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Gigabit Ethernet in Context withOtherLAN Technologies


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Gigabit Ethernet in Context withOtherLAN Technologies


14.1. -

Commonly Installed LAN Technologies


14.2. -

Network Upgrade Comparisons


14.3. -


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Gigabit Ethernet Networking

From: Gigabit Ethernet Networking
Author: David Cunningham; Bill Lane
Publisher: MTP
More Information

14. Gigabit Ethernet in Context withOtherLAN Technologies

As stated in Chapter 13, “Upgrading Ethernet LANs: System and Topology Considerations,” Gigabit Ethernet can be used to upgrade both Ethernet and non-Ethernet local area networks (LANs). Chapter 13 discussed upgrading Ethernet LANs. This chapter briefly reviews some non-Ethernet technologies currently deployed in LANs and then considers the use of Gigabit Ethernet as an upgrade.

There are several reasons why you also should consider upgrading non-Ethernet networks with Gigabit Ethernet technologies:

  • Different divisions within the same organization may have initially chosen to implement different LAN technologies and protocols.

  • Gigabit Ethernet can be the lowest cost long-term alternative for upgrading, even within a non-Ethernet environment. Over 80 percent of the nodes in the world's LANs use Ethernet technology. Furthermore, because this is a highly competitive market, costs can be expected to continue their downward trend.

  • The Ethernet protocol is typically less complex, more easily understood, and simpler to implement than competing protocols.

  • Ethernet can now provide previously unavailable Class of Service (CoS) transmission.

It is widely recognized that the first major application to be addressed by LAN equipment upgrades is the need for increased bandwidth between workgroup, backbone, and campus networking equipment. But, the introduction of new data types requiring CoS and QoS features means that raw bandwidth alone is not sufficient. For this reason, all new switch-based products, whether packet- or circuit-based, already have or are introducing techniques that can provide CoS and QoS features, at least to some degree.

This chapter begins with a review of commonly installed LAN technologies, then continues with a discussion of the higher layer protocols required to both support IP-based traffic on connection-oriented networking technology and to support QoS or CoS over IP-based networks. The chapter concludes with a discussion of the suitability of Gigabit Ethernet as an upgrade for each of the LAN technologies discussed.

14.1. Commonly Installed LAN Technologies

As you should be aware by now, LAN equipment is installed in a hierarchical manner. The hierarchy is related to the building wiring standard levels for horizontal cabling (for connection to desktops), building backbone cabling (for interconnection of workgroup LAN equipment), and campus backbones (for interconnection of building LAN equipment). This hierarchy has evolved because it eases the burdens associated with network design, network management, and fault isolation of both LAN equipment and the supporting cabling infrastructure.

Table 14.1 summarizes some of the important features of commonly installed Layer 2 LAN technologies. ATM is the only connection-oriented LAN technology listed in Table 14.1. All the others are connectionless (packet-based) technologies.

Table 14.1. Multimode Fiber (MMF) Link Lengths for Commonly installed Layer 2 LANs
LAN Type Data Rate
MMF Link
Length (m)
Ethernet100SharedDesktop and Wiring closets412
Ethernet100SwitchedBackbone and Desktop2,000
Token Ring4SharedDesktop2,000
Token Ring16Shared and SwitchedDesktop2,000
Token Ring100SwitchedBackbone and Desktop2,000
FDDI100Shared and SwitchedBackbone2,000
100VG100Shared and SwitchedDesktop and Backbone2,000

a This distance could be increased to 1,600 m if mode-conditioned 1000BASE-LX Gigabit Ethernet optical transceivers were used for this application.

Although Table 14.1 states the maximum multimode fiber link length, you should remember that practically all desktop links use Category-5 UTP cable. Also, as noted in Chapter 13, if single-mode optical fiber is used for switched links, the maximum link length could be very long indeed—up to 110 km—with commercially available, specialized, non-standards-based, single-mode link extenders.

14 1.1. Layer 2 LAN Summary: Token Ring

Token Ring is a deterministic protocol that was developed in the early 1980s to avoid the random packet collisions that occur in Ethernet. Token Ring has been standardized as IEEE 802.5. Originally the network topology was a physical ring. However, today many Token Ring networks have their wiring installed in the normal building wiring tree or star topology.

In the ring configuration, both tokens (permission to send) and information packets are transmitted between successive stations or nodes on the ring. Only one token is allowed on the ring and only the node holding the token is allowed to transmit a data packet. After the sender has finished transmitting a data packet, it generates a new token that is then circulated around the ring. The token will be captured by the next node on the ring needing to send a packet.

The Token Ring frame format is different from the Ethernet format both in how the information is encoded and in the field definitions for the frame format. The data field length can be up to 4,500 bytes, and two additional fields provide control information that is not available in a basic Ethernet frame:

  • A 1-byte access control field in the frame header contains 3 bits for packet priority designation, 3 bits to indicate a token reservation and its priority level, plus a token bit to differentiate a data/command frame from a token.

  • A 1-byte end delimiter identifies the end of the frame and contains control bits to indicate whether the frame was damaged in transmission and whether this frame is the last frame in a logical sequence.

Currently available Token Ring products operate at 4, 16, and 100 Mbps (100 Mbps Token Ring is in the final stages of standardization; also, a task force has been set up to draft a Gigabit Token Ring standard). Both shared and switched versions of Token Ring are available.

14.1.2. Layer 2 LAN Summary: FDDI

The Fiber Distributed Data Interface (FDDI) is a Token Ring protocol similar to IEEE 802.5 that operates at 100 Mbps. FDDI was developed in the 1980s as a high speed backbone and desktop replacement for IEEE 802.5 and was standardized through ANSI by the X3T9.5 working group. Originally, FDDI was designed as a fiber optic network; however, copper UTP and STP versions were developed. The FDDI physical layers were reused by the IEEE 802.3 committee for 100BASE-FX and TX Fast Ethernet. The packet format used by FDDI is similar to that of Token Ring.

FDDI actually uses two counter-rotating rings to provide a high degree of fault tolerance. Comprehensive station management was developed as part of the standard and FDDI's main role has been as a highly reliable backbone LAN. Both shared and switched FDDI products are available. A higher data rate version of FDDI has not been developed.

14 1.3. Layer 2 LAN Summary: 100VG-AnyLAN

100VG-AnyLAN uses the Demand Priority Access Method (DPAM), a deterministic protocol that was developed in the early 1990s as a high speed upgrade for both 10BASE-T and 4 Mbps and 16 Mbps Token Ring LANs. As such, a single DPAM-based network can operate in either an Ethernet mode, where it transmits Ethernet frames, or a Token Ring mode, where it transmits Token Ring frames, but not in both modes at the same time. DPAM also provides two levels of packet priority. DPAM was standardized as IEEE 802.12.

100VG-AnyLAN was specifically designed to operate over the lowest grade UTP cable used by 10BASE-T: 25 pair, voice grade UTP cabling. Because of this, it has a significant operational margin on better grades of UTP, STP, and optical fiber cabling. With shared media, 100VG-AnyLAN obeys the same topology rules as 10BASE-T. Both shared and switched versions of 100VG-AnyLAN are available.

There was an initial effort to develop a Gigabit version of IEEE 802.12; however, with the advent of switch-based networks, it was recognized that Gigabit Ethernet would be a very effective upgrade for 100VG-AnyLAN. Gigabit activity within IEEE 802.12 was terminated, and many of the contributors from that committee transferred to the IEEE 802.3z committee, where they played key roles in the development of Gigabit Ethernet.

14 1.4. Layer 2 LAN Summary: ATM

Asynchronous Transfer Mode (ATM) is probably the best known alternative to Gigabit Ethernet. For this reason, this section provides more coverage than is provided for the other LAN examples. ATM was developed by the telecommunications industry as part of the Broadband Integrated Services Digital Network (BISDN). It provides a means to integrate various data types having disparate transmission requirements, such as computer data traffic, voice traffic, and video traffic. In contrast to the examples considered thus far (Ethernet, Token Ring, FDDI, and 100VG-AnyLAN), ATM is connection-oriented.

ATM transmits a continuous stream of small fixed length packets called cells. An ATM cell is only 53 bytes long, consisting of a 5-byte header and a 48-byte payload field. ATM cells were purposely kept short to reduce transmission delay variations and to enable efficient switching.

The header contains the control and next destination routing information for the cell, including:

  • A generic flow control (GFC) field that is seldom used, but typically set to a default value

  • A virtual path identifier (VPI) that is used with the VCI to identify the next route destination of the cell

  • A virtual channel identifier (VCI) that is used with the VPI

  • A payload type (PT) field that indicates whether the cell contains data or control information

  • A congestion loss priority (CLP) bit that indicates how the cell should be treated with respect to other cells in cases of network congestion

  • A header error control (HEC) field containing a checksum for the header information

The purpose of the ATM layer is to provide transparent transfer of cells across pre-established network connections according to a pre-arranged traffic contract. ATM systems generally use a Synchronous Optical Network (SONET) physical layer. When this is the case, the ATM layer must insert unassigned (idle) cells into the stream of cells during times when there is no data to transmit. The result is a continuous stream of cells, a process called cell rate decoupling.

The VPI and VCI are used by ATM switches, which form part of the ATM layer, to manage connections across the ATM network. Because ATM is a connection-oriented protocol, end-to-end signaling must be used to establish a virtual circuit before any data is transmitted. The signaling establishes the path through the ATM switching network and any required QoS agreements.

Another important part of the ATM layer is the ATM adaptation layer (AAL). The ATM adaptation layer usually resides in end stations. The AAL layer in the transmitting station segments frames received from higher protocol layers into cells for transmission; the peer AAL layer in the receiving station collects the cells and reassembles them into frames for forwarding to the higher protocol layers. Various types of AAL have been standardized for use with different traffic types. AAL5 is commonly used for packet-based communication.

14 1.5. Higher Layer Protocols for IP-Based Networks

The discussion in sections 14.1.1-14.1.4 was limited to issues relating to layer 1 (the physical layer) and layer 2 (the data link layer). For a more complete comparison of Gigabit Ethernet with the other networking options, you need to consider some of the higher layers. This discussion begins with a brief review of the most commonly used LAN model: the TCP/IP suite. The TCP/IP protocol suite doesn't have the same layered model as the OSI reference model. Roughly speaking, however, IP can be associated with the network layer and TCP with the transport layer, although TCP also includes some of the OSI session layer functionality. In Ethernet implementations, the bottom two layers consist of the IEEE 802.2 LLC and IEEE 802.3 MAC (data link layer) and the IEEE 802.3 physical layer, as shown in Table 14.2.

Table 14.2. The Relationship of the TCP/IP/Ethernet Protocol Stack to the OSI Model
OSI LayerOSI Layer NameProtocol Implementation
2Data LinkIEEE 802.2 LLC and IEEE 802.3 MAC
1PhysicalIEEE 802.3 PHY

Because IP is a connectionless protocol, no end-to-end signaling or control information needs to be exchanged before packet transmission begins. Also, because IP only provides a best effort service, no QoS or CoS is provided. TCP is the transport layer protocol of the TCP/IP suite and it provides connection-oriented reliable transmission (see Chapter 4, “Gigabit Repeaters, Bridges, Routers, and Switches”). TCP is a protocol that is best suited to data communications applications. TCP/IP is not a good choice for real-time video transmission.

An IP-based network has two types of nodes: hosts and routers. A router is a node with at least two network interface connections and is capable of forwarding packets from one network to another network. A host is simply a node that is not a router.

Three other important protocols that support the IP protocol are the Internet Control Message Protocol (ICMP), the Internet Group Management Protocol (IGMP), and the Simple Network Management Protocol (SNMP), all of which are standardized. ICMP is used mainly for fault diagnosis, error reporting, and control messages. IGMP is used to form and manage multicast groups. SNMP enables network administrators to manage the network.

In an IP-based internet, routes are calculated by the network routers in a distributed fashion. The IP protocol does not define how the routers should do this. However, there are standard routing protocols for this purpose.Some well-known examples of standards-based IP routing protocols include the following:

  • Routing Information Protocol (RIP)

  • Open Shortest Path First (OSPF)

  • Border Gateway Protocol (BGP)

QoS and CoS in IP-Based Networks

Until the advent of multilayer switching, routing was performed by general purpose processors under software control. But because software routines are typically slower than ASIC hardware routines, software-based routers can be congestion points in a network. As such, software-based routers could make it difficult to provide QoS/CoS service; however, IP doesn't provide either QoS or CoS anyway.

As section 13.2 in Chapter 13 stated, the Internet Engineering Task Force (IETF) has been working for some time to define standards-based mechanisms and protocols for providing QoS and CoS—especially for switch-based networks. They are also finalizing the Resource Reservation Protocol (RSVP), which will enable networks to reserve dedicated bandwidth. In addition, the IP-based multicast backbone (Mbone), which was based on the Real-time Transport Protocol (RTP), has proven that real-time, multicast video conferencing is possible over the Internet—the Mbone was originally designed to broadcast the meetings of the IETF.

Even before these standards are finalized, many manufacturers of high-end multilayer switches have implemented the common IP-based routing algorithms (RIP, OSPF, BGP, and so forth) in hardware to produce routing-switches. These routing switches can process packets at the line rate of each port (even for Gigabit Ethernet!). Usually, each port has dedicated routing hardware so that packets entering the port can either be switched, based on layer 2 information or, if need be, routed, based on layer 3 information. Many of these routing switches can also provide CoS and QoS features based on some or all of the following: IEEE 802.1p/Q, IP datagram Type of Service (ToS), and the source or destination TCP/UDP port number. But not all protocols are routed in hardware. Those that aren't are usually forwarded to a software-based router. However, the network performance for these other protocols is also greatly improved because the software-based routers are only required to deal with non-IP traffic.

Several manufacturers are using cut-through switches to gain performance improvement, compared to software routing. Cut-through switches usually gain performance improvements by switching based on the examination of only the packet header of each packet in a transmission. However, some routers gain a further increase in performance by only examining the first few packets of an identified transmission. Several of the schemes used for IP cut-through routing actually involve using a switch with an underlying ATM-based switching fabric. Cut-through switch-routers tend to use proprietary methods, although standards are beginning to emerge.

14 1.6. Higher Layer Protocols for ATM-Based Networks

As stated in section 14.1.4, ATM is a connection-oriented switch-based layer 2 technology that requires end-to-end signaling before any data packets are transmitted. However, the end-to-end signaling can also be used to set up connections with guaranteed levels of QoS and CoS.

While ATM is ideal for voice and video applications, it has some difficulties when transmitting IP-based data. These difficulties arise because IP was predicated on a connectionless service model. Broadcast and multicast traffic is commonly used in Ethernet and IP-based networks for functions such as address discovery and routing table updates. ATM requires additional higher layer protocol support to cope with these broadcast and multicast requirements, but because over 80 percent of the world's network connections are through Ethernet ports, support of IP traffic is no longer optional.

Even before the growth of the World Wide Web, it would have been difficult to change the software associated with networking to a connection-oriented model simply because most applications assume a connectionless network model.

Table 14.3 compares the TCP/IP/ATM layer-2 protocol stack to the OSI model.

Table 14.3. The Relationship of the TCP/IP/ATM Protocol Stack to the OSI Model

a MPOA is Multiprotocol over ATM, LANE is LAN Emulation, MPLS is Multiprotocol label switching, and RFC 1477 is the IETF's classical IP-over-ATM standard. To simplify the table, ATM end-to-end signaling protocols are not shown.

OSI LayerOSI Layer NameProtocol Implementation
3NetworkIP, LANE, MPOA, MPLS, or RFC 1477a
2Data LinkATM AAL5 and ATM switching layer

So, it seems we've discovered a Catch-22 situation:

  • ATM has native QoS, but requires complex higher layer protocol support for traditional IP-based traffic.

  • Ethernet is ideal for IP-based traffic, but requires complex higher layer protocol support in order to provide CoS for emergent multimedia traffic.

In general, there is no solution to this Catch-22 situation. The best that can be done is to use the appropriate technology for the situation at hand. However, it is now widely recognized that Ethernet technology is best suited for the LAN, and ATM is best suited for the WAN; it is expected that both will compete for use in MANs. The next few sections introduce some of the higher layer protocols commonly used to support IP traffic on ATM-based networks.

14 6.1. LANE

The LAN emulation (LANE) protocol defines the operation of a single emulated LAN (ELAN). LANE uses ATM connection-oriented switches in a way that emulates a traditional connectionless LAN. For example, in the case of an Ethernet emulated LAN, the LANE interface behaves the same as a standard Ethernet layer 2/3 interface. This means that all data sent through the ATM network consists of ATM-encapsulated Ethernet packets.

Multiple ELANs can be simultaneously operated on a common ATM network. However, a separate ELAN must be dedicated to each traditional LAN protocol in operation. LANE was intended to accelerate the introduction of ATM networks. A disadvantage of LANE is that because it emulates a traditional connectionless layer 2/3 LAN interface it cannot use ATM's native QoS features.

LANE is based on a client-server model. The LAN emulation client (LEC) is an end-station function that performs data forwarding, address resolution, and registering of the MAC address with the LAN emulation server. For traditional LANs, the LEC also provides a standard layer 2/3 network interface to higher layer protocols. If an ATM end system is connected to multiple ELANs, there must be a separate LEC for each ELAN supported.

LANE requires three servers as follows:

  • LAN Emulation Server (LES). One LES is required for each ELAN. It is a central point that controls the admission of LECS to the ELAN. It is also responsible for registering client MAC and ATM addresses, and for address resolution.

  • LAN Emulation Configuration Server (LECS). One LECS serves all ELANs within an administrative domain. It maintains a database of the LECs and the ELANs to which they are registered. When queried by an LEC, the LECs will respond with the ATM address of the LES that serves the ELAN appropriate to that LEC.

  • Broadcast and Unknown Server (BUS). Each LEC is associated with one BUS per ELAN. The BUS is responsible for delivering broadcast frames and multicast frames. Additionally, unicast frames containing addresses that are unregistered or as yet unresolved are delivered by the BUS.

As illustrated in Figure 14.1, the LANE protocol is deployed in two types of ATM equipment: ATM network interface cards (NICs) and internetworking equipment—a layer2 LAN switch is shown in the figure. Because an ELAN emulates a specific traditional LAN (Ethernet, Token Ring, FDDI, and so on), it can support various higher layer protocols such as IP, IPX, SNA/APPN, and NetBIOS.

Figure 14.1. Example of How LANE Protocol Can Be Implemented in ATM Network Devices

14 6.2. Classical IP over ATM: RFC 1477

Because IP over ATM as specified in RFC 1477 does not change the basic nature of the IPprotocol, it is called Classical IP over ATM. Classical IP over ATM has the following characteristics:

  • It is based on the concept of a logical IP subnetwork (LIS).

  • An LIS contains hosts and routers having the same IP subnetwork mask (the bits of an IP address that are being used for the common subnetwork address) and the same subnetwork address.

  • Any two hosts of the same LIS may communicate directly using virtual channels (VCs).

  • Hosts from different LISs may only communicate through a router.

  • The maximum transmission unit (MTU) is the same for VCs in an LIS and is a maximum of 9,180 bytes.

  • Within a single LIS, IP addresses are resolved to ATM addresses by an ATM address resolution protocol (ATMARP) server. The inverse resolution is also performed by the inverse ATMARP on the server.

Classical IP over ATM has the obvious advantage that it allows IP data to be routed over an ATM LAN or WAN. However, its main disadvantages are that it only supports IP routing, it has no multicast support, ATM's intrinsic QoS properties may be lost passing through routers, and it is difficult to scale to a large number of LISs.

14 1 6.3. Multiprotocol over ATM (MPOA)

In an MPOA environment, connectionless LAN traffic is directed to shortcut ATM paths. For this reason, MPOA is classed as a shortcut or cut-through routing scheme. MPOA uses three technologies or concepts:

  • LANE protocol

  • IETF's Next Hop Resolution Protocol (NHRP)

  • The concept of the virtual router

LANE is basically used for configuration purposes, NHRP is used to determine shortcut paths through the ATM switch-based network, and the virtual router concept allows partitioning of routing functions among various physically separated elements of the network.

The routing calculations are performed by the MPOA Servers (MPSs), while traffic forwarding is the responsibility of the MPOA Clients (MPCs) and the ATM switching layer. An MPS could be a standalone ATM attached route server, or it may reside within an ATM switch-router. MPCs reside in network edge devices and in ATM attached hosts. Traditional (non-ATM) hosts connect to the ATM network through the edge devices that support traditional LANs on the LAN side of the device and ATM on the backbone side.

Native ATM devices may be directly connected to the ATM layer. The MPOA routers or route servers within the ATM network provide routing information to the edge devices. The edge devices transmit data from their connected hosts to other edge devices using the shortcut ATM layer paths identified by the route servers. Again, although QoS is available to all ATM-attached devices, traditional connectionless-oriented LAN devices may not be able to take full advantage of this feature. Multiprotocol Label Switching (MPLS)

MPLS is a form of tag switching. Tag switching was introduced in Chapter 4 and will not be discussed here. In an ATM context, VCs would be identified with particular labels ortags.


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