Frames Packets and PDU's

There can be confusion about Frames, Packets and other subsets of network communication.

A frame can be defined as the unit of data transferred across the network, defined at the datalink (network access) layer of the protocol stack.

A packet can be defined as the unit of data at any layer of the protocol stack, prior to, or after transmission.

Application layer is called a message.
Transport layer data is sometimes referred to as TCP or UDP segments. Layer 4
Internet layer, data is sometimes talked of in terms of IP, ARP and RARP datagrams. Layer 3
The datalink layer data is referred to as frames. Layer 2
Physical layer, data is sometimes referred to as bits. Layer 1

Frames simply encapsulate the data with a header and addressing information. In ethernet networks the source and destination adress are defined by the MAC. MAC or Media Access Control address is a hardware address that uniquely identifies each node of a network.

In computer networking, "encapsulation" is to include data from an upper layer protocol into a lower layer protocol.

In telecommunication, encapsulation is the inclusion of one data structure within another structure so that the first data structure is hidden for the time being. For example, a TCP/IP-formatted data packet can be encapsulated within an ATM frame (another kind of transmitted data unit). Within the context of transmitting and receiving the ATM frame, the encapsulated packet is simply a stream of bits beween the ATM data that describes the transfer.

As data descends the source protocol stack, successive levels encapsulate the data with additional information. After a frame has been received at its destination, successive layers then de-encapsulate the data as it travels up the destination protocol stack.

Therefore, a packet can be discussed in terms of its subsets - messages, segments and datagrams.

Some good links to review on this are below:

Encapsulation:
Encapsulation

Frames and Packets:

Ethernet Frames and Packets

Packets inside Frames or Frames inside Packets

Frames, Packets, Datagrams and PDU's

Voice over IP and Convergence

With VoIP (Voice over IP) forecasted to eventually replace POTS (plain old telephone service) in the ensuing years it has spawned the term convergence. Where in the past there was a phone network and a data network, with VoIP it converges to a single network.

This is the first in a series of posts that will deal with implementing VoIP. The link below will familiarize you with the terms of telephony.

Top 25 Voice Terms

The link below take you to an excellent article on converegnce.

Convergence

TCP the Details

As we now know TCP will setup a "full-duplex" communication between two applications. The purpose of this post is to provide information on how that communication works.

John Kristoff has put together this excellent piece on the Web that covers the fundamental concepts behind TCP and how it is used to transport data between two endpoints.
http://condor.depaul.edu/~jkristof/technotes/tcp.html

If you go back to Daryl Banttari primer he will walk through an example of a TCP/IP communcation via packet sniffer.
http://www.ipprimer.com/packets.cfm

The link below provide a technical breakdown of the TCP Header Format:
http://www.freesoft.org/CIE/Course/Section4/8.htm

Understanding TCP/IP

Up to now the postings have covered basic tutorials on Networking. The purpose of this entry is to provide more detailed information on how TCP/IP works.

The link below provide a good overview of the protocol and importatnt sub protocols:
http://www.w3schools.com/tcpip/default.asp

Daryl Banttari has put together a very good TCP/IP primer. He has targeted it toward LAN administrators just moving to TCP/IP, however it should help anyone who wants to know a little (more) about how TCP/IP works.
http://www.ipprimer.com/overview.cfm

If you have reviwed all of the material and tutorials in this Web Log the animated movie below should be an excellent review. This is a great animated movie on TCP/IP and how the communications move through network devices on the internet.
http://www.warriorsofthe.net/movie.html

Monitoring Fully Switched Networks

So often we have confusion regarding what happens with packets on a switched network.

Below are some good definitions:

What is a switch:

http://www.pcwebopedia.com/TERM/s/switch.html

In a fully switched network, switches replace all the hubs of an Ethernet network with a dedicated segment for every node. Definition of a segment:

http://www.pcwebopedia.com/TERM/s/segment.html

Every port on a fully switched network is by definition segmented. A great link that explains this is on HowStuffWorks.com:

http://computer.howstuffworks.com/lan-switch5.htm

To understand the information that is contained in the packets on the network such as the device IP adresses, MAC adresses or the protocols requires taping into and capturing information as the packets move across the wire. In a fully switched network managed switches give you that capability by creating monitoring ports that capture the packets from a segmented port and duplicate them to the device monitoring and capturing the packet information.

The following link has a great discussion on how to do this on a Cisco switch. This is referred to as a Span port.

Cisco Link

However, the price of Tap's can get expensive. But today low end managed switches are available that can be purchased for the purpose of creating a monitoring port.

Information on Taps can be found at:

http://www.operativesoft.com/html/taps.htm

Basic Network Information

HowStuffWorks.com is an excellent source for getting good basic information about Networks. It does a very good job presenting the information in a logical organized fashion. These link provide information about networking technologies that is necessary for deploying network test, security, management and monitoring software. Some of the links I like for the basics are listed below:

Howstuffworks "How Ethernet Works
How LAN Switches Work
How Internet Infrastructure Works
How OSI Works

How Firewalls Work
How Routers Work
How WiFi Works
How Radio Works

After you get the basics from HowStuffWorks the about.com is another great resource and you can get more detailed network information from a singe link. It is a great resource that should be bookmarked.
about.com basic networking concepts

Blog Purpose

The Purpose of this BLOG is to provide content that will facilitate visitors knowledge about computer networks and how to monitor and manage those networks. I have created it as a resource for my Sales Team, but want to make it available to others.