Network Devices

The Network Devices are Repeaters, Bridges, Routers, and Gateways.

Network Repeater

A repeater connects two segments of your network cable. It retimes and regenerates the signals to proper

amplitudes and sends them to the other segments. When talking about, ethernet topology, you are probably talking about using a hub as a repeater. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row. Repeaters work only at the physical layer of the OSI network model.

Bridge

A bridge reads the outermost section of data on the data packet, to tell where the message is going. It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks. Bridging occurs at the data link layer of the OSI

model, which means the bridge cannot read IP addresses, but only the outermost hardware address of the

packet. In our case the bridge can read the ethernet data which gives the hardware address of the destination address, not the IP address. Bridges forward all broadcast messages. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. The hardware address is also called the MAC (media access control) address. To determine the network segment a MAC address belongs to, bridges use one of:

• Transparent Bridging - They build a table of addresses (bridging table) as they receive packets. If the address is not in the bridging table, the packet is forwarded to all segments other than the one it came from. This type of bridge is used on ethernet networks.

• Source route bridging - The source computer provides path information inside the packet. This is used on Token Ring networks.

Network Router

A router is used to route data packets between two networks. It reads the information in each packet to

tell where it is going. If it is destined for an immediate network it has access to, it will strip the outer packet, readdress the packet to the proper ethernet address, and transmit it on that network. If it is destined for another network and must be sent to another router, it will re-package the outer packet to be received by the next router and send it to the next router. The section on routing explains the theory behind this and how routing tables are used to help determine packet destinations. Routing occurs at the network layer of the OSI model. They can connect networks with different architectures such as Token Ring and Ethernet. Although they can transform information at the data link level, routers cannot transform information from one data format such as TCP/IP to another such as IPX/SPX. Routers do not send broadcast packets or corrupted packets. If the routing table does not indicate the proper address of a packet, the packet is discarded.

Brouter

There is a device called a brouter which will function similar to a bridge for network transport protocols that are not routable, and will function as a router for routable protocols. It functions at the network and data link layers of the OSI network model.

Gateway

A gateway can translate information between different network data formats or network architectures. It can translate TCP/IP to AppleTalk so computers supporting TCP/IP can communicate with Apple brand computers. Most gateways operate at the application layer, but can operate at the network or session layer of the OSI model. Gateways will start at the lower level and strip information until it gets to the required level and repackage the information and work its way back toward the hardware layer of the OSI model. To confuse issues, when talking about a router that is used to interface to another network, the word gateway is often used. This does not mean the routing machine is a gateway as defined here, although it could be.

Network Categories

TDP/IP includes a wide range of protocols which are used for a variety of purposes on the network. The set of protocols that are a part of TCP/IP is called the TCP/IP protocol stack or the TCP/IP suite of protocols.

Considering the many protocols, message types, levels, and services that TCP/IP networking supports, I believe it would be very helpful to categorize the various protocols that support TCP/IP networking and define their respective contribution to the operation of networking. Unfortunately I have never seen this done to any real extent, but believe it would be worthwhile to help those learning networking understand it faster and better. I cannot guarantee that experts will agree with the categorizations that will be provided here, but they should help the reader get the big picture on the various protocols, and thus clarify what the reason or need is for each protocol.

As mentioned previously, there are four TCP/IP layers. They are link, network, transport, and application. The link layer is the hardware layer that provides ability to send messages between multiple locations. In the case of this document, ethernet provides this capability. Below I define several categories some of which fit into the 4 layer protocol levels described earlier. I also define a relative fundamental importance to the ability of the network to function at all. Importance includes essential, critical, important, advanced, useful.

1. Essential - Without this all other categories are irrelevant.
2. Critical - The network, as designed, is useless without this ability.
3. Important - The network could function, but would be difficult to use and manage.
4. Advanced - Includes enhancements that make the network easier to use and manage.
5. Useful - Functionality that you would like to be able to use as a network user. Applications or some       functionality is supported here. Without this, why build a network?

The categories are:

There are exceptions to my categorizations that don't fit into the normal layering scheme, such as IGMP is normally part of the link layer, but I have tried to list these categorizations according to network functions and their relative importance to the operation of the network. Also note that ethernet, which is not really a protocol, but an IEEE standard along with PPP, SLIP, TokenRing, and ArcNet are not TCP/IP protocols but may support TCP/IP at the hardware or link layer, depending on the network topology.

The list below gives a brief description of each protocol

• ethernet - Provides for transport of information between physical locations on ethernet cable. Data is passed in ethernet packets
• SLIP - Serial line IP (SLIP), a form of data encapsulation for serial lines.
• PPP - Point to point protocol (PPP). A form of serial line data encapsulation that is an improvement over SLIP.
• IP - Internet Protocol (IP). Except for ARP and RARP all protocols' data packets will be packaged into an IP data packet. Provides the mechanism to use software to address and manage data packets being sent to computers.
• ICMP - Internet control message protocol (ICMP) provides management and error reporting to help manage the process of sending data between computers.
• ARP - Address resolution protocol (ARP) enables the packaging of IP data into ethernet packages. It is the system and messaging protocol that is used to find the ethernet (hardware) address from a specific IP number. Without this protocol, the ethernet package could not be generated from the IP package, because the ethernet address could not be determined.
• TCP - A reliable connection oriented protocol used to control the management of application level services between computers.
• UDP - An unreliable connection less protocol used to control the management of application level services between computers.
• DNS - Domain Name Service, allows the network to determine IP addresses from names and vice versa.
• RARP - Reverse address resolution protocol (RARP) is used to allow a computer without a local permanent data storage media to determine its IP address from its ethernet address.
• BOOTP - Bootstrap protocol is used to assign an IP address to diskless computers and tell it what server and file to load which will provide it with an operating system.
• DHCP - Dynamic host configuration protocol (DHCP) is a method of assigning and controlling the IP addresses of computers on a given network. It is a server based service that automatically assigns IP numbers when a computer boots. This way the IP address of a computer does not need to be assigned manually. This makes changing networks easier to manage. DHCP can perform all the functions of BOOTP.
• IGMP - Internet Group Management Protocol used to support multicasting.
• SNMP - Simple Network Management Protocol (SNMP). Used to manage all types of network elements based on various data sent and received.
• RIP - Routing Information Protocol (RIP), used to dynamically update router tables on WANs or the internet.
• OSPF - Open Shortest Path First (OSPF) dynamic routing protocol.
• BGP - Border Gateway Protocol (BGP). A dynamic router protocol to communicate between routers on different systems.
• CIDR - Classless Interdomain Routing (CIDR).
• FTP - File Transfer Protocol (FTP). Allows file transfer between two computers with login required.
• TFTP - Trivial File Transfer Protocol (TFTP). Allows file transfer between two computers with no login required. It is limited, and is intended for diskless stations.
• SMTP - Simple Mail Transfer Protocol (SMTP).
• NFS - Network File System (NFS). A protocol that allows UNIX and Linux systems remotely mount each other's file systems.
• Telnet - A method of opening a user session on a remote host.
• Ping - A program that uses ICMP to send diagnostic messages to other computers to tell if they are reachable over the network.
• Rlogin - Remote login between UNIX hosts. This is outdated and is replaced by Telnet.

Each protocol ultimately has it's data packets wrapped in an ethernet, SLIP, or PPP packet (at the link level) in order to be sent over the ethernet cable. Some protocol data packets are wrapped sequentially multiple times before being sent. For example FTP data is wrapped in a TCP packet which is wrapped in a IP packet which is wrapped in a link packet (normally ethernet). The diagram below shows the relationship between the protocols' sequential wrapping of data packets.

Network Hardware Connections

Ethernet uses star topology for the physical wiring layout. A diagram of a typical ethernet network layout is
shown below.

On a network, a hub is basically a repeater which is used to re-time and amplify the network signals. In this
diagram, please examine the hubs closely. On the left are 4 ports close to each other with an x above or below them. This means that these ports are crossover ports. This crossover is similar to the arrangement that was used for serial cables between two computers. Each serial port has a transmitter and receiver. Unless there was a null modem connection between two serial ports, or the cable was wired to cross transmit to receive and vice versa, the connection would not work. This is because the transmit port would be sending to the transmit port on the other side.

Therefore note that you cannot connect two computers together with a straight network jumper cable between their network cards. You must use a special crossover cable that you can buy at most computer stores and some office supply stores for around 10 dollars. Otherwise, you must use a hub as shown here.

The hub on the upper left is full, but it has an uplink port on the right which lets it connect to another hub. The
uplink does not have a crossover connection and is designed to fit into a crossover connection on the next hub. This way you can keep linking hubs to put computers on a network. Because each hub introduces some delay onto the network signals, there is a limit to the number of hubs you can sequentially link. Also the computers that are connected to the two hubs are on the same network and can talk to each other. All network traffic including all broadcasts is passed through the hubs.

In the diagram, machine G has two network cards, eth0 and eth1. The cards eth1 and eth0 are on two different networks or subnetworks. Unless machine G is programmed as a router or bridge, traffic will not pass between the two networks. This means that machines X and Z cannot talk to machines A through F and vice versa. Machine X can talk to Z and G, and machines A though F can talk to each other and they can talk to machine G. All machines can talk to machine G. Therefore the machines are dependent on machine G to talk between the two networks or subnets.

Each network card, called a network interface card (NIC) has a built in hardware address programmed by its manufacturer. This is a 48 bit address and should be unique for each card. This address is called a media access control (MAC) address. The media, in our specific case will be the ethernet. Therefore when you refer to ethernet, you are referring to the type of network card, the cabling, the hubs, and the data packets beingsent. You are talking about the hardware that makes it work, along with the data that is physically sent on the wires.

There are three types of networks that are commonly heard about. They are ethernet, token-ring, and ARCnet. Each one is described briefly here, although this document is mainly about ethernet.

Ethernet:

The network interface cards share a common cable. This cable structure does not need to form a structure, but must be essentially common to all cards on the network. Before a card transmits, it listens for a break in traffic. The cards have collision detection, and if the card detects a collision while trying to transmit, it will retry after some random time interval.

Token Ring:

Token ring networks form a complete electrical loop, or ring. Around the ring are computers, called stations. The cards, using their built in serial numbers, negotiate to determine what card will be the master interface card. This card will create what is called a token, that will allow other cards to send data. Essentially, when a card with data to send, receives a token, it sends its data to the next station up the ring to be relayed. The master interface will then create a new token and the process begins again.

ARCnet:

ARCnet networks designate a master card. The master card keeps a table of active cards, polling each one
sequentially with transmit permission.