Data and Computer Communications Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011 Chapter 15 – Local Area Network Overview

What is a computer network? most basic version is two computers that are connected by a cable a number of independent computers linked together to share data and peripherals, such as hard disks and printers

Advantages of a computer network Share information (or data) Share hardware and software Centralize administration and support

4 LAN Applications LAN consists of a shared transmission medium and a set of stations (computers, printers, …) —Today, some LANs don’t use shared transmission medium anymore in order to improve the transmission performance LANs usually are owned by the organization that is using the network to interconnect equipment. There are different types of LANs, each with its own protocols. Applications of LAN —Personal computer LANs Share resources (e.g., printers) Share information (e.g., files) “Limited” data rate (10Mbps – 1000Mbps) —Backend networks Interconnecting large systems (mainframes and large storage devices) High data rate —Storage Area Networks A separate network to handle storage needs Hard disks, tape libraries, CD arrays Detaches storage tasks from specific servers

Frame Transmission on Bus LAN

Bus and Tree Bus: stations attach through tap to bus full duplex allows transmission and reception transmission propagates throughout medium heard by all stations terminator at each end Tree: a generalization of bus branching cable with no closed loops tree layout begins at headend and branches out heard by all stations

Frame Transmission Ring LAN

Ring Topology a closed loop of repeaters joined by point-to-point links receive data on one link & retransmit on another – links unidirectional – stations attach to repeaters data transmitted in frames – circulate past all stations – destination recognizes address and copies frame – frame circulates back to source where it is removed Medium Access Control determines when a station can insert frame

Star Topology

each station connects to common central node usually via two point-to-point link, one for transmission and one for reception operate in broadcast fashion only one station can transmit at a time (hub) can act as frame switch central node

11 Star Topology Each station is directly connected to a central node —Usually via two point-to-point links Two types of central node —Simple one: operate in a broadcast fashion Transmission of a frame from one station to the central node is retransmitted on all of the outgoing links Only one station can transmit at a time The central node is referred to as a hub —Complex one: act as frame-switching device An incoming frame is buffered in the central node and then retransmitted on the outgoing link to the destination station Intelligent and powerful More than one stations can transmit at the same time –Buffers are required at the central node to resolve conflict (if more than one frames are destined to the same station at the same time) The central node is referred to as a switch

12 Hubs Central element of a star topology Each station connects to hub by two lines —Transmit and receive —So a link consists of two unshielded twisted pairs (UTP) Hub acts as a repeater —When one station transmits, hub repeats signal to each station —Physically star, logically bus Limited to about 100 m —High data rate and poor transmission qualities of UTP —Optical fiber may be used for about 500 m Transmission from any station received by all other stations —No privacy, security issues —If two stations transmit at the same time, collision

LAN Topologies

Bus LAN Transmission Media cont… early LANs used voice grade cable scaling up for higher data rates not practical twisted pair uses digital signaling original Ethernet baseband coaxial cable

Bus LAN Transmission Media For bus topology, only baseband coaxial cable has achieved widespread use used in cable TV systems analog signals at radio and TV frequencies expensive, hard to install and maintain broadband coaxial cable expensive taps better alternatives available optical fiber

Ring and Star Topologies Ring very high speed links over long distances potential of providing best throughput single link or repeater failure disables network Star uses natural layout of wiring in building best for short distances high data rates for small number of devices

Media Available

Choice of Medium  constrained by LAN topology  capacity to support the expected network traffic  reliability to meet requirements for availability  types of data supported tailored to the application

Choice of Topology – medium – wiring layout – access control factors: reliabilityexpandabilityperformance

20 Choice of Topology Transmission medium —Twisted pair: popularly used by today’s Ethernet —Baseband coaxial cable (digital signaling): was used by the original Ethernet —Broadband coaxial cable (analog signaling): not popular due to the cost —Optical fiber: popularly used by Ethernet —Air: Wireless LAN becomes very popular today Installation and maintenance —For bus and ring topology, installation also means removing some existing links, so it is costly —For ring topology, a failure of one link disable the entire network —For star topology, it can take advantage of the natural layout of wiring in a building, and installation/maintenance of one link does not affect other links Star topology is the most popular one today

Section15.2

LAN Protocol Architecture Open Systems Interconnection (OSI) 1.Logical link control (LLC) 2.Medium access control (MAC) 3.Physical

Layers Physical Layer —Encoding/decoding of signals —Preamble generation/removal for synchronization —Bit transmission/reception —Transmission medium and topology Medium Access Control —Manage access to a shared-access medium —Not found in traditional point-to-point layer 2 data link protocol Logical Link Control —Provide interfaces (called “services”) to higher layers —Perform flow and error control

IEEE 802 Layers Logical Link Control Layer (LLC) – provide interface to higher levels – perform flow and error control Media Access Control – on transmit, assemble data into frame – on reception, disassemble frame, perform address recognition and error detection – govern access to transmission medium – for same LLC, may have several MAC options

LAN Protocols in Context

MAC addresses Sometimes called Ethernet address Uniquely identify each computer, printer, or device in a network Stored in the network interface card (NIC) read- only memory, burned-in address by manufacturer – Interface is where 2 systems meet and interact Used by – Ethernet – wireless networks – Bluetooth

Logical Link Control  transmission of link level PDUs ( Protocol data unit ) between stations  must support multi-access, shared medium  relieved of some details of link access by the MAC layer  addressing involves specifying source and destination LLC users referred to as service access points (SAPs)

LLC Services unacknowledged connectionless service data-gram style service delivery of data is not guaranteed connection-mode service logical connection is set up between two users flow and error control are provided acknowledged connectionless service datagrams are to be acknowledged, but no logical connection is set up

LLC Service Alternatives unacknowledged connectionless service requires minimum logic avoids duplication of mechanisms preferred option in most cases connection-mode service used in simple devices provides flow control and reliability mechanisms acknowledged connectionless service large communication channel needed time critical or emergency control signals

Bridge Function

Bridges  connects similar LANs with identical physical and link layer protocols  minimal processing  reasons for use: reliability performance security geography

Bridge Design Aspects  no modification to frame content or format  no encapsulation  exact bitwise copy of frame  buffering to meet peak demand  contains routing and address intelligence  may connect more than two LANs  bridging is transparent to stations

Connection of Two LANs

35 Why Not One Large LAN? There are several reasons for the use of multiple LANs connected by bridges —Reliability For a single large LAN, a fault on the network may disable communication for all devices. By using bridges, the network can be partitioned into self-contained units —Performance Performance on a LAN declines with an increase in the number of devices or the length of the wire. A number of smaller LANs will often give improved performance if devices can be clustered so that intranetwork traffic exceeds internetwork traffic. —Security Multiple LANs may improve security of communications. —Geography A single LAN is always limited by its diameter. Two separate LANs are needed to support devices clustered in two geographically distant locations. In summary —Bridges provides an extension to the LAN that requires no modification to the communications software in the stations attached to the LANs. —It appears to all stations on the two (or more) LANs that there is a single LAN.

Bridges and LANs with Alternative Routes

Fixed Routing simplest and most common suitable for internets that are stable a fixed route is selected for each pair of LANs usually least hop route  only changed when topology changes  widely used but limited flexibility