1.6 Network Devices (Layer 1)

1.6 Network Devices (Layer 1)

Repeaters

OSI Model: Layer 1 Device
Protocol Data Unit(PDU): Bits

Due to the effects of attenuation some method is required to maintain signal integrity over longer distances. Installing a repeater enables the data signals to travel further. A repeater works at the physical layer of the OSI reference model and regenerates the network signals. The repeater takes a weak signal from one segment, regenerates it, and then passes it on.

A repeater does not allow for the joining of two different types of network, for example, a repeater will not work between an 802.3 LAN (Ethernet) and an 802.5 LAN (Token Ring).

Repeaters do not translate or filter signals. For a repeater to work, both network sections that the repeater joins must use the same access method. A repeater cannot connect part of a network using CSMA/CD to another part of a network using the token passing access method, ie: a repeater cannot translate an Ethernet packet into a Token Ring packet.

Repeaters can, however, move bits from one type of physical media to another, eg: they can take Ethernet bits from a thinnet coaxial cable segment and pass it on to a Ethernet fibre optic segment (obviously the repeater must be capable of accepting the physical connections).

Repeaters are, perhaps, the least expensive way to expand a network. Repeaters send every bit of data from one cable segment to another even if the data consists of corrupt packets or packets not meant for use on the network. Effectively, this means that problems with one segment can disrupt every other segment. Repeaters do not act as filters to restrict the flow of problem traffic.

Repeaters will also pass a broadcast storm along from one segment to the next along the network. A broadcast storm occurs when so many broadcast messages are on the network that the number is approaching the network bandwidth limit.

Hubs

OSI Model: Layer 1 Device
Protocol Data Unit(PDU): Bits

Hubs operate at the Physical Layer of the OSI model and can be generally divided into two types, active and passive.

Passive hubs do not amplify the electrical signal of incoming packets before broadcasting them out to the network, whereas, active hubs retime and regenerate signals in a similar way to a repeater. Some people use the term concentrator when referring to a passive hub and multi-port repeater when referring to an active hub.

Active hubs and repeaters may be used to provide signal amplification and regeneration to restore a good signal level before sending it on. Another name for an active hub is an ‘active star network’. With any type of hub each attached system has to share a proportion of the available network bandwidth.

A very important fact about hubs (and repeaters) is that they allow users to share an Ethernet LAN. A network hub is therefore called a ‘Shared Ethernet’ or a ‘Collision Domain’. The various hosts sharing the Ethernet all compete (contend) for access using the CSMA/CD access protocol. This means that only one system is allowed to proceed with a transmission within a Collision Domain at any one time.

Each port of a hub has an RJ-45 connector able to accept a patch cable to connect to a wall outlet. The other end of the cable is normally connected to a NIC with a 10BASE-T outlet. Many hubs also have an AUI connector, which may be used to connect an external transceiver to which other types of Ethernet Media may be attached. (Sometimes a 10Base2 connector is also provided to allow connection directly to a coaxial cable thin Ethernet network.) Whatever the type of connector, a single hub is only able to connect a group of equipment operating at the same speed (ie: all equipment connected to a 10BASE-T hub must operate at 10 Mbps).

Each port (or interface) allows one piece of equipment to be connected to the hub. In the case of the system connected via port 2 wanting to send a frame of data to the system connected at port 5 can not be achieved as point to point. This is because a hub is not able to recognise the addresses in the header of a frame, and is therefore unable to identify which port to send to. Therefore, it sends the frame to every port except the one it received it from. (This is in contrast to a bridge, switch or router, each of which only forwards a packet if the destination address of the frame or packet corresponds to a system reachable via the output interface.)

Next:

Bridges

OSI Model: Layer 2 Device
Protocol Data Unit(PDU): Frames

A bridge can be used to join segments or workgroup LANs. A bridge can also be used to divide a network to isolate traffic or problems. If the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down the entire LAN, a bridge could isolate those computers or that department. Bridges can be used to:

  • Expand the length of a segment
  • Provide for an increased number of computers on the network
  • Reduce traffic jams resulting from an excessive number of attached computers
  • Split an overloaded network into two separate networks, reducing the amount of traffic on each segment and making each network more efficient
  • Link unlike physical media such as twisted pair and coaxial Ethernet

Because bridges work at the data link layer of the OSI reference model all information contained in the higher levels of the OSI reference model is unavailable to them. Bridges do not distinguish between one protocol and another. They pass all protocols along the network. Because all protocols pass across bridges it is up to the individual computers to determine which protocols they can recognise.

The data link layer has two sub layers: the Logical Link Control (LLC) sub layer and the Media Access Control (MAC) sub layer. Bridges work at the MAC sub layer.

A bridge works at layer 2 of the OSI model and its primary task is to keep local traffic local.

A bridge works by doing the following:

  • Listens to all traffic
  • Checks the source and destination addresses of each frame
  • Builds a switching table, as the information becomes available
  • Forwards packets if not local

A bridge works on the principle that each network node has its own address. A bridge forwards packets based on the address of the destination node. Bridges do have a degree of intelligence in that they learn where to forward data to. As traffic passes through the bridge, information about the computer addresses is stored in the bridge’s RAM. The bridge uses this RAM to build a switching table based on source addresses. The bridge’s routing table is initially empty. As packets are transmitted the source address is copied to the routing table. With this address information the bridge learns which computers are on which segment of the network.

If a bridge knows the location of the destination node it forwards the frame to it. If it does not know the destination it forwards the frame to all segments.

Bridges work at layer 2 of the OSI model whereas repeaters and hubs work at layer 1. This means that bridges have more intelligence than repeaters and hubs. Whilst it is true to say that bridges resemble repeaters in that they can regenerate data, bridges do this at the frame level. This means that bridges can send frames over long distances using a variety of long distance media.

The key advantages of a Bridges are that they are easy to install and set up, invisible to users and relatively inexpensive. Generally, Switches have surpassed Bridges as the choice of a layer 2 device.

Switches

OSI Model: Layer 2 Device
Protocol Data Unit(PDU): Frames

A switch is an OSI layer 2 device that allows network microsegmentation.

LANs can be segmented to limit network traffic and therefore to reduce collisions. Traffic flows within a segment but only leaves that segment if it is really necessary. A segment can be a number of computers such as a department or it may be a single computer.

The purpose of a switch is to provide network connectivity, while making data transmission more efficient. A switch provides multiple connections like a hub but behaves like a bridge by providing traffic regulation. A switch switches frames from incoming ports to outgoing ports and provides each connection with full bandwidth.

A switch works by examining the MAC address (layer 2 address) of incoming frames. Switches learn MAC addresses as traffic is generated, a switching table built, very quickly the switch has enough information to operate effectively. A switch examines MAC addresses of frames. If the frame is local ie: the MAC address on the same network segment as the incoming port of the switch then the frame is not forwarded across the bridge. If the frame is not local ie: with a MAC address not on the incoming port of the switch then it is forwarded to the appropriate network segment. All the decision-making is carried out by the switching circuits based on MAC addresses.

A switch cannot be used to connect different types of network, that job falls to the router. A segment is a logical division of devices; in order for communication to take place all hosts must all share the same network address. All hosts connected to the switch are still in the same broadcast domain. Therefore, all nodes connected through the LAN switch can see a broadcast from any node.

 

The diagram shows a network with three switches. Each port on a switch is its own collision domain and unlike a hub there is no contention and no collisions.

All hosts in the diagram are in the class B network 172.16.0.0 with a subnet mask of 255.255.0.0. Hosts in each segment can communicate with each other without creating traffic over the entire network. If a host wishes to send a broadcast message the broadcast address of 172.16.255.255 will be used, all hosts on the network will see the message.

Moving to a switched LAN environment is very cost-effective as existing hardware and cabling can be utilised with minimum disruption, ie: a one for one change from a hub to a switch can be made.

Switching Technology

A LAN that uses a Switched Ethernet topology creates a network that behaves as though it has only two nodes: the sending node and the receiving node. These two nodes share the maximum available bandwidth between them

Each node is directly connected to one of the switches ports or a segment that is connected to one of the switch’s ports. This creates a maximum bandwidth connection between each node and each segment on the switch. A computer connected directly to an Ethernet switch is its own collision domain. A LAN switch allows many users to communicate in parallel through the use of virtual circuits and dedicated network segments in a collision-free environment.