5. NETWORKING LANs

Since the advantage of LANs became apparent to users, the need to extend networks further and further has arisen. This capability has grown to such an extent that many networks can no longer really be termed “local”, and the following paragraphs outline some of the devices used to extend the local area network.

Network Repeaters

These devices simply connect two lengths of cable, possibly some distance apart and regenerate the signal so that the two parts form one LANs. These days Hubs and Ethernet switches include the repeaters on each port.

Network Hubs

An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fibre optic Ethernet devices together, making them act as a single segment. It works at the physical layer of the OSI model, repeating the signal that comes into one port and out on each of the other ports. If a signal comes into two ports at the same time a collision occurs, so every attached device shares the same collision domain. Hubs support only half duplex Ethernet, providing bandwidth which is shared among all the connected devices. Ethernet hubs have been largly replaced by network switches, which operate at the data link layer and improve performance by separating the connected devices into separate collision domains.

Network Bridges

Bridges are simply devices which bridge the gap between two remote LANs. The distance between the two LANs depends upon the bridges capabilities. Today its more common to use ethernet switches to inter-connect different LAN segments and thus if a bridge is used its more likely to extend the LAN over a wide area network, either via a leased circuit or by a dial up service such as ISDN. With a bridge the network is the same at each end of the network, with the same addressing scheme, therefore bridges allow the network to become one large network. Care must be taken to ensure no two devices on the network have the same IP Address.

The intelligence of the bridge can vary, but usually a bridge will examine the address of each packet of information on the network. Often bridges have a learning capability so they can develop a knowledge of all addresses and whether they are local or remote. Any locally addressed packets are ignored, and all packets with remote addresses related to that bridge will be passed across to the remote bridge which will place them on its network.

A Bridge operates at the data link level of the LAN, usually the Media Access Control (MAC) level. Operation is not as efficient as a router as all the lower level information must also be passed over the link between the networks. However,a benefit of operating at level 2 in the OSI stack is that it is possible to bridge two different networks, for example TCP/IP and OSI, as the bridge simply passes data packets and ignores the higher level protocols.

Spanning Tree

The principle limitations of remote bridges are that a ring cannot be formed in the network (This is no duplication of trunks or links between bridges) and two bridges cannot connect the same two networks for resilience, as the same data packet could be forwarded in an endless loop. However, a standard has been defined called the “Spanning Tree Algorithm” which allows bridges to form loops. What this does is create a protocol for bridges to use when starting up or when failure is detected.

This is based on a learning or listening stage when bridges will intercommunicate with each other but will not transmit live data. This allows each bridge to determine its position and ‘priority’ in the network. Any bridge which detects that it is linking the same two network segments, or is causing a loop and is the lowest priority bridge in that structure, will block its link between the two segments thus preventing duplication of data on the network. After a specified time period, the bridges will start sending live data. Periodically a bridge will then send data to other bridges, and any failure of one of these transmissions will automatically start a new learning process to re-establish the network structure, which may have changed due to the failure of some equipment in the network. Any new bridge introduced into the network will similarly start by listening to determine its position in the network. While this facility increases the flexibility of bridges, care should be taken in selection of systems, as not all devices will have this facility implemented.

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Network Routers

Routers operate in a similar way to bridges except that they operate at a higher level, the Network Layer, in the OSI model. This provides the great advantage of allowing dissimilar networks , for instance CSMA/CD and Token Ring, to interwork. Today routers are mainly used to inter-connect two or more LANS over a wide area network.

Apart from interconnection of dissimilar networks, routers also allow high performance and resilience to be built into network through triangulation, multiple links between the same two networks and additional features such as traffic analysis and accounting. As routers operate at the Network Layer, they have access to the addressing information of data packets and sophisticated routers can select a path between networks, such as the fastest or cheapest route.

The principle limitation of routers is that, as they operate at the Network Layer, it is impossible for them to interwork between dissimilar Network Layer protocols, without the relevant software. For example a TCP/IP router cannot route OSI, DECLAT or any other non TCP/IP protocols without having a software stack for each protocol, and this means a reduction of performance. However as TCP/IP is almost universal today its is no longer such a problem and most routers no longer need to support anything other than TCP/IP.

Many routers also provide Bridging facilties for protocols they don’t recognise, that is to say they will route TCP/IP but if they encounter OSI on the LAN they will bridge it.

Transport of WAN Protocols Over Routers

As the world moves towards TCP/IP and major Telecos move away from pure digital circuits to IP transport only, a new breed of router has to emerge to provide support for non IP devices.

X25 Over IP (XOT)

Where legacy X.25 systems existed, X.25 Over TCP/IP allows the IP Router to emulate an X.25 network, and for the router to transport X.25 over the IP network.

HDLC Over TCP (HOT)

Case Communications HOT (HDLC Over TCP) technology uses a card which fits into routers allowing legacy products which utilise HDLC (such as X.25, Frame Relay and stat muxes etc.) to be transported over an IP network, completely transparently.

Voice Over IP

More common are Voice Over IP Routers which transport telephone calls and faxes over an IP network, saving the cost of expensive long distance calls.

TDM Over IP

A newer but fast growing technology is TDM Over IP (Time Division Multiplexing) which allows virtually any serial devices to operate over an IP network, and which emulate a traditional PDH/SDH Time Division Multiplexer network, over IP.

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Ethernet Switches

Ethernet Switch Introduction

A network switch is a computer networking device that connects network segments. It uses the logic of a Network bridge but allows a physical and logical star topology. It is often used to replace network hubs. A switch is also often referred to as an intelligent hub.

Originally Ethernet switches switched data at layer two, but increasingly switch at layer 3, 4 and beyond. This means that not only can Ethernet switches switch between LAN segments but they can join two different LANs and switch by traffic type.

Switch Operation

A switch can connect Ethernet, Token Ring, or other types of packet switched network segments together to form a heterogeneous network operating at OSI Layer 2. While a number of switches can switch at Layer 3 and beyond, for the purpose of this section we will focus on more common layer two switches. For more detailed information on Ethernet Switching, please refer to the ‘Case Communications pocket book of Ethernet Switches’.

Layer Two Switching

As a frame comes into a switch, the switch saves the originating MAC address and the originating port in the switch’s MAC address table. The switch then selectively transmits the frame from specific ports based on the frame’s destination MAC address and previous entries in the MAC address table. If the MAC address is unknown, or a broadcast or multicast address, the switch simply floods the frame out of all of the connected interfaces except the incoming port. If the destination MAC address is known, the frame is forwarded only to the corresponding port in the MAC address table. If the destination port is the same as the originating port, the frame is filtered out and not forwarded.

Switches, unlike hubs, use divide collision domains, one per connected segment. This way, only the NICs which are directly connected via a point-to-point link, or directly connected hubs are contending for the medium.

By eliminating the possibility of collisions, full-duplex point-to-point connections on the switch become possible.

Virtual LANs can be used in switches to reduce the size of the broadcast domains and at the same time increase security.

In redundant architectures, spanning tree protocol can be used in switches to prevent loops.

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Forwarding Methods

There are four forwarding methods a switch can use:

1. Cut through – starts forwarding the frame (or packet) before the whole frame has been received, normally as soon as the destination address is processed. This technique reduces latency through the switch. In packet switched networks such as Ethernet, cut-through switching can only be used where the outgoing interface is equal in speed to, or slower than the incoming interface.

Cut through routing in IP networks presents some problems since the IP checksum in the packet is supposed to be checked by every router in the path. Since the checksum of a packet cannot be checked until the entire packet has been received, the cut-through router is at risk of forwarding a packet with an incorrect checksum. Provided that there are other routers in the path which are not doing cut-through routing, or that the end system is correctly verifying checksums, this should only result in the occasional loss of a small amount of traffic capacity.

Cut through routing was one of the important features of ATM networks since the edge routers of the ATM network were able to use cell switching through the core of the network with low latency at all points. With higher speed links, this has become less of a problem since packet latency has become much smaller.

2. Store and forward – the switch, unlike cut through, buffers and typically, performs a checksum on each frame before forwarding it on. Store and Forward is typically has greater latency as the switch has to look at and process the packets, but it also provides more reliable data transmission as errors can be removed, and it also allows for speed mis-matching. This is useful where, for example a server could sit on a 1 Gbps port of an Ethernet switch and be connected to by a number of users residing on 10Mbps and 100Mbps ports.

3. Fragment-free switching – is suitable for backbone applications in a congested network, or when connections are allocated to a number of users. The switching device checks the source and destination MAC address of a packet, and sends the packet to the port corresponding to the destination.

The packets are sent through the switch as a continuous flow of data, and the transmit and receive rates are always the same. Because of this, fragment-free switching cannot pass packets to higher speed networks, for example, to forward packets from a 10 Mbit/s to a 100 Mbit/s Ethernet network. Therefore, if you opt for fragment-free switching, you cannot make direct connections to higher speed networks from that port.

Fragment-free switching offers a compromise between cut through (which offers the fastest possible forwarding at the expense of any error checking) and store-and-forward (which offers maximum error checking at the expense of latency), to provide an average latency of approximately 60µs and sufficient error checking to eliminate most common errors.

4. Adaptive switching – mode is a user-defined facility to maximize the efficiency of the switch. Adaptive switching starts in the default switch forwarding mode you have selected (cut-through if you selected adaptive mode as the default switching mode). Depending on the number of runts and CRC errors at that port, the mode changes to the “best” of the other two switching modes. As the numbers of runts and CRC errors change, so does the forwarding mode.

An Adaptive switch will automatically switch between the various modes, and will adopt the best method of operation according to the prevailing conditions. The table below gives an example of this.

Flaws

Switches provide difficulties in monitoring traffic because each port is isolated until it transmits data, and even then only the sending and receiving ports are connected.

Two popular methods that are specifically designed to allow a network manager to monitor traffic are:

• Port mirroring – the switch sends a copy of network packets to a monitoring network connection.
• SMON – “Switch Monitoring” is described by RFC 2613 and is a protocol for controlling facilities such as port mirroring.

Other “methods” (a.k.a. attacks) have been devised to allow snooping on another computer on the network without the cooperation of the switch:

• ARP spoofing – fooling the target computer into using your own MAC address for the network gateway, or alternatively getting it to use the broadcast MAC.

MAC flooding – overloading the switch with a large number of MAC addresses, so that it drops into a “failopen mode”.

Gateways

While not strictly used to connect two LANs or LAN segments Gateways can be used to connect a LAN to a host computer or even to a WAN service.

A Gateway is a specialised form of access device. It is designed to create access between systems or environments running different, often proprietary protocols. It may also enable proprietary systems to be connected to a common backbone LAN, running TCP/IP for instance. For example the Case Communications ‘T.Gate’ interconnects an Ethernet LAN operating TCP/IP to an X.25 network,and the X.25 network to the TCP/IP network.

This may be confusing, but reference to the OSI model is helpful. A Gateway is often a device which operates at a high layer in the OSI model. (Usually above layer 3, and thus beyond the capability of repeaters, bridges and routers.). Alternatively, a Gateway may operate at any level of the model with a dissimilar level at each side of the Gateway. There is no hard and fast rule as to what forms a true Gateway, but principally it is a device concerned with conversion of incompatible protocols, networks and applications.

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