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HP 9000 Networking: Installing and Administering HP FDDI/9000 Software > Chapter 1 FDDI Resources
FDDI Concepts |
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The Fiber Distributed Data Interface (FDDI) is a high speed local area network which has been defined as a standard by an American National Standards Institute committee, ANSI X3T9.5 and by ISO. This second generation LAN is characterized by a fiber-optic dual token ring transmission medium which is capable of transmitting data at 100 megabits per second, ten times the speed of Ethernet. Figure 1-1 “FDDI Standards and the OSI Model” shows the four ANSI FDDI standards and how they combine to form a completely functional fiber optic network. This figure also shows how these standards fit into the OSI model. The physical layer includes two pieces, the Physical Medium Dependent (PMD) layer that provides the point-to-point communications between stations in the network, and the Physical Layer Protocol (PHY) layer that handles synchronization between higher layer data and control symbols, and the code bit representation which is transmitted on the medium. The data link layer includes the Media Access Control (MAC) standard and the Logical Link Control (LLC) standard. The MAC's primary function is the scheduling, routing and delivery of Frames, the vehicles used to transmit information on and off the ring. In an FDDI network, information is transmitted sequentially, within frames, as a stream of encoded symbols from one station to the next. The order of the symbols within the frames is predetermined by the MAC standard. The LLC provides a common protocol between the MAC and the network layer. In addition to FDDI, the LLC standard also applies to 802.3, 802.4, and 802.5 The Station Management (SMT) standard is a layer management entity which interfaces with the other sublayers. It manages connections with the ring as well as station configuration and ring configuration. HP FDDI/9000 supports SMT version 7.2. The example network map in Figure 1-2 “FDDI Network Map” shows HP Series 700 workstations connected to an FDDI dual ring through a concentrator. The FDDI network is connected to an 802.3/Ethernet LAN via a router. During normal operation, the two rings in an FDDI network are independent and the primary ring, represented by the solid line, actively transmits data in accordance with a timed-token protocol. The secondary ring, represented by the dotted line, remains inactive, providing a redundant LAN capability, until a fault occurs to break the primary ring. In the example above, the mainframe, concentrator and router are connected to both the primary and secondary rings simultaneously via A and B port types. These stations can access either or both rings for data transmission and can reconfigure the network, if necessary. Stations directly connected to the dual fiber optic ring are referred to as dual-attached stations (DAS) or dual-attached concentrators (DAC). HP FDDI/9000 supports dual attach and dual homing capabilities on Series 800 systems. HP S700 workstations connect to the dual ring network through a device called a concentrator which is directly attached to both rings via A ports and B ports. The workstations connect to the concentrator via M ports. Stations connected to a ring via a concentrator are referred to as single-attached stations (SAS). The FDDI concentrator provides another type of fault tolerance. When a station is removed from the network via a concentrator, the station is bypassed within the concentrator and the topology of the FDDI network is not affected. Ring access is controlled by a Token, a frame comprised of a unique sequence of symbols, which circulates according to the value of a Target Token Rotation Time (TTRT). The TTRT is negotiated by all stations during ring initialization. During the initialization process, each station on the ring submits a Requested Token Rotation Time (T_Req). All active stations then negotiate a TTRT value (T_Neg) that becomes the final TTRT value upon successful ring initialization. When the ring is operational, the token rotates continuously and sequentially around the ring. The token is the means by which the right to transmit is passed from one station to another. If a station has data to transmit, it must first capture the token from the ring. The station may then transmit data until all its data has been sent or the Token Holding Timer (THT) expires. The THT limits the length of time a station may have the token before issuing a new one. After a transmission is complete, the station creates and transmits a new token onto the ring. The token continues to circulate, providing other stations the chance to gain access to the ring, until it is captured by another (or same) station waiting to transmit data. The portion of the network map shown in Figure 1-3 “FDDI Dual Ring During Normal Operation” illustrates the data path through the primary ring and dual-attached stations during normal operation. In this example, the value of the Ring Management State (RMT), (the function of FDDI Station Management (SMT) which manages MAC layer components) is RingOp for every MAC station on the ring, indicating that every station is part of an operational FDDI ring. (Refer to the fddistat manpage for more information.) Data enters each dual-attached station or concentrator on the primary ring via Port A and leaves via Port B. Data is transmitted to the single-attached stations via the concentrator M ports. The secondary ring remains inactive during normal operation. By definition, information is always received by a station from the nearest station upstream, the Upstream Neighbor (UNA), and transmitted to the nearest station downstream, the Downstream Neighbor (DNA). In the example in Figure 1-3 “FDDI Dual Ring During Normal Operation”, the concentrator with MAC address 080009001023 receives data from its Upstream Neighbor at MAC address 080009005034 (see Figure 1-5 “FDDI Single Attached Station When Bypassed” for that section of the network map) and transmits to its Downstream Neighbor, the concentrator at MAC address 080009004056. In this example, when the ring is up and functioning normally, the Station State of the concentrator is listed as Rooted, indicating that the concentrator has active A, B, or S ports in tree mode, and the Station State of the single- attached stations is listed as Wrapped indicating the MAC in that station can transmit data frames on the FDDI network. (Refer to the fddinet for more information.) During normal operation, the two rings in an FDDI network are independent. The primary ring actively transmits data in accordance with timed-token protocol, while the secondary ring remains inactive, providing a redundant LAN capability, until a fault occurs to break the primary ring. When stations on an FDDI network are connected to both the primary and secondary rings simultaneously, these stations can access either or both rings for data transmission and can reconfigure the network, if necessary. Stations directly connected to the dual fiber optic ring are referred to as dual-attached stations. Dual-attached stations do not require connection to the FDDI network through a concentrator. Dual homing improves network availability by allowing the dual fiber connections on the FDDI adapter card to be connected into two separate concentrators simultaneously. If one connection is broken, the system will still be able to communicate on the FDDI ring. HP FDDI/9000 supports dual attach and dual homing capabilities for Series 800 systems. In Figure 1-4 “FDDI Dual Ring In Wrap Mode” a fault has occurred on the primary ring because the mainframe computer is not operational. The FDDI fault tolerant capability allows the rings to reconfigure automatically and form a new ring between the two concentrators and the router. An optical bypass could also be used. In that case, the ring would not wrap. The dotted lines illustrate connections to the mainframe which are no longer active. In this new configuration, the concentrator at MAC Address 080009001023 will have an attachment state (CF_State) of Wrap_A, indicating frames are now being transmitted via the A port, and a new downstream neighbor, the router at MAC Address 080009002034. The CF_State of individual stations on the ring varies depending on the ring configuration at any given time. In Figure 1-3 “FDDI Dual Ring During Normal Operation”, which shows an FDDI ring in normal operation, all the dual-attached stations on the ring have a CF_State of Thru_A, with the data stream entering each MAC through the A port and exiting through the B port. Together the A and B ports determine the attachment configuration of the station or concentrator. In Figure 1-4 “FDDI Dual Ring In Wrap Mode” showing an FDDI ring in wrap mode, the CF_State of both the inactive and active stations has changed to reflect the new ring configuration. The CF_State of the mainframe, which is no longer operational, is now Isolated, the CF_State of the concentrator at MAC address 080009001023 is Wrap_A, because its MAC is now transmitting the data stream back out the A port, and the CF_State of the concentrator at MAC address 080009005067, is Wrap_B because its MAC is now transmitting the data stream back out the B port. The possible attachment states of single-attached stations are Wrap_S or Isolated. Wrap_A and Wrap_B are fault recovery states. Wrap_S is a normal state. (Refer to Figure 1-5 “FDDI Single Attached Station When Bypassed” for an illustration of single-station attachment configurations. Refer to the fddistat(1M) manpage for more information.) Figure 1-5 “FDDI Single Attached Station When Bypassed” shows the data path through the concentrator after one of the workstations has been detached from the FDDI network. In this case the fault tolerant capability of the concentrator enables the station that is not operational to be bypassed. The FDDI Link Error Monitor (LEM) is a fault detection service that continually examines the Link Error Rate (LER) of an active link. The detector function of the LEM monitors the link quality and identifies link error events. Whenever the link error rate exceeds the predetermined cutoff threshold, the link is automatically removed from the ring. The Link Error Rate Estimate (LER_Estimate) provides the long term average link error rate. (Refer to the fddistat(1M) manpage for more information.) For error detection and performance analysis, the MAC entity in each station maintains statistics on all frames received at and transmitted from the station. The MAC counter, Frame Count (Frame_Ct), specifies the total number of frames received at the station. To be counted as a frame, the frame must conclude with an ending delimiter, the symbol that indicates the ending of tokens and frames. The MAC also monitors the Receive Count (Receive_Ct), which indicates the total number of SMT or LLC frames successfully received by the station, the Transmit Count (Transmit_Ct), which indicates the total number of frames originated by the station, the Lost Count (Lost_Ct), which indicates the total number of frames received with a format error detected, and the Error Count (Error_Ct), which indicates the total number of frames received with the error indicator as E and not yet set to S. All stations on the ring inspect repeated frames for errors. If an error is detected and the received E indicator was not set to S, then an error is counted. The E indicator is set to S by a station when either an error to be counted is detected or the received E indicator is set to S. (Refer to the fddistat(1M) manpage for additional details about these fields and how to display them at your workstation.) |
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