Repeater and Switch written by Adrian Lienhard, Lukas Renggli, Stefan Reichhart 1 / 11 Table of Contents Introduction............................................................................................................. 3 Overview.............................................................................................................. 3 Structure of packets and MAC Address.................................................................... 3 Repeater ................................................................................................................. 3 Basics .................................................................................................................. 3 Standards and common implementations ................................................................ 4 Multiport-Repeater ................................................................................................... 5 Basics .................................................................................................................. 5 Common Multiport-Repeater implementations.......................................................... 5 Switch ..................................................................................................................... 6 Basics .................................................................................................................. 6 Common switch implementations............................................................................ 7 Summary................................................................................................................. 8 Multiple-Choice Questions ......................................................................................... 9 Answers to the Multiple-Choice Questions..................................................................11 2 / 11 Introduction Overview The following paragraphs of this document should give an impression of what repeaters, hubs and switches are, explain their functionality and features as well as show differences and things in common. Beside that, advantages and disadvantages of the certain technologies are shown and how, why and where you should use these technologies in practice. Aspects like the OSI/ISO-Reference-model, TCP/IP, international standard and physical hardware are broached. First of all this document should give you an overview about packets and MAC addresses, followed by extensive descriptions and examples about repeaters and switches. At the end there is a short summary and a sheet of questions to test yourself if you have understood eve- rything. Structure of packets and MAC Address A Packet consists of five basic parts beginning with a leading 8 byte preamble as a start delimiter: The first three parts are reserved for the “MAC information” which is linked to the Datalink-Layer of the OSI/ISO-Model. The forth part is interpreted by the Network-layer and consists of “IP information”. At the end, there is a CRC that belongs to the “MAC information” to ensure the correctness of a packet. IP Datagram 48-1500 byte Preamble Destination Source Packettype Packet Data CRC Address Address Frame Header / SDU Frame Data / PDU Ethernet Frame The “MAC information” itself consists of four parts. The first one has a size of 6 bytes and is called “MAC Destination-address”. It can be of type unicast, multicast or broadcast. The sec- ond one is the “MAC Source-address” and consists of 6 bytes. The third part decides of which “Length” (IEEE 802.3) or “Type” (Ethernet) the containing packet (Packet Data) is. This part is also called SAP (Service Access Point) and needs 2 bytes. The last part, the CRC, encapsulates the “IP Information” and provides a checksum of the frame. Repeater Basics A repeater is a signal generator, regenerator (digital or analogue) and amplifier that pro- vides two ports which can be connected with standard BNC technology (means: coaxial ca- bling). It realizes (broadcasted) forwarding of data respectively packets from one port to an- other and regenerates or amplifies them if they're faded. While analogue repeaters can almost only amplify signals, digital repeaters can also reconstruct the signal to near its original quality. Simultaneously they are filtering electrically invalid signals to guarantee the security and stabil- ity of the net and to avoid unnecessary stress. 3 / 11 Nevertheless, repeaters do forward invalid or damaged data as they do not have any ca- pability to ensure their correctness. It is the same with packet collisions. These have to be solved by the end-user systems. So, repeaters act as an only-forwarding medium for data. As a result of this, it has a very short latency time. Signal-Processor Repeaters divide the network in two parts or segments. Each segment can be a computer or a network of computers. The topology of the entire network is a bus topology which always addresses to the “weakest” participant. Important is that both segments do have identical quali- ties or characteristics (as a result of a bus topology). They have both the same bandwidth, se- curity level, topology, etc. Example: segment A has a maximum bandwidth of 100Mbit/s, seg- ment B only 10Mbit/s. The resulting maximum bandwidth of the net- work is 10Mbit/s. Both segments have the same characteristics. So, using a repeater does not increase the network's bandwidth or capac- ity. As both segments have the same characteristics, it is not possible to connect different or incompatible networks. To realize this it is necessary to plug in a media-converter. An important characteristic that has to be mentioned explicitly is that a repeater forward any incoming packets to all ports respectively segments. This results of the bus topology that repeaters cause. The whole network consisting of subnets act in the same way and can actually be seen as just one big segment. Repeaters are assigned to the first layer of the OSI/ISO-Model. It is totally “invisible” for the user. As it is on the bottom of the layer model, it is only seen as a primitive physical trans- port medium. Extended versions provide further functionalities like multiple ports (e.g. hubs, see section below), multiple protocols and standards and higher layer interpretation (e.g. bridges and switches, switches are explained in a later section of this document). Standards and common implementations The general functionality of repeaters are quite restricted and depend mostly on interna- tional standards like “Ethernet IEEE802.3”. It defines all the variations of Ethernet and their characteristics. Therefore, there are fixed rules that describe the maximum size of a segment and the whole network, the maximum amount of repeaters within a network, distance between repeaters (inter-repeater-link), bandwidth, etc. When building up a network it is important to know which standards a repeater supports as repeaters do not automatically cope with other standards. Actually there are three main branches of repeaters. The first one belongs to the rather old “Ethernet” standard (10Mbit/s), the second one is called “Fast-Ethernet” (100-200Mbit/s) which is itself branched into Class I (max. 1 repeater/net) and Class II (more than 1 repeater, but max. amount is restricted depending on network topology) and the last one is the “Gigabit Ethernet” (1000Mbit/s). Main differences between these branches are bandwidth, maximum size of bandwidth, cabling, enhanced topologies, etc1. 1 To read more about these standards and their characteristics contact the "IEEE802.3 Reference Guide" that can be found on the Internet. 4 / 11 Multiport-Repeater Basics A special kind of a repeater is the “Multiport-Repeater”. Colloquially, it is almost always described as hub or common hub. However the term “hub” is in literature also used for other network devices or objects such as switches, routers, bridges, airport-hubs and so on. In the following sections we use the term “hub” or common hub instead of “Multiport-Repeater” but meaning “Multiport-Repeater”. Hubs do have a lot in common with repeaters. They have the same characteristics and functionalities, but hubs do normally have more than just two ports. Normally they have multi- ple ports (this is why it was originally called “Multiport-Repeater”) and are able to connect to other hubs (using an uplink-port) and to a number of hosts (using a downlink-port). So it is still a broadcast device sharing the same network topology and performance in any segment and sub-segment. Incoming Outgoing The only difference of a repeater and a hub is that a hub is normally connected with Twisted-Pair which increases bandwidth, guarantees security of disturbance and allows a more reliable topology (broadcast star). Moreover, primitive collision domains are integrated to han- dle collisions to relieve the network of useless packets. All that makes this extended technology more reliable, faster but also more expensive. However, as a repeater does divide the network only in two parts, both having the same characteristics, it is quite the same with hubs. All its ports act like one segment, called “Shared Segment”. Therefore, it is neither possible to increase the network's performance with a hub. On the other hand it is possible to enlarge the network by connecting a hub to another one. Such configurations are called “x-logical repeater”2, “Stackable Multiport Hub” or shorter and more common “Micro Hub”. Today it is common to enhance the standard functionalities of hubs to make them more powerful. Hardware producer provide them with additional