Internetworking Fundamentals (Lecture #1) Andres Rengifo Copyright 2008.

Internetworking Fundamentals

(Lecture #1)

Andres Rengifo Copyright 2008

What is a communication system?

• It is composed of both hardware and software that allows users to exchange information.

• Information is referred to as graphics, text, video, audio, data, etc.

• First communication system developed was the telephone system.

– It was used as the model to all data communication systems.

– The idea behind the system is that it should be very reliable, efficient and it should provide the services that it advertises.

What is a communications system? (Cont.)

• What devices are necessary to make up a communication system?

– A source

– A transmitter

– A Transmission system or cloud

– A receiver

– A destination

What is a communication system? (Cont.)

• A source creates the actual data to be transported across a network. Applications such as email software, word processors, etc will create the information which will traverse the network.

• A transmitter takes this information and encodes it using the right signaling mechanism according to the physical topology. It merely encodes the information but it does not change it.

What is a communication system? (Cont.)

• A transmission system is usually referred to as the cloud.

• A receiver takes that encoded signal that has all the information content and decodes it so that it can be provided to the final destination.

• A destination takes the information provided by the network and interprets it using applications accordingly.

What are the key functions needed when designing a communication

network?

• Utilization• Congestion Control mechanisms• Flow control• Error detection and error correction mechanisms• Hand-shake processes• Recovery mechanisms• Security schemes• Network management

Key functions (Cont.)

• Maintaining the proper utilization on a network is important. Multiple users will be sending data at the same time.

– These communication flows cannot interfere with each other so as not to drop any information.

– When a system is heavily utilized, data is lost due to oversubscription. It is important to provide the proper bandwidth allocation to allow data not to be dropped.

– Usually the interconnection between networks is design to be a very high speed link between two points.

Key Functions (Cont.)

• Congestion Control Mechanisms are necessary in order to prevent data loss.

– Data is throttle back between systems in order to prevent the overflowing and possible data corruption.

– It is basically an advanced buffering scheme which is agreed to between two systems.


• Flow control is a method where two devices or entities “agree” ahead of time to exchange data at a specific rate given a specific window or buffer size without dropping data.

– One device tells the other “stop sending me data because I am processing data in my buffer space. When I am done, I will tell you to send”

– The other side responds with “ok, I will wait, just let me know when to send the data..”


• Error detection and error correction mechanisms are necessary to make sure the integrity of the data sent between two points is maintained. The error mechanisms will detect that a specific bit that changed from a 1 to a 0 and the process will be able to change it back to the original bit as it was intended.


• Handshaking is a very important process in order to provide a clean and optimal connection between two devices.

– It is a mutual agreement ahead of time (before the exchange of data) as to what buffer sizes will be in used, how fast is the data going to be sent, how much data will be sent, what protocol or language will be used, etc.

– After these parameters are exchanged and agreed too, the conversation will begin.


• Recovery mechanisms are important in case there is a “break” in the path of the data flow.

– This process should allow the communication flow to continue from the point it left off during the break without having to start again from the beginning.


• Security schemes are important because there is always the threat to a network with regards to attacks, viruses, denial of services, etc. Firewalls usually are implemented to protect the infrastructure both inside and outside.


• Network management is important because it is the process or mechanism that provides the administrator with a “health” view of the actual network.

– Network management tools look for outages, possible issues on the network and ways to correct them.

– Being proactive is probably the best way of managing a network because it tells the network administrator of the possibility of an outage before the users realized the issue.

• If the user calls complaining about the problem, it is already too late and it now becomes a reactive measure.

How is information exchanged on a given network?

• Data is usually converted to 1’s and 0’s as computer systems only understand this language.

• These data bits are then sent across different types of physical mediums such as electrical wires, fiber optic cables, microwave links, etc.

– Data is not sent across the physical media as 1’s or 0’s but instead as voltages, light, or frequencies. For example, if there are wires connected to the computer, the 1’s and 0’s will be converted using a pulse mechanisms where a 1 is -3V and a 0 is +3V and this in turn creates a sinusoidal wave using electrons carried across the wiring.

How are these networks interconnected?

• International organizations such as the IEEE and EIA (Electronic Industries Alliance) came up with different standard ways of interconnecting devices using cabling and specific protocols to allow for that communication

– For example, a printer connected to a comm port on a computer. The standard says how fast data can be transfer between the two devices, etc.

– In early stages, the speeds were slow such as 2400 bps or 1200 bps.

– The need to have faster communication between two network nodes gave way to improved protocols. These were referred to as data link protocols.

Campus Design (Interconnecting nodes to form a network)

• Cisco refers to a campus design to a network that is broken into different layers. Each layer has a function and provides specific services.

– There are usually three layers to this architecture:

• Access Layer• Distribution Layer• Core Layer

Campus Design (Interconnecting nodes to form a network)

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Core LayerRouters/Switches

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Access LayerSwitches

PCClients

PCClient

Buildling A LANs Buildling B LANs

MAN/WANconnectivity acrossbuildings (usuallyvery high speed

connections via FiberOptics, GigEthernet,

etc.

Corporate Network example whereconnectivity from the Access Layer

(User level) is fully redundant. There isalways a path between PC clients andservers on either building, unless theentire building A or B is affected by a

major catastrophe.

Large bandwidth uplinksbetween user switchesand the network. These

"pipes" will allow traffic toflow without saturation and

overutilization

Store and Forward Mechanism

• When user A sends a message to user B, this information is digitized and segmented. These segments are divided into packets and sent across the network.

• Each intermediate node between users will take in, process the packets, and pass them along to the next node without having to wait for the entire message to arrive.

• When user “A” sends packet 1 to intermediate node 1 and it is received, packet 2 is already leaving user “A”. At the same time, packet 1 is leaving intermediate node 1 to intermediate node 2 while it receives packet 2 from user “A”.

HeaderUserData

Trailer

- A typical packetHeader - usually contains source and

destination address as well as sequencenumbers to identify and verify that all packets

are received by a given destinationUser Data - actual information from sender to

receiverTrailer - contains control information specific

for detection and correction of errors

Why Packetized data vs Streaming 1’s and 0’s

• Store and Forward process is much faster than streaming. This means user A sends the entire message to intermediate node 1, where node 1 waits for the whole message to arrive.

• Streaming– When data is not grouped into packets but instead is sent as a stream

of 1’s and 0’s, it will only forward to intermediate node 2 once the entire message has arrived. If the string of 1’s and 0’s takes one minute to go between two directly connected nodes and each node has to wait for all the 1’s and 0’s of the stream to arrived, it will take the number of nodes in minutes plus one minute to transfer the message from user A to user B.

• Packetized– Instead, when the message is broken into equal size packets (i.e. 60

packets) and each packet takes one second to get between two nodes, it will take only one minute plus the number of intermediate nodes in seconds for the message to reach its destination.

Internet Topology

• This system is made up of millions of computers that interconnect and use common networks to transport everyone’s data. These common networks are made up of interconnections between Internet Service Providers’ (ISPs) high-speed routers and switches, which create what it is called the Internet backbone.

• These “backbones” allow for the transfer of data at very high speeds and provide connections to other smaller networks, which then provide connectivity to end-users. On huge communication systems like the Internet, there are also centralized points called Network Access Points (Naps), strategically located around the world to allow for the exchange of Internet traffic specific to the region.

• In the United States, there are multiple Naps that interconnect multiple ISPs, which in turned, connect to other networks.

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ISPs connect to Network Access Points(NAPs) all over the world. There are

very high speed switches thatinterconnect these routers. These NAPsthen interconnect with each other usingvery high speed backbones that allowfor vast amount of data to be transfer.

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