary ring. Frame Transmission FDDI stations communicate on the ring using the following message formats: • Frames: provide information concerning ring management, network problems, and statistics. • Token: The token is a special frame that controls access to the ring.
Chapter 2 FDDI DEVICES This chapter describes devices that are common to an FDDI network. All devices attached to an FDDI ring must comply with The ANSI X3T9.5 Standards outlined in Appendix A. Typical FDDI devices include stations, concentrators, bridges. and optical bypass switches. Figure 2-1 shows various devices attached to an FDDI ring. The following sections provide a description of each of these devices and their network functions.
FDDI DEVICES FDDI STATIONS FDDI stations are addressable nodes on an FDDI network capable of transmitting, receiving, and repeating information. Workstations, Fileservers, and Printers are examples of FDDI stations. Stations connect to the ring using one of the following configurations: • Single-Attachment Station (SAS): connects to one ring. • Dual-Attachment Station (DAS): connects to both rings. Figure 2-2 shows each of the station configurations.
FDDI CONCENTRATORS FDDI CONCENTRATORS FDDI concentrators are central hubs that provide connections to the ring for single attached stations. Concentrators may or may not have a MAC entity and connect to the ring using one of the following configurations: • Null-Attachment Concentrator - Does not connect to the Dual Rings. • Single-Attachment Concentrator (SAC) - connects to one ring. • Dual-Attachment Concentrator (DAC) - connects to both rings.
FDDI DEVICES FDDI BRIDGES FDDI bridges connect multiple FDDI networks. They also link FDDI rings to similar networks such as Token Ring or Ethernet. Similar networks have the same upper five layers of the OSI model but have different Link and Physical layers. A bridge does not expand an existing FDDI ring, it connects rings. Figure 2-4 is an example of an FDDI bridge configuration. ETHERNET NETWORK FDDI TO ETHERNET BRIDGE FDDI DUAL COUNTER-ROTATING RING NETWORK Figure 2-4.
OPTICAL BYPASS SWITCHES OPTICAL BYPASS SWITCHES Optical bypass switches maintain ring continuity if an FDDI station fails or becomes removed from the ring. This device is inserted between a station and the FDDI ring connection and provides passive optical switching of both the primary and secondary ring cables. For example, if an FDDI station fails, the optical bypass switch automatically diverts FDDI frames through the switch instead of through the station. This prevents a wrap condition on the FDDI ring.
Chapter 3 FDDI PHYSICAL CONNECTIONS This chapter describes FDDI connector types and FDDI connection rules. Multimode fiber and single mode fiber optic cable use Media Interface Connectors (MICs) to attach to FDDI ports while Twisted pair cable attaches to concentrators using RJ45 ports and connectors. The following section describe each connector type. FDDI FIBER CONNECTORS Multimode fiber and singlemode fiber optic cable use Media Interface Connectors (MICs) to attach to FDDI ports.
FDDI PHYSICAL CONNECTIONS MIC Connector Ports MIC connectors have keys which distinguish port types. Types A, B, and M are precision connectors, mechanically keyed to ensure proper connections to Primary Ring-In and Primary Ring-Out fibers. The Type S connector has a wide, centrally located key and is considered a non-precision connector for use at the station end of a Single Attachment Station lobe cable.
FDDI TWISTED PAIR CONNECTORS FDDI TWISTED PAIR CONNECTORS The Twisted Pair Physical Layer Medium Dependent (TP-PMD) ANSI specification is a working draft and many of the standards proposed by the TP-PMD have not been approved by the networking industry. The twisted pair specifications listed in this document are specific to Cabletron only. Cabletron’s FDDI products use twisted pair cabling to connect Single Attached Stations to FDDI concentrators.
FDDI PHYSICAL CONNECTIONS FDDI PORT CONNECTION RULES In a typical FDDI ring, the following rules apply: • A ports should only connect to B ports. • B ports should only connect to A ports. • M ports should only connect to S ports. All other port-to-port connections are either Illegal or Undesirable because they may result in unexpected ring topologies. The Station Management entity checks for Illegal or Undesirable connections when any link is established.
Chapter 4 FDDI FRAME FORMATS This chapter describes FDDI frame formats. The MAC entity generates two basic message formats, Tokens and Frames. The following sections describe each message format.
FDDI FRAME FORMATS Table 4-1. FDDI Data Frame Layout Field Name Field Definition Preamble 16 + symbols Signals the start of a valid frame. Start Delimiter 2 Symbols Signals that FC is next field. Frame Control 2 Symbols Identifies the type of frame (MAC, LLC, etc.). Destination Address 4 or 12 Symbols Address of the destination of the packet. Source Address 4 or 12 Symbols Address of the packets origin. Information (Data) ≤ 8956 Symbols Contains the data to be transferred.
TOKEN FRAMES TOKEN FRAMES The Token is made up of 22 symbols. Table 4-2 explains each of the Token Frame fields. Table 4-2. FDDI Frame Type Field Name Field Size Field Definition Preamble 16 + symbols Maintains clock synchronization Start Delimiter 2 Symbols Signals the start of a valid token. Frame Control 2 Symbols Distinguishes a Token from a Frame. ED (Ending Delimiter) 1 Symbol Signals the end of the Token.
Chapter 5 FDDI RING TOPOLOGY This chapter describes FDDI ring topologies as well as FDDI design considerations that may be useful to a network designer. DUAL RING WITHOUT TREES The Dual Ring Without Trees configuration consists of dual attachment stations that form a ring by connecting A ports to B ports and B ports A ports. This configuration is commonly used in small engineering/research environments to localize FDDI rings.
FDDI RING TOPOLOGY DUAL RING WITH TREES A Dual Ring with Trees configuration uses dual attachment concentrators, single attachment concentrators, and single attachment stations to form tree structures. Single attachment stations and single attachment concentrators connect to the dual attachment concentrator M ports instead of the ring.
WRAPPED RING WRAPPED RING A Wrapped Ring is the result of a broken cable or a faulty Dual Attachment Concentrator or Dual Attachment Station. Figure 5-3 shows a cut cable between concentrator 3 (downstream neighbor) and concentrator 4 (upstream neighbor). Both Concentrator 3 and Concentrator 4 wrap. This scenario repeats for a failed station or concentrator. Both the upstream and downstream neighbors wrap.
FDDI RING TOPOLOGY SINGLE TREE The Single Tree topology does not use the dual ring, only the single ring. All of the network devices are single attachment stations and single attachment concentrators. Since the Single Tree topology only uses the single ring, there is no back up path if a cable fails. If an individual single attachment station fails, it does not affect the rest of the network.
DUAL HOMING FDDI DUAL ATTACHED STATION FILESERVER DUAL ATTACHED CONCENTRATOR 3 DUAL HOMED CONNECTION PRIMARY RING SECONDARY RING FDDI DUAL COUNTER-ROTATING RING NETWORK DUAL ATTACHED CONCENTRATOR 4 DUAL ATTACHED CONCENTRATOR 2 DUAL ATTACHED CONCENTRATOR 1 Figure 5-5.
Chapter 6 FDDI RING OPERATION This chapter describes basic FDDI ring operation, including: • Station Initialization • Ring Initialization Proper operation of the FDDI ring requires interaction between the Station Management (STM), Media Access Control (MAC), Physical Layer Protocol (PHY), and Physical Layer Media Dependent (PMD) entities. The Station Management entity is responsible for coordinating station initialization and the normal operation of the FDDI ring.
FDDI RING OPERATION STATION C STATION B Link B/C PHY B PHY B MAC Link C/A MAC Link A/B PHY A PHY A PHY A PHY B MAC STATION A Figure 6-1.
STATION INITIALIZATION Table 6-1 explains the station initialization procedure. Table 6-1. Station Initialization Sequence Description Break State Figure 6-1 shows the PCM of station C entering the Break State. During the Break State, Station C sends Quiet Symbols to the PHY in station B. As a result, Station B stops transmitting any data or symbols and enters the Break State. During the Break State, Station B breaks all existing connections and enters the Quiet Line State.
FDDI RING OPERATION Table 6-1. Station Initialization Sequence Description Link Confidence Test Link Confidence Test information is also exchanged between stations. This information indicates the length of the test and determines if a MAC is involved. If a MAC is not involved in the test, Idle Symbols are transmitted during the Link Confidence Test.
RING INITIALIZATION After the Station Initialization process is complete and the stations are physically attached to the ring, as shown in Figure 6-3, the ring must be initialized. The Ring Initialization procedure determines which station transmits the first token. It also sets the Operational Token Rotation Time. The Operational Token Rotation Time determines how long a station must wait before it receives a token.
FDDI RING OPERATION Transmitting the First Token Stations on an FDDI ring bid for the right to issue the first token. Each station has a preset timing requirement called a Token Rotation Timer (TRT). The TRT determines how often the token must visit the station. This time is compared with the timing requirements of the other stations on the ring and the stations bid for the Target Token Rotation Time (TTRT). The station with the lowest TRT wins the right to issue the first token.
RING INITIALIZATION Synchronous Transmission Transmission of normal Protocol Data Units is controlled by a Timed Token Rotation Protocol. This protocol supports two classes of service, synchronous and asynchronous. Synchronous service gives each station a guaranteed bandwidth and response time. Each station can be assured that they see a token once every 2 x T_OPR (two times the winning TTRT bid).
FDDI RING OPERATION Restricted/Non-Restricted Token Mode Asynchronous bandwidth is controlled by a two tier allocation mechanism, enforced by two classes of tokens. Normal operation is achieved using a Non-Restricted Token being issued by a station. Restricted Token Mode may be entered when a station wishes to initiate an extended dialogue requiring substantially all of the unallocated ring bandwidth (e.g., an extended burst data transfer from a high speed device).
BASIC RING OPERATION Total Transmission Time The total allowable transmission time is the negotiated value of T_OPR minus the maximum ring latency. The formula for Total Transmission Time is: Total Transmission Time = T_OPR - Maximum Ring Latency. Token Rotation Timer Each station has a timer called the Token Rotation Timer (TRT) that is used to control ring scheduling during normal operation and to detect and recover from serious ring error situations.
FDDI RING OPERATION The Beacon Process The Beacon process is used to recover from serious ring faults. These faults include a failed Claim Process, a broken ring, re-configured ring, or the joining of two logical rings into one. The purpose of the Beacon Process is to signal to all other stations that a significant logical break in the ring has occurred and to provide diagnostics or other assistance to recover the ring using SMT.
BASIC RING OPERATION Ring Fault Recovery An FDDI network consists of two distinct separate rings., the primary ring and the secondary ring. Under normal conditions data frames travel on the primary ring and the secondary ring is used as a back-up path. If a fiber is cut between two Dual Attached nodes its upstream and downstream neighbor will wrap. When a node wraps a port it internally connects the primary ring to the secondary ring.
FDDI RING OPERATION OPTICAL BYPASS SWITCH In a network design where the wrap function is undesirable, an Optical Bypass Switch (OBS) is used to maintain the integrity of the data flow through the network while not combining the primary and secondary rings. When a node is de-powered or is malfunctioning, the OBS is activated and diverts the frames through the switch instead of to the node, This eliminates the need for upstream and downstream nodes to wrap.
Chapter 7 BRIDGING WITH THE FDMMIM This chapter explains how Cabletron’s FDMMIM performs translational bridging from Ethernet to FDDI and back to Ethernet. ETHERNET FRAME TYPES There are four basic Ethernet frame types: • 802.3 "raw" • Ethernet II (DIX) • Ethernet 802.2 • Ethernet SNAP Figure 7-1 shows each of the Ethernet frame types. The Ethernet 802.2 and Ethernet SNAP frames are extensions of the 802.3 "raw" frame format, while the Ethernet II frame is formatted slightly different.
BRIDGING WITH THE FDMMIM Ethernet II Frame Preamble Destination Address Source Address Type Data FCS 8 bytes 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes 802.3 "Raw" Frame Preamble Start Frame Delimiter Destination Address Source Address Length Data FCS 7 bytes 1 byte 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes Ethernet 802.
ETHERNET FRAME TYPES Ethernet II Frame Type The Ethernet II or (DIX) frame format was developed by DEC, Intel Corporation, and Xerox Corporation. Table 7-1 describes each of the Ethernet II Frame fields. Table 7-1. Ethernet II Frame Type Field Name Field Size Field Definition Preamble 8 bytes Signals beginning of the packet. Destination Address 6 bytes Address of the destination of the packet. Source Address 6 bytes Address of the packets origin.
BRIDGING WITH THE FDMMIM Ethernet 802.2 Frame Type The Ethernet 802.2 frame format builds upon the 802.3 "raw" frame structure. Table 7-3 describes each of the Ethernet 802.2 frame fields. Table 7-3. Ethernet 802.2 Frame Type Field Name Field Size Field Definition Preamble 7 bytes Signals beginning of the packet. Start Frame Delimiter 1 byte Signals start of data. Destination Address 6 bytes Address of the destination of the packet. Source Address 6 bytes Address of the packets origin.
ETHERNET FRAME TYPES Ethernet SNAP Frame Type The Ethernet Sub-Network Access Protocol (SNAP) frame type is an extension of the Ethernet 802.2 frame structure. The Ethernet SNAP frame adds a 5 byte Protocol Identification Field immediately following the LLC header. The Protocol Identification Field is made up of a 3 byte Protocol ID or Organizational Code Field followed by a 2 byte Type Field "Ethertype".
BRIDGING WITH THE FDMMIM FDDI FRAME TYPES Figure 7-2 shows an FDDI Token Frame and an FDDI Data Frame: TOKEN Preamble ≥ 16 Symbols Starting Delimiter 2 Symbols Frame Control 2 Symbols Preamble ≥ 16 Symbols Starting Delimiter 2 Symbols Frame Check Sequence Coverage T T J K FRAME Ending Delimiter 2 Symbols Frame Control Destination Address 2 Symbols 4 or 12 Symbols Source Address 4 or 12 Symbols T Information ≥ 0 Symbol Pairs Frame Check Ending Delimiter Sequence 8 Symbols 1 Symbol Frame Statu
FDDI FRAME TYPES FDDI 802.2 Frame Type There are two basic FDDI frame types that are used for the transmission of data: • 802.2 • FDDI SNAP The FDDI 802.2 frame type contains the same 802.2 (LLC) header set up as the Ethernet 802.2 frame type. The differences between the two frames are due to their technological differences. These differences can be seen when comparing the 802.3 header with the FDDI header.
BRIDGING WITH THE FDMMIM FDDI SNAP Frame Type The FDDI SNAP frame builds upon the 802.2 (LLC) layer of the FDDI 802.2 frame, just as the Ethernet SNAP frame builds upon the LLC layer of the Ethernet 802.2 frame. The SNAP header consists of the 802.2 header plus a Protocol Identification Field. The FDDI and Ethernet SNAP frame differ in the same way the FDDI and Ethernet 802.2 frame types differ. The Ethernet SNAP frame does not have a Frame Control Field, but does have a Length Field.
ETHERNET TO FDDI BRIDGING ETHERNET TO FDDI BRIDGING When bridging a frame from Ethernet to FDDI the FDMMIM has to deal with 4 different types of Ethernet frames, and translate them into one of two FDDI frame formats*. The FDMMIM's first task is to determine what type of frame it is receiving on Ethernet. To do this the FDMMIM goes directly to the 2 byte field immediately following the Source Address of the packet.
BRIDGING WITH THE FDMMIM Ethernet II to FDDI SNAP Frame Bridging When an FDMMIM bridges an Ethernet II frame onto FDDI it will insert a SNAP header into the frame and format it as an FDDI SNAP frame. The following is an illustration of this process. In this particular example there is a workstation located on an Ethernet segment (00001D09535D) which is transferring a file from a Novell server (00001D08A333) which is located on another Ethernet segment.
FDDI TO ETHERNET BRIDGING Ethernet SNAP Frame to FDDI SNAP Frame Bridging Finally, we have the Ethernet SNAP frame type. Again the FDMMIM will go to the 2 byte field immediately following the Source Address. It contains a hex value of 40. The hex value 40 is equal to 64 decimal, which is less than 1500. Since the FDMMIM knows that the frame is not an Ethernet II frame it will simply take out the Length Field, add the Frame Control, Start of Frame Sequence, and End of Frame Sequence.
BRIDGING WITH THE FDMMIM Case 1 A frame originated as an 802.3 "raw" frame on Ethernet and has been bridged onto FDDI. The FDMMIM that is going to forward this frame onto an Ethernet segment is responsible for forwarding the frame as an 802.3 "raw" frame. In reality the format of the frame on FDDI is not a legal FDDI data frame. Because Novell is using the 802.3 "raw" format, Ethernet to FDDI bridge vendors had to develop a way to pass these frames onto FDDI.
FDDI TO ETHERNET BRIDGING Case 4 This is a special case that is handled by the FDMMIM. The frame originates as an Ethernet SNAP frame and is bridged to FDDI. Case 3 tells us that when the FDMMIM bridges a FDDI SNAP frame to Ethernet it translates the frame into an Ethernet II frame. There is an exception to that rule.
BRIDGING WITH THE FDMMIM The following combinations of frame types may cause problems: 7-14 • If a station generates an Ethernet SNAP frame when using any protocol other than the AppleTalk or AppleTalk ARP protocols there will be problems with frame type consistency. The Ethernet SNAP frame will get bridged to FDDI as a FDDI SNAP frame, however it will get bridged back to Ethernet as an Ethernet II frame.
Appendix A ANSI STANDARDS FOR FDDI This chapter describes the American National Standards Institute (ANSI) standards for FDDI. ANSI is the governing body of FDDI standards and all devices on an FDDI ring must comply with these standards. The ANSI standards committee defines the following entities: • Station Management (SMT) - ANSI X3T9.5 • Media Access Control (MAC) - ANSI X3.139 • Physical Layer Protocol (PHY) - ANSI X3.148 • Multimode Fiber Physical Layer Medium Dependant (PMD) - ANSI X3.
ANSI STANDARDS FOR FDDI THE OSI NETWORK MODEL The OSI network model defines standards for communication between computer equipment and networks.The FDDI entities defined by ANSI perform many of the functions required in layer 1 (Physical) and Layer 2 (Data Link) of the OSI network model. Figure A-2 shows the relationship between the OSI Model and the ANSI FDDI entities.
STATION MANAGEMENT (SMT) STATION MANAGEMENT (SMT) SMT is the management entity. It communicates with the MAC, PHY, and PMD entities to ensure proper station and ring operation. SMT communicates with the SMT of each station on the FDDI network to ensure proper ring operation. The ANSI X3T9.5 specifies three distinct SMT functions: • • • SMT Frame Services Connection Management Ring Management Each FDDI Station may have several instances of MAC, PMD, or PHY but may have only one instance of SMT.
ANSI STANDARDS FOR FDDI SMT Frame Based Management Protocols SMT Frame Based Management Protocols allow FDDI stations to communicate with the SMT of other FDDI stations on a ring. It gathers network statistics as well as detects, isolates, and resolves network faults. The SMT Management Protocols consist of six basic frame types; NIF, SIF, DCF, RDF, PMF, and SRF. Table A-1 describes each SMT frame type. Table A-1.
STATION MANAGEMENT (SMT) Physical Connection Management (PCM) PCM performs the following functions: • Controls the physical connection (link) of the station onto the FDDI ring. • Runs a line-state communications protocol between its PHY and the PHY at the other end of the link. The line-state communications protocol tests the integrity of the link, and checks for valid FDDI topology connections before it allows the link to become active.
ANSI STANDARDS FOR FDDI MEDIA ACCESS CONTROL (MAC) The FDDI Media Access Control (MAC) specifies the lower sublayer functions of the Data Link Layer of the OSI Model. The MAC performs the following functions: • Controls access to the medium (single mode fiber, multimode fiber, shielded twisted pair, unshielded twisted pair). • Addresses frames. • Specifies token and MAC frame formats. • Generates MAC frames. The MAC entity is the lower sublayer of the Data Link Layer.
PHYSICAL LAYER PROTOCOL (PHY) 4-Bit/5-Bit Encoding/Decoding Scheme The PHY receives data frames from the MAC as a series of 4-bit symbols and encodes each 4-bit MAC symbol as a 5-bit symbol for transmission. The 5-bit symbols are encoded so that each symbol has at least two bit transitions to assure bit-cell synchronization at the remote receiver. Decoding reverses this process for the received frames. This process is referred to as 4B/5B or NRZI (Non-Return to Zero Invert on Ones) encoding/decoding.
ANSI STANDARDS FOR FDDI PHYSICAL MEDIUM DEPENDANT (PMD) The FDDI Physical Medium Dependant (PMD) specifies the lower sublayer functions of the Physical Layer of the OSI model. The PMD establishes the physical interface to the FDDI ring and converts optical energy symbols into electrical symbols, as well as electrical energy symbols into optical energy symbols. The PMD performs the following functions: • Controls optical transmit/receive levels. • Controls optical jitter.
Appendix B FDDI SPECIFICATIONS This appendix outlines FDDI specifications and design considerations. Table B-1. General Rules and Specifications Max. Number of Connections 1000 (500 sations) Stations are connected in series on an optical fiber ring. Since fiber optics is a point to point media, no taps are allowed between stations. Data Rate 100 Megabits per second Max. Total Ring Length 100km (or 200 km in wrap state) Drive Length (Max. Distance between Stations) -Multimode Fiber: 2 km (1.
FDDI SPECIFICATIONS FDDI DESIGN CONSIDERATIONS The main variables that are of interest to the FDDI network designer are: • ring length • drive distance (distance between nodes) • maximum number of stations on the ring The following sections outline basic FDDI design considerations as well as critical specifications. Ring Length The maximum FDDI Ring Length is 100 km. Although ANSI standard X3T9.
FDDI DESIGN CONSIDERATIONS Attenuation Attenuation is the level of optical power loss measured in decibels (dB). The maximum attenuation (attenuation budget) between any two active connections to the ring, as defined by the FDDI standard, is 11 dB. The budget includes the attenuation of the cabling, splices, connections, and optical bypass switches. For example, the attenuation of the typical multimode fiber optic cable used in FDDI networks is 2.5 dB/1km or 5 dB for the 2 km maximum node separation.
FDDI SPECIFICATIONS DAS 2 16 PHYSICAL CONNECTIONS DAC 1 +1 2 2 SAC 2 2 SAS 2 SAS 2 SAS 2 SAS 2 SAS Figure B-1.
