IEEE 802.3 and IEEE 802.11

To increase compatibility, international standards for Local and Metropolitan Area Networks were released by the IEEE Computer Society. These are updated on an ongoing basis.

IEEE 802.3 ~ CSMA/CD
IEEE 802.3 is the IEEE international standards for Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical layer specifications. IEEE 802.3 is the standard of the Ethernet network.

This standard is one part of a family of standards governing local and metropolitan area networks. IEEE 802.3 is part 3 and provides conformance test information to meet requirements for implementations of 10BASE-T. It was originally based on the Ethernet standard version 1.0 specification. The first draft of this standard was released in 1983, with the first official release following in 1985. Since 1985 numerous supplements have been released that have either added to, redefined or retired sections within the standard as technology has moved forward.

Under IEEE 802.3 there are currently 4 data rates defined governing operation of twisted-pair and optical fibre cables.

• 10 Mbps - 10Base-T Ethernet (IEEE 802.3)
• 100 Mbps - Fast Ethernet (IEEE 802.3u)
• 1000 Mbps - Gigabit Ethernet (IEEE 802.3z)
• 10-Gigabit - 10 Gbps Ethernet (IEEE 802.3ae)

The IEEE 802.3 standard defines requirements of layer 1, the Physical layer, and layer 2, the Data Link layer of the OSI.

IEEE 802.11 ~ Wireless LANs
IEEE 802.11 is the IEEE international standards for Wireless LANs, Medium Access Control (MAC) and Physical Layer specifications. IEEE 802.11 is the standard of Wi-Fi.

It denotes a set of standards that incorporate all Wireless LAN/WLANs and was first implemented in 1997. Similar to IEEE 802.3, it specifies the requirements of CSMA/CD though for data communications via the “air”, radio or infrared. Medium Access Control supports access point control and connection between independent stations. Included in the protocol is authentication, association and re-association services. There is also optional procedures for encryption and decryption as well as power management reducing power consumption in mobile devices.

IEEE 802.11 defines requirements of layer 1, the Physical layer, and layer 2, the Data Link layer of the OSI.

The Client Network

A Client-Server network connects a group of devices to a DHCP server, forming a LAN. The DHCP server controls the network and resolves all addressing requirements. It allocates a range of IP Addresses, so that each device connected to the network has a unique IP Address for that network.


Alongside a DHCP server, there may be another server providing internet connection sharing (ICS) functionality allowing each of the LAN devices to connect to the wide area network (WAN). Depending on requirements of the network, these two different servers may co-exist on the same hardware server.

A big advantage of the client-server network is the fact that the configuration of the network is centralised.

A limitation of the client-server network is the availability of servers that control the network. If for some reason a DHCP server is taken off-line, then the network it controls is effectively also out of action.

The SOHO Network

A single office or home office (SOHO) network uses peer-to-peer technology to create a local area network (LAN), these are often referred to as a WORKGROUP by Microsoft.

In a SOHO network each device (a device being a computer, printer or other device) takes an even role in controlling the network with no device taking a lead role. There are generally no more than 10 devices in a SOHO network.

Building a SOHO network using peer-to-peer technology is very quick an easy, and for a small network can be both low cost and low maintenance.

A Limitation of the SOHO network is that each device has to broadcast it existence across the network, allowing 'others' to see them. This creates large amounts of traffic on the network and thus limits the size. The amount of network traffic produced in this way can be reduced by using a WINS service. The WINS service basically acts as an address book containing each of the devices on the network, and so reducing the need to broadcast.

What is meant by data encapsulation?

Data encapsulation is the process of adding a header to wrap data as it flows down the OSI model, this is done to add additional information required to process the data as it moves from source to destination.

Encapsulation begins at layer 4 of the OSI as the Transport layer converts the data into segments. Here the header contains port numbers of source and destination.

When the segments are converted into packets, at the network layer, another header is added containing the IP addresses of both source and destination.

The data link layer adds another header containing the source and destination MAC addresses. It also adds a trailer which contains a frame check sequence that is used to verify the data on receipt.

Why is a data stream broken into data packets for transmission over a network?

Dividing a data stream into smaller data packets avoids transmissions consuming large amounts of bandwidth for long periods of time. The smaller packets means the bandwidth can be shared and the length of time each opportunity for a device to transmit data is reduced.

In addition to this, transmitting data in packets reduces the effect of data corruption or data loss between devices as each individual packet can be retransmitted instead of having to send the entire data stream.

RING / FDDI / STAR topologies

RING topology

The RING network consists of a closed loop of networked devices. Each device is directly connected to another device in the loop. The RING network can cover greater distances than some other types of networks (e.g. BUS networks) due to each host’s ability to regenerate the message as it passes through it and the fact that there is no central hub requiring each device to connect to.


The most common implementation of the RING topology is the Token Ring network. These networks use a 'Token' that is passed along the RING to prevent data collision. Data can only be sent by the computer (host) with the Token, and the Token can only be held for a set period of time before being passed on to the next computer, if when the Token is received no data is required to be sent, then the Token is passed on immediately.

Original RING networks were implemented using coaxial cable limiting the maximum bandwidth achievable by the network to either 4Mbps or 16Mbps.

RING networks can be expensive to implement and costly to run – any work on the structure of the ring, or fault resulting in an effective break in the ring, results in a period of downtime (all ends of the RING network must be terminated in order for the RING to function). Also, if there is a break in any of the cables, the whole network is down. On the upside, they have the potential to deliver fantastic bandwidth over large distances.


FDDI topology

The Fiber Distributed Data Interface (FDDI) network builds on the basis of the single RING network, but consists of two RING networks running side by side. This architecture is know as 'dual-ring', and the data on each RING flows in an opposite direction (counter-rotating).


The two rings are known as ‘Primary’ and ‘Secondary’. The Secondary ring is generally used as a back-up RING, with the Primary delivering up to 100Mbps capacity. In some circumstances where the Secondary ring is not being used for backup, the capacity can be doubled and therefore effectively deliver 200Mbps of bandwidth.

An advantage of FDDI over a standard RING network is the fact that FDDI uses fibre optics, and so is capable of far higher bandwidth and further distances between hosts (and so overall distance).


Also, by using two rings instead of one, FDDI allows for an element of redundancy that a RING network cannot. If a computer is taken out of the network, the secondary ring forms a loop using the remaining computers and thus maintains the network between the remaining computers.


STAR topology

The STAR network is built using a central 'connection' known as the hub, off which all other network devices are connected. This 'hub' can be a hub, switch or router, and depending on the device, ranges from a simple data link to actively processing which messages are sent to which machine.

The basic hub design consists of messages being sent from a single computer to the hub, and the hub then forward the message onto all other computers connected to the hub.


STAR networks have a typical bandwidth of 100Mbps, though this does vary, and use Ethernet cabling (CAT 5).

Data collision can be high on a STAR network, and this increases as the network grows. Whenever there is a data collision, the whole network is 'frozen' for an instance and then effectively 'reset'. As data collision increases as more and more devices are added to the network, this can have a major impact on performance.

Devices such as switches and routers reduce data collision, as these actively manage data direction, effectively reducing the traffic on the network and therefore reducing the possibility of data collision.


Comparison of STAR and RING topologies

Below is a table showing comparison of RING and STAR networks:

	RING	STAR
Reliability	The RING network is limited by design, as the whole network is down whenever there is a fault in the line	Good, the network remains up, even if there is a failure taking out a computer.
Cost	RING networks are reasonably easy to implement, as there is no central hub.	A central hub is require and cabling needs to be routed from all attached devices to this hub.
Speed	The RING network can offer high speeds on reasonable size networks as data only goes in one direction. Though there on larger networks speed becomes an issue do to data having to pass through every computer between the sender and receiver.	Can offer excellent speed, even on larger networks if combined will switches and routers. Data is only sent from host to receiver, with few other devices involved.
Maintainability	Network has to be down to add or remove a computer from the network. Though only cabling is required between to a computer on the network to the new computer	Very good as new computers can be added to the network without any down time. Though if new cabling is required this can prove to be an issue, as it has to be routed to the nearest hub.
Expandability	There is a limit to the number of computers that can be included on a RING network before there is a drop in performance.	There is good potential for expansion on a STAR network, as with the use of switches and routers, additional network activity is minimal.
Data Collision	Data Collision is not an issue on a RING network, as only the computer that currently holds the token can transmit data.	Can become an issue when there is a large amount of network activity either on large networks or those that the functionality of switches and routers.

The CSMA/CD Protocol

Each PC connected to an Ethernet network connects using a Network Interface Card (NIC), which controls data connectivity between the host PC and the network. Note: Each NIC has a unique identifier called the MAC address (which can be seen as the ‘Physical Address’ when using the ‘IPCONFIG /ALL command in MSDOS).

The Carrier Sense Multiple Access / Collision Detection protocol (CSMA/CD) means that all NICs connected to the network must listen and wait until there is no traffic on the network.

At this point each NIC has an equal chance to transmit data across the network. If any of the other NICs are transmitting data across the network, there will be a signal called a 'carrier' on the network that informs other NICs that the network is busy. This is defined as 'Carrier Sense'.

When there is no traffic on the network, each NICs has an equal chance of transmitting data, and this is defined as 'Multiple Access'.

As data can taken a defined period of time to travel across the network, and isn't instant (though to us it may appear to be so), there is a chance that two NICs may transmit data at the same time.

When this is detected by the nearest NIC at the point where the two transmissions meet, a signal is sent across the network that causes all transmissions to cease for a period of time before transmissions resume.

This is defined as 'Collision Detection', and when this happens each NIC resets an internal random delay that avoids the same collision occurring when transmissions can resume.

The CSMA/CD protocol by design allows an equal chance for each NIC to transmit across the network allowing the network to be shared by several NIC connected devices.