A Local Area Network (LAN) is a communications network that interconnects a variety of data communications devices within a small geographic area and transmits data at high data transfer rates. The strongest advantage of a local area network is its capability of allowing users to connect their computers to the internet and share hardware and software resources. Since appeared in 1970, LAN 's use has become widespread in commercial and academic environments.
PRIMARY FUNCTION OF LOCAL AREA NETWORKS
The majority of users expect a local area network to provide access to hardware and software resources that will allow them to perform one or more of the following:
- Access to the internet
- File serving: a large storage disk drive acts as a central storage repository
- Database and application serving
- Print serving: providing the authorization to access a particular printer, accept and queue print jobs, and providing a suer access to the print queue to perform administrative duties
- email serving
- Process control and monitoring
- Distributed processing
- Manufacturing support
- Academic support
ADVANTAGES AND DISADVANTAGES OF LOCAL AREA NETWORK
Advantages:
- Ability to share hardware and software resources
- Individual workstation might survive network failure
- Component and system evolution are possible
- Support for heterogenous forms of hardware and software
- Access to other LANs and WANs
- Private ownership
- Secure transfer at high speed with low error rates
Disadvantages:
- Equipment and support can be costly
- Level of maintenance continues to grow
- Private ownership
- Some types of hardware may not interoperate
- Just because a LAN can support 2 different kinds of packages does not mean data can interchange easily
- LAN is only as strong as its weakest link
THE FIRST LOCAL AREA NETWORK: THE BUS/TREE
- Bus/tree local are network: simply called bus LAN was the first physical design when LANs became commercially available in the late 1970s that consisted of a simple coaxial cable, or bus, to which all devices attached.
- Connecting to the cable requires a simple device called Tap, a passive device
- Passive device: does not alter the signal and does not require electricity to operate
- Network interface card (NIC) is an electronic device, sometimes in the form of a computer circuit board or part of a larger circuit board that performs the necessary signal conversions and protocol operations that allow the workstation to send and receive data on the network
- Can be used with baseband signal and broadband signal
- Baseband signals are bidirectional and more outward in both directions from the workstation transmitting
- Broadband signals are usually uni-directional and transmit in only one direction, however, special wiring considerations are necessary
- It is also to split and join broadband cables and signals to create configurations called Tree
A MORE MODERN LAN
- Star-wired bus LAN: logically acts as a bus, but physically looks like a star
- Logical design: determines how the data moves around the network from workstation to workstation
- Physical design: refers to the pattern formed by the location of the elements of the network, as it would be drawn on s sheet of paper
- All workstations connect to a central device such as a hub
- Hub: is a relatively non-intelligent device that simply and immediate;y retransmits the data it receives from any workstation out to all other workstations connected to the hub
- Twisted pair cable has become the preferred medium
- Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus
- Hubs can be interconnected with other cables
- Biggest disadvantage: when one station talk, everyone hears it, this is called Shared Network-all devices on the network are sharing the one bandwidth.
- Medium access control protocol: is the software that allows a device to place data onto a sub-based LAN and allows workstations to "take turn" at transmitting data
a) Contention-based protocols, such as carrier sense multiple access with collision detection
b) Round-robin protocols such as token passing
1. Contention-based protocols
- Essentially first-come, first-served: the first station to recognize that no other station is transmitting data and place its data onto the medium is the first station to transmit
- The most popular is Carrier sense multiple access with collision detection (CSMA/CD)
- If no workstation is transmitting, a workstation can transmit
- If another workstation is transmitting, the workstation wanting to transmit will wait and try again to transmit
- If two workstation transmit at the same time, collision occurs
- when two workstation hear collision, they stop transmitting immediately
- Each workstation backs off a random amount of time and tries again - CSMA/CD is a nondeterministic protocol, at which cannot calculate the time at which a workstation will transmit
SWITCHES
- The hub is a simple device that transmits an incoming frame out to all connections
- the Switch: uses addresses and processing power to direct a frame out of a particular port, thus reducing the amount on traffic on the network
- A switch primary function is to direct the data frame only to the addressed receiver
- Switches have eliminated many hubs
- Most switches are transparent- which means they observe the addresses of the frames in transmission on the current network and creates an internal port table to be used for making future forwarding decisions.
- The switches create internal port by using a form of backward learning- they observe each frame that arrives at a port, extracts the source address from the frame, and places that address in the port's routing table
- Workstation that connect to a hub are on shared segment
- Workstations that connect to a switch are on a switched segment
- The backplane of a switch must be fast enough to support multiple data transfers at one time
- In a cut-through architecture, the data frame begins to exit the switch almost as soon as it begins to enter the switch
- In contrast, a store-and-forward device holds the entire frame for a small amount of time while various fields of the frame are examined, a procedure that diminishes the overall network throughput
- Shared segment network: a switch may be connected to a hub, which then connects multiple workstation
- Dedicated segment network: a switch may be directly connected to one or more workstations. Each workstation then has a private or dedicated connection that can increase the bandwidth , which is a very efficient way to isolate heavy users from the network
1. Isolating traffic patterns and providing multiple access
- Whether shared or dedicated segments are involved, the primary goal of a switch is to isolate a particular pattern of traffic from other patterns of traffic or from the remainder of the network
- Switches, because of their backplane, can also allow multiple paths of communications to simultaneously occur
2. Full-duplex switches
- Allow for simultaneous transmission and reception of data to and from a workstation
- This full-duplex connection helps to eliminate collisions
- To support a full-duplex connection
- NIC in the workstation must be capable of supporting, and then configured to support a full duplex connection
- A switch must be configured for a full duplex connection as well
- The cable connecting must also be able to support full duplex connection
3. Virtual LANs
- Virtual LAN (VLAN) – logical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to another
- Even though employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network
- A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN
- Instead of sending technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator
4. Link Aggregation
- Allow you to combine two or more links into one higher-speed link
- An IEEE protocol (802.3ad-2000) which typically runs in most LAN devices can support link aggregation
- Link aggregation can also be used in the event of a link failure
- Can be used to to allow multiple parallel links to a server
5. Spanning Tree algorithm
- The spanning tree algorithm (used in Spanning Tree Protocol and now Rapid Spanning Tree Protocol) runs in switches and can identify loops and remove them
- The way spanning tree algorithm works:
- Identify a switch as the root switch
- Visit each switch and identify the one port that has the shortest path back to the root switch. Mark these ports with RP (root port)
- Visit each LAN and identify the port that provides the shortest path back to the root switch. Mark these ports with a DP (designated port).
- Are there any ports remaining that don’t have either an RP or DP designation? Mark those ports as Removed. (They aren’t physically removed, only removed in the forwarding tables)
6. Quality of service
- On a standard Ethernet LAN, all frames were created equal, or first come first served protocol
- Wireless Ethernet provides a level of priority
- The 802.1p standard adds a 3-bit field to the front of each Ethernet frameThis 3-bit field can be used to establish a priority
WIRED ETHERNET
- Ethernet: was the first commercially available local area network system and remains, the most popular LAN system today
- Primarily on the star-wired bus topology and uses essentially the CSMA/CD medium access protocol
- Comes in many forms depending upon the medium used and transmission speed and technology
- One additional improvement to Ethernet is Power over Ethernet (PoE), can place a NIC in a device, but dont have to connect the device to an electrical source
WIRED ETHERNET FRAME FORMAT
- To better support local area networks, the data link layer of the OSI model was broken into two sublayers:
- Logical link control sublayer
- Medium access control sublayer
- Medium access control sublayer works more closely with the physical layer and contains a header, computer addresses, error detection codes, and control information
- Logical Link Control (LLC) sublayer: is primarily responsible for logical addressing and providing error control and flow control info.
- EEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3)
- The two frames do not have the same layout
- If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another
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