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Basic Communications Systems Class 4. Today’s Class Topics. Local Area Networks What is a LAN? LAN Topologies Ethernet LANs Token Ring and FDDI LANs LAN Interconnection Bridges Switches Routers. Data Communications and Computer Networks Chapter 7. Introduction
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Today’s Class Topics • Local Area Networks • What is a LAN? • LAN Topologies • Ethernet LANs • Token Ring and FDDI LANs • LAN Interconnection • Bridges • Switches • Routers
Data Communications and Computer Networks Chapter 7 Introduction A local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates. Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.
What is a LAN? • Key Elements: • High Communications Speed • Very Low Error Rate • Limited Geographic Boundaries • Simple Cabling System • Provides resource sharing (files, printers, disks, applications, etc.)
LAN Components • Workstations (PCs, etc.) • Network Interface Card (NIC) or Network Adapter Card provides LAN interface. • Interconnecting Cable • Typically twisted copper wire or optical fiber. • Network Operating System (NOS) Software
Data Communications and Computer Networks Chapter 7 Advantages of Local Area Networks Ability to share hardware and software resources. Individual workstation might survive network failure. Component and system evolution are possible. Support for heterogeneous forms of hardware and software. Access to other LANs and WANs (Figure 7-1). Private ownership. Secure transfers at high speeds with low error rates.
Data Communications and Computer Networks Chapter 7 Basic Network Topologies Local area networks are interconnected using one of four basic configurations: 1. Bus/tree 2. Star-wired bus 3. Dual Ring 4. Star-wired ring 5. Wireless
Data Communications and Computer Networks Chapter 7 Bus/Tree Topology 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. Because of this, special wiring considerations are necessary. Buses can be split and joined, creating trees, but this is not commonly done.
Bus Topology • Used in early LANs (1970s, 1980s) • All workstations and servers connected to same physical cable. • Each transmission was broadcast to all other devices on LAN. • Problems: • One cable problem downs the network • Hard to locate problems when they occur
Data Communications and Computer Networks Chapter 7 Star-wired Bus Topology Logically operates as a bus, but physically looks like a star. Star design is based on hub. All workstations attach to hub. Hub takes incoming signal and immediately broadcasts it out all connected links. Hubs can be interconnected to extend size of network. This is THE most common LAN topology used today, in the form of 10BASE-T Ethernet.
Star Topology • Advantages: • Hub can isolate any network problems that occur • Intelligent (managed) hub can collect network performance statistics
Ring Topology • Devices connect sequentially in a ring. • Each device transmits to the next. • Problems: • One cable problem downs the network • One workstation problem downs the network • Used today only with dual-ring systems that can survive cable cuts
Dual-Attached Workstations in Normal Operation Dual-attached counter rotating rings Dual Ring Example:Fiber Distributed Data Interface (FDDI) Self healed after Link Failure Self-healing FDDI dual- link failure attached adapters GOLDMAN: LAN
Data Communications and Computer Networks Chapter 7 Star-wired Ring Topology Logically operates as a ring but physically appears as a star. Star topology is based on MAU (multi-station access unit) which functions similarly to a hub. Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion. Like hubs, MAUs can be interconnected to increase network size.
Data Communications and Computer Networks Chapter 7 Wireless Topology Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point. Newer IEEE 802.11 and 802.11b standard defines various forms of wireless LAN connections. Speeds up to 11 Mbps with 802.11b standard. Workstations reside within a basic service set, while multiple basic service sets create an extended service set.
Data Communications and Computer Networks Chapter 7 Wireless Topology Two basic components necessary: the client radio, usually a PC card with an integrated antenna, and the access point (AP), which is an Ethernet port plus a transceiver. The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions. Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.
Data Communications and Computer Networks Chapter 7 Wireless Topology With directional antennae designed for point-to-point transmission, 802.11b can work for more than 10 miles. With an omnidirectional antenna on a typical AP, range may drop to as little as 100 feet. Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.
Data Communications and Computer Networks Chapter 7 Wireless Topology In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps (from a July 2000 test by Network Computing). To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. Management of roaming clients not defined well by 802.11b. What will Bluetooth’s impact be on 802.11b?
Data Communications and Computer Networks Chapter 7 Medium Access Control Protocols How does a workstation get its data onto the LAN medium? A medium access control protocol is the software that allows workstations to “take turns” at transmitting data. Three basic categories: 1. Contention-based protocols 2. Round robin protocols 3. Reservation protocols
Data Communications and Computer Networks Chapter 7 Contention-Based Protocols Most common example is carrier sense multiple access with collision detection (CSMA/CD), used in Ethernet If no one is transmitting, a workstation can transmit. If someone else is transmitting, the workstation “backs off” and waits.
Data Communications and Computer Networks Chapter 7 Contention-Based Protocols If two workstations transmit at the same time, a collision occurs. When the two workstations hear the collision, they stop transmitting immediately. Each workstation backs off a random amount of time and tries again. Hopefully, both workstations do not try again at the exact same time. CSMA/CD is an example of a non-deterministic protocol.
Data Communications and Computer Networks Chapter 7 Round Robin Protocols Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation. Most common example is token ring LAN in which a software token is passed from workstation to workstation. Token ring is an example of a deterministic protocol. Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated? Hogged? Token ring LANs are losing the battle with CSMA/CD LANs.
Data Communications and Computer Networks Chapter 7 Reservation Protocols Workstation places a reservation with central server. Workstation cannot transmit until reservation comes up. Under light loads, this acts similar to CSMA/CD. Under heavy loads, this acts similar to token ring. Powerful access method but again losing out to CSMA/CD. Most common example of reservation protocol is demand priority protocol.
Data Communications and Computer Networks Chapter 7 Medium Access Control Sublayer To better support local area networks, the data link layer of the OSI model was broken into two sublayers: 1. Logical link control sublayer 2. Medium access control sublayer Medium access control sublayer defines the frame layout and is more closely tied to a specific medium at the physical layer. Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT.
Data Communications and Computer Networks Chapter 7 IEEE 802 Frame Formats The IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5). Each frame format describes how the data package is formed. Note how the two frames are different. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.
Data Communications and Computer Networks Chapter 7 Local Area Network Systems Ethernet or CSMA/CD Most common form of LAN today. Star-wired bus is most common topology but bus topology also available. Ethernet comes in many forms depending upon medium used and transmission speed and technology.
Data Communications and Computer Networks Chapter 7 Ethernet Originally, CSMA/CD was 10 Mbps. Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps. Then 1000 Mbps (1 Gbps) was introduced. 10 Gbps is in the process of being standardized.
Data Communications and Computer Networks Chapter 7 Ethernet Enhancements Transmission can be full duplex (separate transmit and receive), and collisions can be eliminated if Ethernet switches are used rather than standard Ethernet hubs. (Switches explained further later) Data prioritization is possible using 802.1p protocol.
Data Communications and Computer Networks Chapter 7 Local Area Network Systems IBM Token Ring Deterministic LAN offered at speeds of 4, 16 and 100 Mbps. Very good throughput under heavy loads. More expensive components than CSMA/CD. Losing ground quickly to CSMA/CD. May be extinct soon.
Data Communications and Computer Networks Chapter 7 Local Area Network Systems FDDI (Fiber Distributed Data Interface) Based on the token ring design using 100 Mbps fiber connections. Allows for two concentric rings - inner ring can support data travel in opposite direction or work as backup. Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.
Data Communications and Computer Networks Chapter 7 Local Area Network Systems 100VG-AnyLAN Deterministic LAN based on demand priority access method. Similar to hub topology (star design). Two levels of priority - normal and high. Supports a wide-variety of media types. Losing ground quickly to CSMA/CD. Will be extinct soon.
