Title - Introduction to Data Communications


Network interconnectivity, also known as internetworking, glues together networks from different vendors, with different protocol stacks, and utilizing different communications technologies to form an enterprise wide network. To make these networks useful, they devices need to have the network identify them (names), there must be a way of identifying the cause of troubles in the network (network management), and there must be a way to forward information betwen networks.


The key devices used in network interconnectivity are bridges, routers, and switches. As shown in Figure 1, there are devices that tie these various networks together. In order for these devices to perform their functions, there is the concept of the name of the computers, its address(s) within a network and the route that will be used to transmit the data between them.

Figure 1. Mixture of Data Networking Components

Names, Addresses and Routes

In some respects, a computer network operates with some of the same processes used by people to send information (letters) to eachother. If you want to contact somebody, you need their name, an address of where the letter is to go and then the post office decides on the best route to be used for the delivery of your information.



Names are usually unique identifier that can be associated with either a machine function or a user. Think about names in a phone book. Sometimes the person (name) may be at the address specified in the book. If we wanted to visit that person at their home, we would need to travel a route to get to them. This analogy works well for data networks.

The name and address for a device are unique for each protocol. Layer 2 and 3 of the OSI protocol stack has its own addressing mechanism and a computer may be running more than one protocol at each layer. A good example may be having an IP address (32 bits) running over an Ethernet address (48 bits) over an ATM network (20 bytes).

The association between names and addresses is not static, that is it can change. Sometimes a machine may be a server but other times a new machine can replace the old server. Instead of everyone changing the address, it is possible to use the services of a name to address translator. The device that translates names to address is called a name server. In the Internet, the specific name is the Domain Name Server (DNS).


Once a name is translated into a network address, it becomes possible to find a path to get to that address. One of the things that separates bridges, switches and routers is the way they get a packet to its destination.

In a switch, the address must first sent to an intelligent entity that knows about the entire network. It may find multiple routes through many switches to get the destination. It would then select an the best route identify individual switches that must create the path. Once this path is established, all packets will follow this same route. At the end this entity must be aware of when there is no longer a need to maintain this path.

A bridge will learn about the stations that are on its LAN and forward any packets that have addresses it cannot identify. If there are multiple bridges on a LAN, each bridge will see if there is a return message from that destination. If it does not see a message, there will be no further forwarding. A bridge does not need to know about the entire network, only about the machines directly attached to it.

A router examines each packet and performs one of several functions. It can use a preconfigured table to send it to a destination, it may learn about a destination similar to a bridge, or it may ask adjacent routers for help. Routers usually only have a few ports verses many ports for a switch. It does not need to know when a connection starts and stops. It only needs to know about its part of the network verses the entire network.


In summary:

  • A Name

    • is the identifier for a unit or function

    • needs to be translate to get its address

  • An Address

    • is an identifier for a machine

    • may be a multicast address which represent multiple destinations (IP, Ethernet, ATM)

    • associated with a name may change

  • A Route

    • is the path through the network

    • may have switches which require a set-up for a session

    • may have Routers/Bridges which can automatically learn about the network

An example: How names are translated to address for the Internet

Every computer (host or router) in a well run part of the Internet has a Name. The name is usually given to a device by its owner. Internet names are actually hierarchical, and look rather like postal addresses. For example, the name quote.yahoo.com directs it to the server for stock price quotes (quote). The company it is in is called yahoo. The organization is com (for commercial) and it is in the United States (us). The Internet calls this the Domain Name System (DNS). Names in this system are "Case Insensitive", which means that it makes no difference whether you give them in capitals or not.

Figure 2. Name to Address Translation

In Figure 2, the name from the destination name is typed into the computer and forwarded (step 1) into the network. The message is forwarded to the Domain Name Server (step 2) which sends out a message to the country server (step 3), and forwards it on through the hierarchy of names until the complete address is found. When it finally gets to a DNS server that knows the destination's address, the last DNS server returns the address to the original DNS server (step 7).

One of the benefits of using this hierarchical system is that not every DNS computer needs to know the name of every computer in the world (a rather large database!). Thus a company would only need to alter the list for its servers and not worry about all the other servers in the world. The primary disadvantage is that it takes several steps to resolve the name.

Everything in any part of the Internet that wants to be reached must have an address. The address tells the computers in the Internet (hosts and routers) where something is topologically. Thus the address is also hierarchical. The Conklin Corporation server address is It was assigned by the IANA (Internet Assigned Numbers Authority) for a network number.

For large companies with Class B addresses, it might be given the number 127.99.x.y. The company can fill in the x and y as they like. Too number the computers in a network, divide the computers into groups on different LAN segments, and number the segments 1-256 (x), and then the hosts 1-256 (y) on each segment. When an organization asks for a number for its net, it will be asked how many computers it has, and assigned a network number big enough to accommodate that number of computers.

Everything in the Internet must be reachable. The route to a host will traverse one or more networks. The easiest way to picture a route is by thinking of how a letter to a friend in a foreign country gets there.
You post the letter in a postbox. It is picked up by a postman (LAN), and taken to a sorting office (router). There, the sorter looks at the address, and sees that the letter is for another country, and sends it to the sorting office for international mail. This then carries out a similar procedure. And so on, until the letter gets to its destination. If the letter was for the same 'network' then it would get immediately locally delivered. Notice the fact that all the routers (sorting offices) don't have to know all the details about everywhere, just about the next hop to go to. Notice the fact that the routers (sorting offices) have to consult tables of where to go next (e.g. international sorting office). Routers chatter to each other all the time figuring out the best (or even just usable) routes to places.

One way to picture this is to imagine a road system with a person standing at every intersection who is working for the Road Observance Brigade. This person (Rob) reads the road names of the roads meeting at the intersection, and writes them down on a card, with the number 0 after each name. Every few minutes, Rob holds up the card to any neighbor standing down the road at the next intersection. If they are doing the same, Rob writes down their list of names, but adds 1 to the numbers read off the other card. After a while, Rob is now telling people about the neighbors roads several roads away! Of course, Rob might get two ways to get somewhere! Then, he crosses out the one with the larger number.


The Basics of Bridging, Routing and Switching

Bridges, switches and routers are devices that connect networks together. All three of these technologies are useful and apply to different portions of the network.


Bridges and switches are a data communications devices that operate principally at Layer 2 of the OSI reference model. A principle difference between these two technologies is that switches must have the route of the packet determined before the first packet can traverse the network. Bridges can make packet forwarding decisions dynamically.

Figure 3. A Network Utilizing Bridges, Router and Switches

In Figure 3, all three internetworking devices are shown working simultaneously. A bridge connects two similar or dissimilar LANs to form a larger network at the data-link layer (layer 2). Bridges are simple devices and do not deal with any higher-level issues such as network routing and session control. Bridges require that the networks have consistent addressing schemes and packet sizes. Their primary usage is:

  • Interconnections of LANs with different layer 1 technologies. Bridges are used frequently to connect Ethernet LANs. For example, many bridges connect thin wire Ethernet to thickwire Ethernet. A bridge between two LANs will basically read a message from the first LAN and pass only the messages destined for the second LAN.

  • Bridges can be used to divide a large network into smaller subnets to control traffic. For example, consider an Ethernet LAN with 100 stations. Since all 100 stations chattering simultaneously can cause collisions, a bridge can be used to subdivide the network into two 50 stations LANs.

The biggest advantage of a switch is that it is aware of the presense of a session between two computers. This allows the switch to provide different leverls of service to each type of connection based on the needs of each of the sessions. For example, a voice conncection requires a constant data rate and very low delays between the computers. A computer browsing the Internet can tolerate variations in the delay between the computers.


A router operates at layer 3. It finds a path for a message and then sends the message on the selected path. A router may appear to be the same as a bridge, but the main distinguishing feature of a router is that it knows alternate routes for a message and uses the alternate route to send a message if the primary route is not available. Consequently, a router must know the network topology (a layer 3 issue). Owing to their router algorithms, routers are more complex and expensive than bridges.


In Summary:

  • Switches

    • forward packets based solely on the layer 2 address. (Lowest cost)

    • are session aware (must set-up and tear down)

  • Bridges

    • forwards packets based on the layer 2 address. (Medium cost)

    • learn which devices are on both sides of the its interfaces (usually 2 interfaces)

    • filter packets (hop count)

  • Routers

    • Based on layer 3 information (protocol specific)

    • Finds path through network / shares information

    • Filters out broadcast messages for layer 2

    • Highest cost (factor of 3)

For more details on how these devices work, click on the following:

  • Bridges - A description of the five main types of bridges.

  • Routers - A description of the various types of routers and how they work.

Network Management

In the late 1970's, computer networks had grown from a simple layout of small, separate networks that were not connected to each other to larger networks that were interconnected. These larger networks were called internets and their size grew at an exponential rate. The larger these networks became the more difficult they became to manage (i.e.. monitor and maintain), and it soon became evident that a network management protocol need be developed.

The first protocol used was the Simple Network Management Protocol (SNMP). It was commonly considered to be a quickly designed "band-aid" solution to internetwork management difficulties while other, larger and better protocols were being designed. The five main functions are fault management, configuration management, security management, performance management, and accounting management.

To aid in the task of managing the network, network protocols are used so that the process is automated (i.e. run by computers) as much as possible.

Simple Network Management Protocol (SNMP) and Common Management Information Protocol (CMIP) are two of the network management protocols. Generally, SNMP works under the TCP/IP (Transport Control Protocol/ Internet Protocol) communication stack and CMIP works under the OSI (Open Systems Interconnection) communication stack.

SNMP is designed to facilitate the exchange of management information between network devices. By using SNMP to access management information data (such as packets per second and network error rates), network administrators can more easily manage network performance and find and solve network problems. SNMP is a relatively simple protocol, yet its feature set is sufficiently powerful to handle the difficult problems presented by management of heterogeneous networks.

CMIP is used with the Common Management Information Services (CMIS). CMIS defines a system of network management information services. CMIP was proposed as a replacement for the less sophisticated Simple Network Management Protocol (SNMP) but has not been widely adopted. CMIP provides improved security and better reporting of unusual network conditions.

The information the SNMP and CMIP can attain from a network is defined as a MIB (management information base). The MIB is structured like a tree. At the top of the tree is the most general information available about a network. Each branch of the tree then gets more detailed into a specific network area, with the leaves of the tree as specific as the MIB can get. For instance, devices may be a parent in the tree, its children being serial devices and parallel devices. The value of these may be 6 , 2, 4 accordingly; with the numbers corresponding to the number of devices attached (4 parallel + 2 serial = 6 total devices). Each node in the MIB tree is a variable (hence in the above example, devices, serial devices, and parallel devices are all variables, their values being 6, 2, 4 accordingly). The top of a LAN MIB tree is usually referred to as "Internet".

The CMIP protocol was supposed to be the protocol that replaced SNMP in the late 1980's. Funded by governments and large corporations, many thought that it would become a reality because of its almost unlimited development budget. Unfortunately, problems with its implementation have delayed its widespread availability and it is now only available in limited form from its developers themselves.
CMIP was designed to build on SNMP by making up for SNMP's shortcomings and becoming a bigger, more detailed network manager. Its basic design is similar to SNMP, whereby PDU's are employed as variables to monitor a network. CMIP however contains 11 types of PDU's (compared to SNMP's five).

In CMIP, the variables are seen as very complex and sophisticated data structures, with many attributes.


These include:

  • variable attributes: which represent the variables characteristics (its data type, whether it is writable).

  • variable behaviors: what actions of that variable can be triggered.

  • Notifications: the variable generates an event report whenever a specified event occurs (e.g.. a terminal shutdown would cause a variable notification event.

As a comparison, SNMP only employs variable properties one and three from above.


Additional information is available for:

Bridges - A description of the four main types of bridges.

Routers - A description of the various types of routers and how they work

IP addressing - A description of the four types of IP addresses, public/private addresses, and

    static/dynamic addresses.

Network Managment - How these systems monitor complex networks.


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