Fddi 1
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Shining a Light on FDDI topology design. Fiber-optic cable also offers excellent noise immunity, and is virtually impossible to tap.
Introduction FDDI - the 100-Mbit/s Fiber Distributed Data Interface networking technology is a solution for many of the new problems presented by changing corporate networks: •
Groups that previously had no need for communication now want network connections.
•
Existing token ring and Ethernet backbones that are interconnected are now reaching their capacity.
•
Applications require increasing bandwidth, security, and fault tolerance.
Figure 1
These are examples of some of the network requirements that are met by FDDI. At 100 Mbit/s, FDDI allows high-speed interconnection of all the LANs in an organization's network. FDDI easily overcomes the performance limitations of 10-Mbit/s Ethernet and 16FDDI Ring Mbit/s token ring networks. In addition, FDDI provides the security of fiber-optic cabling as well as fault tolerance that is built into the network design. As a result FDDI is becoming widely accepted, especially as a network backbone technology.
Technology Overview Fiber Distributed Data Interface (FDDI) is an ANSI and ISO specification (X3T9) for the transmission of data at high speeds, typically 100 Mbit/s, using fiber-optic cable as the transmission medium. Optical fiber technology offers networks a great degree of flexibility in bandwidth and July 1993
FDDI is a token-passing technology that uses a timed-token protocol to guarantee network access between network stations (network devices and end nodes). Figure 1 shows a standard dual-attach ring. Network access is negotiated between stations at initialization, and every time a new node is added to the network. The network backbone is constructed of stations interconnected by two counter-rotating rings. These rings are two pairs of fiberoptic cable to which each device is attached. Cable lengths between stations can be anywhere from 2 kilometers (km) with multimode fiber, to 60 km with single-mode fiber. The total ring length cannot exceed a maximum of 200 km. During normal operation, the first ring is the primary data carrier, and the second acts as the backup. This offers the network a greater degree of redundancy and fault tolerance.
FDDI and the OSI Model The FDDI standard is made up of four distinct parts: (1) the Physical Layer Medium Dependent (PMD) and Single Mode Fiber Physical Layer Medium Dependent (SMFPMD), (2) the Physical Layer Protocol (PHY), (3) the Media Access Control (MAC), (4) the Station Management (SMT). Although not part of the FDDI standard, the Logical Link Control (LLC) is required by FDDI to assure transmission
1993 Hewlett-Packard Company
Figure 2
SMT frames carry data and control information for the operation and management of the FDDI network. The MAC, PHY, and PMD standards were approved by ANSI and ISO by 1990. The SMT standard was approved in 1991. FDDI is the only LAN with extensive management capabilities as defined in the SMT.
How FDDI Works
FDDI Protocol Stack of user data. These standards define the 100-Mbit/s fiberoptic dual counter-rotating FDDI ring.
FDDI is based on two counter-rotating 100-Mbit/s fiberoptic token-passing rings. If one ring should fail, FDDI automatically becomes a single, not dual, FDDI ring. The rings consist of point-to-point connections between adjacent stations. Stations negotiate for ring access at initialization and when new stations are added to the ring. The two rings act as a primary carrier and a backup (secondary) carrier. Data flows in opposite directions on the two rings.
FDDI Stations There are three types of FDDI stations: dual-attach stations (DAS), single-attach stations (SAS), and concentrators.
The PMD, SMF-PMD, and PHY are equivalent to the physical layer of the OSI model (see figure 2). The PMD and SMF-PMD correspond to the lower portion of the physical layer. The PMD defines the media requirements for multimode fiber, such as fiber-optic cable, connectors, and driver receiver operation for FDDI stations. Cables, connectors and receivers are discussed below in the “Cabling” section. The SMF-PMD defines similar requirements for single-mode fiber-optic media. The PHY corresponds to the upper portion of the physical layer and defines the symbol set, link states, encoding/decoding, clocking, and framing.
Dual attach stations are devices that connect to both the primary ring and the secondary ring simultaneously. Single attach stations are devices that are connected through a primary ring only, and are usually attached using concentrators. Dual attach stations have two ports, one labeled port A, the other port B. With each port there is a primary and a secondary ring connection; port A is primary IN secondary Figure 3
The MAC corresponds to part of the data-link layer of the OSI model. The MAC standard defines the token-passing method as the means for acquiring access to the ring. It is responsible for frame and token construction, sending and receiving frames on the FDDI ring, and delivering LLC frames. The LLC service provides the transmission of a frame of data between two stations. LLC frames carry user information to stations on the ring and also to the extended LAN. FDDI also has a process that defines protocols for managing the PMD and SMF-PMD, PHY, and MAC, called Station Management (SMT). SMT defines facilities for connection management, station configuration, error recovery, and the encoding of SMT frames. MAC and 2
HP Router Application Note
Dua-Attach Stations on an FDDI Network
OUT, port B is primary OUT secondary IN. If either port fails, the other port then becomes primary IN and OUT on the same port. Concentrators connect directly to the backbone of the ring, and provide indirect access to the ring for other devices. Concentrators support multiple DAS and SAS connections, and add a degree of fault tolerance to the network by isolating end nodes from the ring.
Ring Access When a station is first attached to the FDDI ring, it undergoes an initialization process. During this initialization process, a newly inserted station initiates a connection management process (CMT) with its neighbor. CMT tests port types, performs physical port tests or link confidence tests, and transmits link-state characters upon completion of the tests. Stations initiate CMTs between each upstream and downstream neighbor. Upon successful completion of the initialization phase, the ring begins the claim process to generate a token. The claim process begins with all stations negotiating a target token rotational timer (known as TTRT). Each station makes a bid for the timer, and the one with the lowest value wins the right to initialize the ring by inserting the token. If two stations happen to make the same bid, the station with the highest address wins the right to generate the token. Once the claim process has finished, the ring enters a steady state, to be disrupted only by the insertion of another station, or by a break in the ring.
Access to the FDDI network is controlled by a token that circulates the primary ring. When a station has data to transmit, it must capture the token before it can transmit data onto the ring. Only one token may exist on the ring at any one time. If the token is lost, the ring will enter a phase known as beaconing (a state in which the beaconing station controls all access to the ring). If more than one token has been generated, the ring is then scrubbed (scrubbing is used to remove all data from the ring).
Ring Wrap Ring wrap is one of the three techniques that FDDI uses in order to ensure fault tolerance. If any station on the dual ring fails or if the cable is damaged, the dual ring automatically wraps into a single ring as shown in Figure 4. Multiple failures within a single dual ring network present other challenges. The likelihood of multiple failures greatly increases as the number of nodes within an FDDI network increases. If a single failure occurs, the ring wraps and becomes a single ring. If another failure should occur, the ring is segmented and separated into two rings.
Optical Bypass Optical bypass can be used for fault tolerance to prevent ring segmentation. Optical bypass switches maintain connectivity of the FDDI ring in the absence of power or during fault conditions in a station. Stations bypassed by optical bypass switches are effectively removed from the ring. Figure 5 shows an inoperative station switched out by an optical bypass switch.
Figure 4
Ring Wrap Shining a Light on FDDI
3
Optical bypass switches are recommended when designing a multistation FDDI backbone. In a fault condition, a DAS will cause the ring to wrap over the secondary ring, isolating the bad section of the ring. If this happens to more than any single section of the FDDI ring, the network is sectioned into two independent rings. Stations that have an optical bypass device are physically switched out when a fault condition occurs. Note that they do not prevent the network from segmenting in a cable fault condition. There are several limitations of optical bypass that a network designer must be aware of:
the DAS automatically enables the backup link to the secondary concentrator.
Cabling Optical fiber has many advantages over traditional copper cabling. Fiber doesn't emit electrical signals and is immune to electrical interference, making it secure and reliable. It is easy to manipulate because it is lightweight. Important specifications of fiber-optic cable are the optical signal wavelength and attenuation.
•
When the bypass switch bypasses a station, the station is effectively removed, possibly allowing the maximum segment length to exceed 2 km.
•
The integrity of the ring is only as good as the mechanical integrity of the bypass switch.
Two types of transmission media are currently defined by the FDDI standards: multimode fiber and single-mode fiber. Both operate at 100 Mbit/s. Multimode means that multiple rays of light can enter the fiber from different angles. Multimode uses light-emitting diodes (LEDs) that convert electrical signals into light and transmit the light into the fiber-cable.
•
Optical bypass switches can produce up to 2.5 dB optical power loss. This loss must be considered when calculating attenuation. Attenuation and optical budget are discussed later.
Single-mode means that only one ray of light is allowed to enter the fiber. Single-mode uses laser diodes (LDs) to convert electrical signals into light and transmit the light into the fiber-optic cable.
Dual Homing Dual homing is a fault tolerance technique used for redundancy in concentrator environments where continuous uptime is crucial. Dual-homing allows a DAS to use one link as a backup link for redundancy purposes. Thus, one of the two attachments is active at any given time. Figure 6 shows a dual-homed device attached to concentrators.
Attenuation and Optical Budget Figure 5
A station can be dual-homed to the same concentrator or to two different concentrators. When it is homed to two different concentrators and the primary concentrator fails, Figure 6
DAS with Optical Bypass Switch
Dual Homed Station 4
HP Router Application Note
Signal attenuation through optical fiber is important with FDDI. It describes the amount of energy (optical power) that is lost as the light signal travels from the transmitter through the cable to the receiver. The longer the cable, the
higher the loss of optical power. Energy loss is denoted in decibels (dB), an expression used to mathematically compare the power of two signals. Attenuation is calculated by knowing the unit attenuation and the length of the link. The maximum cable attenuation is the power loss in the cable as well as any loss incurred by splices, connectors, and anything else connected to the cable. PMD specifies an optical power budget between any two stations of 11 dB. SMF-PMD allows for a range of power budgets that extends from a minimum of 10 dB to a maximum of 32 dB. To calculate the cable loss between two adjoining stations, use the formula shown below: Attenuation =
(Cable Len (km) x Attenuation/km) + (Splices x Attenuation per Splice) + (Connectors x Connector Attenuation) + (Loss at Transmit MIC)
For example: 2 km fiber x 2.5 dB/km 2 splices x 0.25 dB/splice 1 connector x 0.5 dB/connector Loss at transmit MIC Total link attenuation
5.0 dB 0.5 dB 0.5 dB 0.5 dB 6.5 dB
Once you know the attenuation, you can calculate the remaining power available for the network. The maximum allowed loss is 11 dB. Following is the formula: Power Available = Optical Power Budget - Attenuation For example: Optical Power Budget Attenuation Total Available
11 dB 6.5 dB 4.5 dB
After calculating the dB loss of a particular link, it is important to compare the resulting power with receiver sensitivity. If the receiver is not sensitive enough, poor link quality will result.
into light signals. In single-mode fiber cable, transmitters are laser-quality light-emitting diodes or laser diodes (LDs).
Internetworking with HP Routers HP currently offers two routers that provide FDDI links: the HP 27270B Router CR, and the HP 27290A Router BR. Both FDDI link modules are compliant with the following ANSI X3T9.5 standards: •
Physical Medium Dependent (PMD)
•
Physical Layer Protocol (PHY)
•
Media Access Control (MAC)
•
System Management (SMT)
The FDDI modules also comply with RFC 1188, which specifies transmission of IP datagrams over FDDI media, and with IEEE 802.1 Parts D & H. Physically, the FDDI modules provide a single MAC connection and support both Class A and Class B attachments to FDDI media. A Class A attachment requires two physical connectors and provides connectivity between a dual-attachment station (DAS) and the FDDI primary and secondary rings. A Class B attachment uses a single physical connector and provides connectivity between a single-attachment station (SAS) and the FDDI primary ring or an FDDI concentrator. The HP FDDI link interface only supports multimode 50-micron or 62.5-micron gradedindex fiber-optic cable.
FDDI Routing Since routers are media-independent, they encapsulate packets to conform to the media type in use. RFC 1188 specifies 802.2 LLC with SNAP (subnetwork access protocol) when routing TCP/IP traffic over FDDI media. RFC 1188 conforming routers provide IP routing between source and destination end systems on Ethernet or 802.3 Figure 7
Optical Transmitters and Receivers Transmitters convert data from electrical signals to light. The receiver converts the light signals back to electrical signals. Receivers contain photo detectors that convert incoming optical signals back into electrical signals. Optical transmitters convert modulated electrical signals into modulated light signals that are transmitted through the fiber-optic cable. In multimode fiber cable, transmitters are light-emitting diodes (LEDs) that convert electrical signals
Encapsulation Bridging Shining a Light on FDDI
5
LANs across an FDDI backbone, as well as to destination systems directly connected to the backbone. Other routed protocols are translated on the FDDI ring according to rules defined by RFC 1188 and IEEE 802.1 bridge standards. Adherence to such rules allows interoperability between multiprotocol routers and translating bridges that may be attached to the same FDDI ring. The protocols supported on FDDI include: •
TCP/IP
•
Novell IPX
•
Appletalk Phase 2
•
DECnet Phase IV
•
Xerox XNS
•
HP Probe
Encapsulation Bridging There are many protocols that are non-routable and therefore must be bridged. These non-routable protocols must be encapsulated in an FDDI frame in order to be transported across the ring. Encapsulation is the bridge's implementation that enables interconnection of similar networks over an FDDI network. Figure 7 shows how non routable protocols might be encapsulated across an FDDI network. Within such a topology, a bridge encapsulates the original Ethernet/IEEE 802.3 frame into a new message type (in this case, an FDDI-specific packet) for travel across the FDDI ring. At the destination, the message is removed from the FDDI packet and transmitted in its original form. It is important to note that encapsulation is not the preferred method of transporting data across an FDDI network. In addition, no standard method of encapsulation exists, preventing multivendor interoperability.
FDDI Translation Bridging To ensure multivendor interoperability, the bridge protocol should be based on the IEEE 802.1 Spanning Tree Protocol. Translation is required when bridging between LANs with different data link layer characteristics. For example, forwarding from an Ethernet to an 802.3 end station Table 1 Source Network Encapsulation Ethernet
Translation Rules
RFC 1042 (SNAP OUI 00-00-00) 802.3/LLC + FDDI LLC + 6 HPSNAP Router ApplicationSNAP Note 802.3/802.2 LLC FDDI 802.2 LLC
Destination Network Encapsulation Ethernet
802.3/LLC + SNAP 802.3/802.2 LLC
Table 2 Protocol XNS DECnet IV
Novell Novell Appletalk Phase 2 Source Routing
IP IP
Source Type
Translation Rules
Destination Type
Ethernet Ethernet
RFC 1042 RFC 1042
Ethernet
Ethernet
Proprietary Ethernet LLC + SNAP 802.2 LLC
RFC 1042 RFC 1042 LLC + SNAP 802.2 LLC
Ethernet
Ethernet
LLC +
SNAP
802.2 LLC
Ethernet RFC 1042 Ethernet
LLC + LLC + LLC +
SNAP SNAP SNAP Protocol Translation and Encapsulation requires a translation. When bridging Ethernet or 802.3 packets to FDDI media, a MAC-layer translation is also required. The 802.1 standards define how a received packet is formatted for transfer across an intervening LAN (802.x or FDDI) and presented to a destination LAN. In general, Ethernet Version 2 frames are encapsulated according to rules specified in RFC 1042, and then converted back to Ethernet format. IEEE 802.3/802.2 LLC and 802.3/LLC+SNAP remain unchanged when transferred across an intervening LAN. Table 1, summarizes translation rules. Table 2 summarizes protocol-specific translation. If an Ethernet frame generated on LAN 1 is destined for LAN 2, the frame is translated as shown in figure 8. Translation by the bridge consists of: 1.
extraction of addressing information from the Ethernet header
2.
incorporation of address information into a newly generated FDDI MAC header
3.
encapsulation of Ethernet data as specified in RFC 1088
4.
FCS recalculation
5.
addition of the FDDI MAC-level trailer
Standalone FDDI Rings Network designers can take advantage of FDDI's highspeed capabilities without an FDDI backbone in a network topology. In the absence of a ring, a network designer can use the HP 27290A Router BR as a standalone FDDI network, interconnecting a maximum of two SAS devices or a single DAS. Figure 9
Standalone FDDI Ring A file server is a very good candidate for a standalone FDDI network. In a standard configuration, a server would be connected directly to an Ethernet segment. Traffic Figure 8
Translation Bridging between segments would be very heavy, with traffic on the server's segment being the heaviest. If servers were connected to the HP 27290A Router BR as shown in figure Shining a Light on FDDI
7
9, two benefits would be: 1. The traffic between segments would decrease drastically, effectively increasing the bandwidth of the ethernet internet. 2. Each server would have 100 Mbit/s bandwidth with which to serve all incoming segments. In general, a device can be connected to the HP Router BR creating a dual-attach ring with only two devices, or as a SAS, creating a single-attach ring.
8
HP Router Application Note
Introduction FDDI - the 100-Mbit/s Fiber Distributed Data Interface networking technology is a solution for many of the new problems presented by changing corporate networks: •
Groups that previously had no need for communication now want network connections.
•
Existing token ring and Ethernet backbones that are interconnected are now reaching their capacity.
•
Applications require increasing bandwidth, security, and fault tolerance.
Figure 1
These are examples of some of the network requirements that are met by FDDI. At 100 Mbit/s, FDDI allows high-speed interconnection of all the LANs in an organization's network. FDDI easily overcomes the performance limitations of 10-Mbit/s Ethernet and 16FDDI Ring Mbit/s token ring networks. In addition, FDDI provides the security of fiber-optic cabling as well as fault tolerance that is built into the network design. As a result FDDI is becoming widely accepted, especially as a network backbone technology.
Technology Overview Fiber Distributed Data Interface (FDDI) is an ANSI and ISO specification (X3T9) for the transmission of data at high speeds, typically 100 Mbit/s, using fiber-optic cable as the transmission medium. Optical fiber technology offers networks a great degree of flexibility in bandwidth and July 1993
FDDI is a token-passing technology that uses a timed-token protocol to guarantee network access between network stations (network devices and end nodes). Figure 1 shows a standard dual-attach ring. Network access is negotiated between stations at initialization, and every time a new node is added to the network. The network backbone is constructed of stations interconnected by two counter-rotating rings. These rings are two pairs of fiberoptic cable to which each device is attached. Cable lengths between stations can be anywhere from 2 kilometers (km) with multimode fiber, to 60 km with single-mode fiber. The total ring length cannot exceed a maximum of 200 km. During normal operation, the first ring is the primary data carrier, and the second acts as the backup. This offers the network a greater degree of redundancy and fault tolerance.
FDDI and the OSI Model The FDDI standard is made up of four distinct parts: (1) the Physical Layer Medium Dependent (PMD) and Single Mode Fiber Physical Layer Medium Dependent (SMFPMD), (2) the Physical Layer Protocol (PHY), (3) the Media Access Control (MAC), (4) the Station Management (SMT). Although not part of the FDDI standard, the Logical Link Control (LLC) is required by FDDI to assure transmission
1993 Hewlett-Packard Company
Figure 2
SMT frames carry data and control information for the operation and management of the FDDI network. The MAC, PHY, and PMD standards were approved by ANSI and ISO by 1990. The SMT standard was approved in 1991. FDDI is the only LAN with extensive management capabilities as defined in the SMT.
How FDDI Works
FDDI Protocol Stack of user data. These standards define the 100-Mbit/s fiberoptic dual counter-rotating FDDI ring.
FDDI is based on two counter-rotating 100-Mbit/s fiberoptic token-passing rings. If one ring should fail, FDDI automatically becomes a single, not dual, FDDI ring. The rings consist of point-to-point connections between adjacent stations. Stations negotiate for ring access at initialization and when new stations are added to the ring. The two rings act as a primary carrier and a backup (secondary) carrier. Data flows in opposite directions on the two rings.
FDDI Stations There are three types of FDDI stations: dual-attach stations (DAS), single-attach stations (SAS), and concentrators.
The PMD, SMF-PMD, and PHY are equivalent to the physical layer of the OSI model (see figure 2). The PMD and SMF-PMD correspond to the lower portion of the physical layer. The PMD defines the media requirements for multimode fiber, such as fiber-optic cable, connectors, and driver receiver operation for FDDI stations. Cables, connectors and receivers are discussed below in the “Cabling” section. The SMF-PMD defines similar requirements for single-mode fiber-optic media. The PHY corresponds to the upper portion of the physical layer and defines the symbol set, link states, encoding/decoding, clocking, and framing.
Dual attach stations are devices that connect to both the primary ring and the secondary ring simultaneously. Single attach stations are devices that are connected through a primary ring only, and are usually attached using concentrators. Dual attach stations have two ports, one labeled port A, the other port B. With each port there is a primary and a secondary ring connection; port A is primary IN secondary Figure 3
The MAC corresponds to part of the data-link layer of the OSI model. The MAC standard defines the token-passing method as the means for acquiring access to the ring. It is responsible for frame and token construction, sending and receiving frames on the FDDI ring, and delivering LLC frames. The LLC service provides the transmission of a frame of data between two stations. LLC frames carry user information to stations on the ring and also to the extended LAN. FDDI also has a process that defines protocols for managing the PMD and SMF-PMD, PHY, and MAC, called Station Management (SMT). SMT defines facilities for connection management, station configuration, error recovery, and the encoding of SMT frames. MAC and 2
HP Router Application Note
Dua-Attach Stations on an FDDI Network
OUT, port B is primary OUT secondary IN. If either port fails, the other port then becomes primary IN and OUT on the same port. Concentrators connect directly to the backbone of the ring, and provide indirect access to the ring for other devices. Concentrators support multiple DAS and SAS connections, and add a degree of fault tolerance to the network by isolating end nodes from the ring.
Ring Access When a station is first attached to the FDDI ring, it undergoes an initialization process. During this initialization process, a newly inserted station initiates a connection management process (CMT) with its neighbor. CMT tests port types, performs physical port tests or link confidence tests, and transmits link-state characters upon completion of the tests. Stations initiate CMTs between each upstream and downstream neighbor. Upon successful completion of the initialization phase, the ring begins the claim process to generate a token. The claim process begins with all stations negotiating a target token rotational timer (known as TTRT). Each station makes a bid for the timer, and the one with the lowest value wins the right to initialize the ring by inserting the token. If two stations happen to make the same bid, the station with the highest address wins the right to generate the token. Once the claim process has finished, the ring enters a steady state, to be disrupted only by the insertion of another station, or by a break in the ring.
Access to the FDDI network is controlled by a token that circulates the primary ring. When a station has data to transmit, it must capture the token before it can transmit data onto the ring. Only one token may exist on the ring at any one time. If the token is lost, the ring will enter a phase known as beaconing (a state in which the beaconing station controls all access to the ring). If more than one token has been generated, the ring is then scrubbed (scrubbing is used to remove all data from the ring).
Ring Wrap Ring wrap is one of the three techniques that FDDI uses in order to ensure fault tolerance. If any station on the dual ring fails or if the cable is damaged, the dual ring automatically wraps into a single ring as shown in Figure 4. Multiple failures within a single dual ring network present other challenges. The likelihood of multiple failures greatly increases as the number of nodes within an FDDI network increases. If a single failure occurs, the ring wraps and becomes a single ring. If another failure should occur, the ring is segmented and separated into two rings.
Optical Bypass Optical bypass can be used for fault tolerance to prevent ring segmentation. Optical bypass switches maintain connectivity of the FDDI ring in the absence of power or during fault conditions in a station. Stations bypassed by optical bypass switches are effectively removed from the ring. Figure 5 shows an inoperative station switched out by an optical bypass switch.
Figure 4
Ring Wrap Shining a Light on FDDI
3
Optical bypass switches are recommended when designing a multistation FDDI backbone. In a fault condition, a DAS will cause the ring to wrap over the secondary ring, isolating the bad section of the ring. If this happens to more than any single section of the FDDI ring, the network is sectioned into two independent rings. Stations that have an optical bypass device are physically switched out when a fault condition occurs. Note that they do not prevent the network from segmenting in a cable fault condition. There are several limitations of optical bypass that a network designer must be aware of:
the DAS automatically enables the backup link to the secondary concentrator.
Cabling Optical fiber has many advantages over traditional copper cabling. Fiber doesn't emit electrical signals and is immune to electrical interference, making it secure and reliable. It is easy to manipulate because it is lightweight. Important specifications of fiber-optic cable are the optical signal wavelength and attenuation.
•
When the bypass switch bypasses a station, the station is effectively removed, possibly allowing the maximum segment length to exceed 2 km.
•
The integrity of the ring is only as good as the mechanical integrity of the bypass switch.
Two types of transmission media are currently defined by the FDDI standards: multimode fiber and single-mode fiber. Both operate at 100 Mbit/s. Multimode means that multiple rays of light can enter the fiber from different angles. Multimode uses light-emitting diodes (LEDs) that convert electrical signals into light and transmit the light into the fiber-cable.
•
Optical bypass switches can produce up to 2.5 dB optical power loss. This loss must be considered when calculating attenuation. Attenuation and optical budget are discussed later.
Single-mode means that only one ray of light is allowed to enter the fiber. Single-mode uses laser diodes (LDs) to convert electrical signals into light and transmit the light into the fiber-optic cable.
Dual Homing Dual homing is a fault tolerance technique used for redundancy in concentrator environments where continuous uptime is crucial. Dual-homing allows a DAS to use one link as a backup link for redundancy purposes. Thus, one of the two attachments is active at any given time. Figure 6 shows a dual-homed device attached to concentrators.
Attenuation and Optical Budget Figure 5
A station can be dual-homed to the same concentrator or to two different concentrators. When it is homed to two different concentrators and the primary concentrator fails, Figure 6
DAS with Optical Bypass Switch
Dual Homed Station 4
HP Router Application Note
Signal attenuation through optical fiber is important with FDDI. It describes the amount of energy (optical power) that is lost as the light signal travels from the transmitter through the cable to the receiver. The longer the cable, the
higher the loss of optical power. Energy loss is denoted in decibels (dB), an expression used to mathematically compare the power of two signals. Attenuation is calculated by knowing the unit attenuation and the length of the link. The maximum cable attenuation is the power loss in the cable as well as any loss incurred by splices, connectors, and anything else connected to the cable. PMD specifies an optical power budget between any two stations of 11 dB. SMF-PMD allows for a range of power budgets that extends from a minimum of 10 dB to a maximum of 32 dB. To calculate the cable loss between two adjoining stations, use the formula shown below: Attenuation =
(Cable Len (km) x Attenuation/km) + (Splices x Attenuation per Splice) + (Connectors x Connector Attenuation) + (Loss at Transmit MIC)
For example: 2 km fiber x 2.5 dB/km 2 splices x 0.25 dB/splice 1 connector x 0.5 dB/connector Loss at transmit MIC Total link attenuation
5.0 dB 0.5 dB 0.5 dB 0.5 dB 6.5 dB
Once you know the attenuation, you can calculate the remaining power available for the network. The maximum allowed loss is 11 dB. Following is the formula: Power Available = Optical Power Budget - Attenuation For example: Optical Power Budget Attenuation Total Available
11 dB 6.5 dB 4.5 dB
After calculating the dB loss of a particular link, it is important to compare the resulting power with receiver sensitivity. If the receiver is not sensitive enough, poor link quality will result.
into light signals. In single-mode fiber cable, transmitters are laser-quality light-emitting diodes or laser diodes (LDs).
Internetworking with HP Routers HP currently offers two routers that provide FDDI links: the HP 27270B Router CR, and the HP 27290A Router BR. Both FDDI link modules are compliant with the following ANSI X3T9.5 standards: •
Physical Medium Dependent (PMD)
•
Physical Layer Protocol (PHY)
•
Media Access Control (MAC)
•
System Management (SMT)
The FDDI modules also comply with RFC 1188, which specifies transmission of IP datagrams over FDDI media, and with IEEE 802.1 Parts D & H. Physically, the FDDI modules provide a single MAC connection and support both Class A and Class B attachments to FDDI media. A Class A attachment requires two physical connectors and provides connectivity between a dual-attachment station (DAS) and the FDDI primary and secondary rings. A Class B attachment uses a single physical connector and provides connectivity between a single-attachment station (SAS) and the FDDI primary ring or an FDDI concentrator. The HP FDDI link interface only supports multimode 50-micron or 62.5-micron gradedindex fiber-optic cable.
FDDI Routing Since routers are media-independent, they encapsulate packets to conform to the media type in use. RFC 1188 specifies 802.2 LLC with SNAP (subnetwork access protocol) when routing TCP/IP traffic over FDDI media. RFC 1188 conforming routers provide IP routing between source and destination end systems on Ethernet or 802.3 Figure 7
Optical Transmitters and Receivers Transmitters convert data from electrical signals to light. The receiver converts the light signals back to electrical signals. Receivers contain photo detectors that convert incoming optical signals back into electrical signals. Optical transmitters convert modulated electrical signals into modulated light signals that are transmitted through the fiber-optic cable. In multimode fiber cable, transmitters are light-emitting diodes (LEDs) that convert electrical signals
Encapsulation Bridging Shining a Light on FDDI
5
LANs across an FDDI backbone, as well as to destination systems directly connected to the backbone. Other routed protocols are translated on the FDDI ring according to rules defined by RFC 1188 and IEEE 802.1 bridge standards. Adherence to such rules allows interoperability between multiprotocol routers and translating bridges that may be attached to the same FDDI ring. The protocols supported on FDDI include: •
TCP/IP
•
Novell IPX
•
Appletalk Phase 2
•
DECnet Phase IV
•
Xerox XNS
•
HP Probe
Encapsulation Bridging There are many protocols that are non-routable and therefore must be bridged. These non-routable protocols must be encapsulated in an FDDI frame in order to be transported across the ring. Encapsulation is the bridge's implementation that enables interconnection of similar networks over an FDDI network. Figure 7 shows how non routable protocols might be encapsulated across an FDDI network. Within such a topology, a bridge encapsulates the original Ethernet/IEEE 802.3 frame into a new message type (in this case, an FDDI-specific packet) for travel across the FDDI ring. At the destination, the message is removed from the FDDI packet and transmitted in its original form. It is important to note that encapsulation is not the preferred method of transporting data across an FDDI network. In addition, no standard method of encapsulation exists, preventing multivendor interoperability.
FDDI Translation Bridging To ensure multivendor interoperability, the bridge protocol should be based on the IEEE 802.1 Spanning Tree Protocol. Translation is required when bridging between LANs with different data link layer characteristics. For example, forwarding from an Ethernet to an 802.3 end station Table 1 Source Network Encapsulation Ethernet
Translation Rules
RFC 1042 (SNAP OUI 00-00-00) 802.3/LLC + FDDI LLC + 6 HPSNAP Router ApplicationSNAP Note 802.3/802.2 LLC FDDI 802.2 LLC
Destination Network Encapsulation Ethernet
802.3/LLC + SNAP 802.3/802.2 LLC
Table 2 Protocol XNS DECnet IV
Novell Novell Appletalk Phase 2 Source Routing
IP IP
Source Type
Translation Rules
Destination Type
Ethernet Ethernet
RFC 1042 RFC 1042
Ethernet
Ethernet
Proprietary Ethernet LLC + SNAP 802.2 LLC
RFC 1042 RFC 1042 LLC + SNAP 802.2 LLC
Ethernet
Ethernet
LLC +
SNAP
802.2 LLC
Ethernet RFC 1042 Ethernet
LLC + LLC + LLC +
SNAP SNAP SNAP Protocol Translation and Encapsulation requires a translation. When bridging Ethernet or 802.3 packets to FDDI media, a MAC-layer translation is also required. The 802.1 standards define how a received packet is formatted for transfer across an intervening LAN (802.x or FDDI) and presented to a destination LAN. In general, Ethernet Version 2 frames are encapsulated according to rules specified in RFC 1042, and then converted back to Ethernet format. IEEE 802.3/802.2 LLC and 802.3/LLC+SNAP remain unchanged when transferred across an intervening LAN. Table 1, summarizes translation rules. Table 2 summarizes protocol-specific translation. If an Ethernet frame generated on LAN 1 is destined for LAN 2, the frame is translated as shown in figure 8. Translation by the bridge consists of: 1.
extraction of addressing information from the Ethernet header
2.
incorporation of address information into a newly generated FDDI MAC header
3.
encapsulation of Ethernet data as specified in RFC 1088
4.
FCS recalculation
5.
addition of the FDDI MAC-level trailer
Standalone FDDI Rings Network designers can take advantage of FDDI's highspeed capabilities without an FDDI backbone in a network topology. In the absence of a ring, a network designer can use the HP 27290A Router BR as a standalone FDDI network, interconnecting a maximum of two SAS devices or a single DAS. Figure 9
Standalone FDDI Ring A file server is a very good candidate for a standalone FDDI network. In a standard configuration, a server would be connected directly to an Ethernet segment. Traffic Figure 8
Translation Bridging between segments would be very heavy, with traffic on the server's segment being the heaviest. If servers were connected to the HP 27290A Router BR as shown in figure Shining a Light on FDDI
7
9, two benefits would be: 1. The traffic between segments would decrease drastically, effectively increasing the bandwidth of the ethernet internet. 2. Each server would have 100 Mbit/s bandwidth with which to serve all incoming segments. In general, a device can be connected to the HP Router BR creating a dual-attach ring with only two devices, or as a SAS, creating a single-attach ring.
8
HP Router Application Note
