Monday, March 17, 2014

Network Cabling

This section may be skipped by those more interested on the software aspects of networking or those
learning networking, but all readers should at some time be aware of the terminology used in this section
since they are used with regard to cabling. If this section is skipped by those learning networking, it
should be read later. This section should be read by those who plan to physically install their own

Types of Transmission

1. Baseband - Data bits are defined by discrete signal changes.
2. Broadband - Uses analog signals to divide the cable into several channels with each channel at its
    own frequency. Each channel can only transmit one direction.

Physical media

1. Twisted pair - Wire is twisted to minimize crosstalk interference. It may be shielded or

  • UTP-Unshielded Twisted Pair. Normally UTP contains 8 wires or 4 pair. 100 metermaximum length. 4-100 Mbps speed.
  • STP-Shielded twisted pair. 100 meter maximum length. 16-155 Mbps speed. Lowerelectrical interference than UTP.

2. Coaxial - Two conductors separated by insulation such as TV 75 ohm cable. Maximum length of 185 to       500 meters.

1. Thinnet - Thinnet uses a British Naval Connector (BNC) on each end. Thinnet is part of the RG-58 family of cable*. Maximum cable length is 185 meters. Transmission speed is 10Mbps. Thinnet cable should       have 50 ohms impedance and its terminator has 50 ohms impedance. A T or barrel connector has no impedance.

2. Thicknet - Half inch rigid cable. Maximum cable length is 500 meters. Transmission speed is 10Mbps. Expensive and is not commonly used. (RG-11 or RG-8). A vampire tap or piercing tap is used with a transceiver attached to connect computers to the cable. 100 connections may be made. The computer has an attachment unit interface (AUI) on its network card which is a 15 pin DB-15 connector. The computer is connected to the transceiver at the cable from its AUI on its network card using a drop cable.

Coax cable types:

  • RG-58 /U - 50 ohm, with a solid copper wire core.
  • RG-58 A/U* - 50 ohm, with a stranded wire core.
  • RG-58 C/U* - Military version of RG-58 A/U.
  • RG-59 - 75 ohm, for broadband transmission such as cable TV.
  • RG-62 - 93 ohm, primarily used for ArcNet.
  • RG-6 - Used for satellite cable (if you want to run a cable to a satellite!).
  • Only these are part of the IEEE specification for ethernet networks.

3. Fiber-optic - Data is transmitted using light rather than electrons. Usually there are two fibers, one for each direction. Cable length of 2 Kilometers. Speed from 100Mbps to 2Gbps. This is the most expensive and most difficult to install, but is not subject to interference. Two types of cables are:

1. Single mode cables for use with lasers.
2. Multimode cables for use with Light Emitting Diode (LED) drivers.

Cable Standards

The Electronic Industries Association and Telecommunications Industries Association (EIA/TIA) defined a standard called EIA/TIA 568 which is a commercial building wiring standard for UTP cable. It defines transmission speed and twists per foot.

Category Speed                                  Notes
1      None                               Used for old telephone systems
2      4Mps
3      10Mps                            The minimum category for data networks
4      16Mps
5      100Mps                          Cat 5 network cable, used by most networks today
6                                             Data patch,Two pair with foil and braided shield
7                                             Undefined
8                                             Flat cable for under carpets with two twisted pair
9                                             Plenum cable with two twisted pair. It is safe if you're having a fire.

The maximum transmission length is 100 meters. This cable is susceptible to interference.


Shielded twisted pair has a maximum cable length of 100 meters (328 feet). Data rate from 16 to 155 Mbps. Cables require special connectors for grounding but this cabling method resists electrical interference and is less susceptible to eavesdropping. Costs more than UTP or Thinnet, but not as much as Thicknet or Fiber-optic.


  • Attenuation - Signal loss due to impedance.
  • Bandwidth - Indicates the amount of data that can be sent in a time period. Measured in Mbpswhich is one million bits per second.
  • Impedance - The amount of resistance to the transmission device.
  • Interference - Electromagnetic Interference (EMI). Crosstalk - When wires pick upelectromagnetic signals from nearby wires also carrying signals.
  • Plenum - Space above a false ceiling in an office area where heat ducts and cables may be run.Plenum cabling is special fire resistant cabling required for use in these areas due to fire hazards.
  • Shielding - Used to minimize interference.

Thursday, March 6, 2014

Internet Control Message Protocol

Internet Control Message Protocol (ICMP) defined by RFC 792 and RFC 1122 is used for network error reporting and generating messages that require attention. The errors reported by ICMP are generally related to datagram processing. ICMP only reports errors involving fragment 0 of any fragmented datagrams. The IP, UDP or TCP layer will usually take action based on ICMP messages. ICMP generally belongs to the IP layer of TCP/IP but relies on IP for support at the network layer. ICMP messages are encapsulated inside IP datagrams.
ICMP will report the following network information:
  •  Timeouts
  •  Network congestion
  •  Network errors such as an unreachable host or network.
The ping command is also supported by ICMP, and this can be used to debug network problems.

ICMP Messages:

The ICMP message consists of an 8 bit type, an 8 bit code, an 8 bit checksum, and contents which vary depending on code and type. The below table is a list of ICMP messages showing the type and code of the messages and their meanings.
Type Codes Description                                                                            Purpose
0 0 Echo reply                                                                                            Query
3 0 Network Unreachable                                                                           Error
3 1 Host Unreachable                                                                                 Error
3 2 Protocol Unreachable                                                                           Error
3 3 Protocol Unreachable                                                                           Error
3 4 Fragmentation needed with don't fragment bit set                               Error
3 5 Source route failed                                                                               Error
3 6 Destination network unknown                                                             Error
3 7 Destination host unknown                                                                    Error
3 8 Source host isolated                                                                              Error
3 9 Destination network administratively prohibited                                 Error
3 10 Destination host administratively prohibited                                      Error
3 11 Network Unreachable for TOS                                                           Error
3 12 Host Unreachable for TOS                                                                 Error
3 13 Communication administratively prohibited by filtering                   Error
3 14 Host precedence violation                                                                  Error
3 15 Precedence cutoff in effect                                                                 Error
4 0   Source quench                                                                                    Error
5 0   Redirect for network                                                                           Error
5 1   Redirect for host                                                                                 Error
5 2   Redirect for type of service and network                                           Error
5 3   Redirect for type of service and host                                                 Error
8 0   Echo request                                                                                      Query
9 0   Normal router advertisement                                                            Query
9 16 Router does not route common traffic                                              Query
10 0 Router Solicitation                                                                            Query
11 0 Time to live is zero during transit                                                      Error
11 1 Time to live is zero during reassembly                                              Error
12 0 IP header bad                                                                                     Error
12 1 Required option missing                                                                    Error
12 2 Bad length                                                                                          Error
13 0 Timestamp request                                                                            Query
14 0 Timestamp reply                                                                               Query
15 0 Information request                                                                          Query
16 0 Information reply                                                                              Query
17 0 Address mask request                                                                       Query
18 0 Address mask request                                                                       Query

ICMP is used for many different functions, the most important of which is error reporting. Some of these are "port unreachable", "host unreachable", "network unreachable", "destination network unknown", and "destination host unknown". Some not related to errors are:
  •  Timestamp request and reply allows one system to ask another one for the current time.
  •  Address mask and reply is used by a diskless workstation to get its subnet mask at boot time.
  •  Echo request and echo reply is used by the ping program to test to see if another unit will respond. 

User Datagram Protocol

User Datagram Protocol (UDP) supports the network at the transport layer. User Datagram Protocol (UDP) is an unreliable connection-less protocol and is defined by RFC 768 and 1122. It is a datagram service. There is no guarantee that the data will reach its destination. UDP is meant to provide serivce
with very little transmission overhead. It adds very little to IP datapackets except for some error checking and port direction (Remember, UDP encapsulates IP packets). The following protocols or services use UDP:
  •  DNS
  •  SNMP
  •  BOOTP
  •  TFTP
  •  NFS
  •  RPC
  •  RIP 

UDP Message Format 

The UDP header includes:
1. Source port number (16 bits) - An optional field
2. Destination port number (16 bits)
3. UDP length (16 bits)
4. UDP checksum (16 bits)
This is followed by data. The UDP checksum includes UDP data, not just the header as with IP message formats. For UDP and TCP checksum calculation a 12 byte pseudo header is included which contains some fields form the IP message header. This header is not transmitted as part of UDP or TCP, but is only used to help compute the checksum as a means of being sure that the data has arrived at the correct IP address. This is the TCP/UDP pseudo header:
1. Source IP address (32 bits)
2. Destination IP address (32 bits)
3. blank filler(0) (8 bits)
4. Protocol (8 bits)
5. UDP length (16 bits)