Local-Area Networks
make use of a shared transmission medium and packet broadcasting. Owned by an organization, used to interconnect equipment. Much greater capacity than wide-area networks to carry what is generally a greater internal communications load.
14.1 Background
I. Personal Computer Local Area Networks
Supports personal computers, but central processing facilities are still required. Used for client/server applications. Key requirement is cost - the cost of attachment to the network must be significantly less than the cost of the attached device.
B. Backend Networks and Storage Area Networks
used to interconnect large systems such as mainframes, supercomputers, and mass storage devices. Found at sites of large companies or research installations with large data processing budgets. A small difference in productivity can mean millions of dollars.
Characteristics:
-high data rate
-high-speed interface
-distributed access
-limited distance
-limited number of devices
Key requirements - high data rates, storage area network - a separate network to handle storage needs. No server sits between the storage devices and the network. Instead, the storage devices and servers are linked directly to the network.
C. High-Speed Office Networks
Fax machines, document image processors, and graphics programs on pcs and workstations require higher speeds. Require LANs with high speed that can support the larger numbers and greater geographic extent of office systems.
D. Backbone Local Networks
lower cost, lower capacity LANs within buildings or departments are interconnect with these networks with a higher-capacity network.
E. Factory LANs
Tie automated systems together. Interconnect devices and provides mechanisms for their cooperation. Key characteristics include high capacity, ability to handle a variety of data traffic, large geographic extent, high reliability, and ability to specify and control transmission delays.

14. 2 LAN Configuration
A. Tiered Local-Area Networks
A single local network cannot serve all equipment efficiently.
Alternative approach - employ two or three tiers of local networks.
A low-cost, moderate speed LAN supports a cluster of personal computers and workstations, lashed together with a backbone LAN of higher capacity.
Evolution Scenario - how a networking implementation comes about in an organization.
Scenario #1: LAN decisions are made from the bottom up, with each department making decisions more or less in isolation. Each department develops its own cluster networks. Over time, departments realize they need to interconnect. LAN backbone (tier 2) is used to provide interconnect capability. Advantage: local interconnect strategies can be responsive to the specific applications of the department. Disadv: suboptimization of resources, duplication, incompatible networks result.
Scenario #2: top-down design of a LAN strategy. Decision is centralized, compatibility results. Problem - not responsive and timely about meeting departmental needs.

14.3 Topologies and Transmission Media
Key elements of a LAN:
-topology (bus, ring or star)
-transmission medium (twisted pair, coaxial cable, optical fiber)
-layout (linear or star)
-medium access control (CSMA/CD or token passing)
I. Topologies
the way in which the end points or stations attached to the network are interconnected.
A. Bus and Tree Topologies
LAN topology in which stations are attached to a shared transmission medium.
Bus and tree use a multipoint medium.
For bus, all stations attach through appropriate hardware interfacing known as a tap to a linear transmission medium or bus.
Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus.
Tree topology - stations are attached to a shared, branching transmission medium. Generalization of the bus topology. Transmission medium is a branching cable with no closed loops. Tree layout begins at a point called the headend. A transmission from any station propagates throughout the medium and can be received by all other stations.
Problems: 1) a transmission from any one station can be received by all other stations 2) a mechanisms is needed to regulate transmission.
Solution: stations transmit data in small blocks called frames, which consists of a portion of the data the station wishes to transmit plus a frame header that contains control info.
B. Ring Topology
network consists of a set of repeaters joined by a point-to-point links in a closed loop. Links are unidirectional; data are transmitted in one direction only.
C. Star Topology
each station is connected directly to a common central node. Each station attaches to a central node, called a star coupler.
D. Choice of Topology
For moderate data-rate requirements, bus/tree is most flexible.
Ring is appropriate for very high speed links over considerable distances.
Star - best for short distances and can support a small number of devices at very high data rates.
Twisted pair, coaxial cable, and optical fiber all used.
A. Baseband Bus
uses digital signaling, transmits signals without modulation. Can extend only a limited distance - l km. Repeaters can extend length of the network.
B. Broadband Bus
data transfer by means of analog (radio-frequency) signals. Frequency-division multiplexing can be used. Separate channels can support separate and independent data traffic, etc.
Unidirectional signaling. Uses two paths, joined at the headend.
C. Choice of Transmission Medium
Determined by:
-capacity
-reliability
-types of data support
-environmental scope
Generalizations:
-Voice-grade unshielded twisted pair is inexpensive, well-understood.
-shielded twisted pair and baseband coaxial cable are more expensive but provide greater capacity.
-broadband cable is even more costly but provides even greater capacity.
In recent years, trend has been toward use of high-performance untwisted pair, especially Category 5 UTP, which supports high data rates for a small number of devices.
Optical fiber is still expensive and requires skilled personnel to install and maintain it.
III. Relationship between Medium and Topology
Ring topology - requires point-to-point links between repeaters, twisted-pair wire, baseband coaxial cable, and optical fiber are all appropriate.
Bus topology - twisted pair, baseband and broadband coaxial cable are appropriate, many products are good for it. Optical fiber until recently was not considered feasible, multipoint configuration was not cost-effective.
Tree topology - can be employed with broadband coaxial cable. Unidirectional nature of broadband signaling allows the construction of a tree architecture.
Star topology requires a point-to-point link between each device and the central node.
IV. Structured Cabling
A. Cabling Standards
Standards called structured cabling systems have been issued that specify cabling types and layout for commercial buildings.
Standards provide guidance for preinstallation of cable in new buildings so that future voice and data networking needs can be met without the need to rewire the building.
Based on the use of a hierarchical, star-wired cable layout.
B. Wiring Closets vs. Hubs
Two general strategies for laying out LAN transmission medium - linear and star.
Linear strategy attempts to provide the desired topology with the minimum cable, subject to the physical constraints of the building.
Star layout uses an individual cable from a concentration point to each subscriber location. Obvious choice for a star topology LAN, but can also be used for the bus and ring LAN topologies. Wiring closet is the concentration point.

14.4 LAN Standards
Key to LAN market is availability of a low-cost interface.
IEEE 802 committee - has developed standards for the development of local area networks. Needed is a LAN standard that assures volume and also enables equipment from a variety of manufacturers to intercommunicate.
A. Structure of the LAN Standards
The task of communication across LANs needs to be broken up into more manageable subtasks, and no single technical approach will satisfy all requirements.
Standards are organized in a 3-layer protocol hierarchy.
- logical link, medium access control, and physical layers.
B. Logical Link Control (IEEE 802.2)
specifies the mechanisms for addressing stations across the medium and for controlling the exchange of data between two users. A common link protocol for all the LANs.
C. Medium Access Control
determines which station on a local area network has access to the transmission medium at any time.
I. Medium Access Control
Determines which station has access to the transmission medium at any time.
Uses carrier-sense multiple access with collision detection (CSMA/CD) - a medium access control technique in which a station first senses the medium and transmits only if the medium is idle. Station ceases transmission if it detects a collision.

14.5 Bridges
an internetworking device that connects two similar local area networks that use the same LAN protocols. Bridges improve:
a. Reliability (network can be partitioned into self-contained units.
b. Performance - enables clustering of devices so that intranetwork traffic significantly exceeds internetwork traffic.
c. Security - different types of traffic can be kept on physically separate media which are connected by bridges.
d. Geography - can be used to connect physically distant devices.

14.6 Layer 2 and Layer 3 Switches
Hubs - acts as a repeater. Repeats signal on the outgoing line to each station. A hub uses a star wiring arrangement to attach stations to the hub. A transmission from any one station is received by the hub and retransmitted on all of the outgoing lines. Only one stations can transmit at a time.
Multiple levels of hubs can be cascaded in a hierarchical configuration.
Layer 2 Switch or Switching hub - the central hub acts as a switch. An incoming frame from a particular station is switched to the appropriate output line to be delivered to the intended destination. Other unused lines can be used for switching other traffic.
Advantages of layer- switches -
a. No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN.
b. Each attached device has a dedicated capacity equal to that of the entire
original LAN.
c. The layer 2 switch scales easily.
Layer 2 switches provide increased performance to meet the needs of high-volume traffic generated by pcs, workstations, and services.
But, limitations of layer 2 switches include:
Flat address space - all users share a common MAC broadcast address. In a large network, frequent transmission of broadcast frames can create a broadcast storm, in which numerous broadcast frames clog the network and crowd out legitimate traffic.
Current standards for bridge protocols dictate that there be no closed loops in the network - only one path between any two devices, making it impossible to provide multiple paths through multiple switches between devices.
To overcome these limitations, vendors have developed layer 3 switches, which implement the packet-forwarding logic of the router in hardware.
Two types of layer 3 switches:
1. pack-by-packet - operates just like a router.
2. A flow-based switch tries to enhance performance by identifying flows of IP packets that have the same source and destination. Once a flow is identified, a predefined route can be established through the network to speed up the forwarding process.