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Chapter 7 Local Area Networks: The Basics. Primary Function of a LAN. File serving – large storage disk drive acts as a central storage repository
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Primary Function of a LAN • File serving – large storage disk drive acts as a central storage repository • Print serving – Providing authorization to access a particular printer, accept and queue print jobs, and user access to print queue to perform administrative duties • Video transfers – High speed LANs are capable of supporting video image and live video transfers • Manufacturing support – LANs can support manufacturing and industrial environments • Academic support – In classrooms, labs, and wireless • E-mail support • Interconnection between multiple systems
Advantages of LAN • 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 • Private ownership • Secure transfers at high speeds with low error rates
Disadvantages of LAN • 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 two different kinds of packages does not mean their data can interchange easily • A LAN is only as strong as it weakest link, and there are many links
Basic LAN Topologies • Bus/tree • Star-wired bus • Star-wired ring • Wireless
Bus/Tree Topology • The original topology. • Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap. • Data can be transferred using either baseband digital signals or broadband analog signals. • Baseband signals are bidirectional (broadcast) and move 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.
Baseband Broadband 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 • Unshielded twisted pair usually used to connect workstation to hub • Hub takes incoming signal and immediately broadcasts it out all connected links • Hubs can be interconnected to extend network size • Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus • Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable • Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium
Star-wired Ring Topology • Logically operates as a ring but physically appears as a star • 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
Wireless LANs (I) • Not really a specific topology • Workstation in wireless LAN can be anywhere as long as within transmitting distance to access point • Several versions of IEEE 802.11 standard defines various forms of wireless LAN connections • Two basic components necessary: • Client Radio - usually PC card with integrated antenna installed in a laptop or workstation • Access Point (AP) - Ethernet port plus transceiver • AP acts as bridge between wired and wireless networks • Can perform basic routing functions • Single-cell - Workstations reside within a basic service set • Multiple-cell - Multiple basic service sets create an extended service set • Ad-hoc - Wireless LANs configured without access point
Wireless LANs (II) • IEEE 802.11 – The original wireless standard, transmitting data at 2 Mbps • IEEE 802.11b – The second wireless standard, transmitting data at 11 Mbps • IEEE 802.11a – One of the more recent standards, transmitting data at 54 Mbps using 5 GHz frequency range • IEEE 802.11g – The other recent standard, also transmitting data at 54 Mbps but using the same frequencies as 802.11b (2.4 GHz) • Backwards compatible with 802.11b • IEEE 802.11n (100 Mbps) is last standard that has been widely implemented. • Available at both 2.4 & 5 GHz • Latest wireless Ethernet is using MIMO technology (multiple input multiple output) • Sender and receiver have multiple antennas for optimum reception • IEEE 802.11ac is the latest standard that is gaining momentum • Operates only on 5 GHz band with data rate up to 6.9 Gbps
Medium Access Control Protocols • How does a workstation get its data onto the LAN medium? • Medium access control protocol - software that allows workstations to “take turns” at transmitting data • Two basic categories: • Contention-based protocols • Round robin protocols
Contention-Based Protocols (I) • Essentially first come first served • Most common example: • Carrier sense multiple access with collision detection (CSMA/CD) • If no one is transmitting, a workstation can transmit • If someone else is transmitting, workstation “backs off” and waits • If two workstations transmit at same time, collision occurs • When two workstations hear 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 exact same time • CSMA/CD is an example of a nondeterministic protocol
Contention-Based Protocols (II) • Wireless CSMA/CA (Collision avoidance) • Protocol does not listen and detect collisions • Instead, tries to avoid collisions before they happen • How does CSMA/CA do this? • All devices, before they transmit, must wait an amount of time called an interframe space (IFS) • Some applications have a short IFS, while others have a long IFS • If two applications want to transmit at same time, the application with shorter IFS will go first. If medium is idle after IFS, a random backoff counter is selected and transmission starts after the countdown.
Round Robin Protocols • Each workstation takes turn transmitting: 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
IEEE 802 • To better support local area networks, data link layer of the OSI model was broken into two sublayers: • Logical link control sublayer • Medium access control sublayer • Medium access control sublayer defines the frame layout • More closely tied to specific medium at physical layer • Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT
IEEE 802 Frame Formats • IEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3), CSMA/CA (IEEE 802.11), and token ring (IEEE 802.5) • Each frame format describes how data package is formed • If a CSMA/CD network connects to a token ring network, frames have to be converted from one to another
Frame Formats IEEE 802.3 CSMA/CD IEEE 802.11 CSMA/CA IEEE 802.5 Token Ring
LAN Systems • Ethernet or CSMA/CD • IBM Token Ring • FDDI (Fiber Distributed Data Interface)
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. • Transmission is full duplex (separate transmit and receive), thus no collisions. • Prioritization is possible using 802.1p protocol. • Topology can be star or mesh (for trunks). • Cabling can be either UTP or optical. • Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queuing. • Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps is limited only by media. • 10 Gbps is now beginning to appear.
Power & Ethernet • What if you have a remote device that has an Ethernet connection? • It will require a power connection • What if you don’t have an electrical outlet nearby? • Use PoE • Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals • Ethernet over power line • Uses existing power lines in the building • No new wiring needed • Slower
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.
FDDI • 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.
Interconnection • Necessary to connect a local area network to another local area network or to a wide area network. • LAN-to-LAN connections are often performed with a bridge-like device. • LAN-to-WAN connections are usually performed with a router. • A switch can be used to interconnect segments of a local area network.
Why Segment or Interconnect? • To separate / connect one corporate division with another • To connect two LANs with different protocols • To connect a LAN to the Internet • To break a LAN into segments to relieve traffic congestion • To provide a security wall between two different types of users
Hubs • Interconnects two or more workstations into a local area network. • When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links. • A hub can be managed or unmanaged. • A managed hub possesses enough processing power that it can be managed from a remote location.
Hub issues • Maximum distance between devices (100m in 10Base-T) • Must avoid loops between connected hubs • message would circulate endlessly • Number of devices on network increases collision risks • collisions during peak traffic periods can crash the network (200 devices)
Bridges • Connect two similar LANs, such as two CSMA/CD LANs. • Connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN. • Examines the destination address in a frame and either forwards this frame onto the next LAN or does not. • Examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.
Transparent Bridge • Does not need programming but observes all traffic and builds routing tables from observation. • The observation is called backward learning. • Each bridge has two connections (ports) and there is a routing table associated with each port. • Observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table. • Found with CSMA/CD LANs. • Can also convert one frame format to another. • Sometimes refereed to as a gateway or sometimes a router. • Removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format.
Remote Bridge • Passing a data frame from one LAN to another when the two LANs are separated by a long distance and there is a wide area network connecting the two LANs. • Takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers. • When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame.
Switches (I) • Combination of a hub and a bridge. • Can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns. • When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection. • Workstations that connect to a hub are on a shared segment. • Workstations that connect to a switch are on a switched segment.
Switches (II) • The backplane of a switch is fast enough to support multiple data transfers at one time. • A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch. • Multiple workstations connected to a switch use dedicated segments. • This is a very efficient way to isolate heavy users from the network. • A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server. • Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network
Virtual LANs • 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
Full Duplex Switches • Allows for simultaneous transmission and reception of data to and from a workstation • This full duplex connection helps eliminate collisions • To support a full duplex connection to a switch, at least two pairs of wires are necessary • One for the receive operation • One for the transmit operation • Most people install four pairs today, so wiring is not problem
Link Aggregation • Combining multiple physical connection into one logical connection • Increase connection speed • Fault tolerance • IEEE 802.3ad-2000
Spanning Tree Algorithm • In large network, a loop can be created where a frame can circle through the network and back to the originating device • 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 • Identify a switch as the root switch • Visit each switch and identify the one port (RP) that has the shortest path back to the root switch. • Visit each LAN and identify the port (DP) that provides the shortest path back to the root switch. • Mark the remaining unidentified ports as Removed in the forwarding tables.
Quality of Service (QoS) • Set priority for each frame • The 802.1p adds a 3-bit field (PCP) to each Ethernet frame PCP Value Traffic Type 0Best effort 1Background (lowest priority) 2Excellent effort 3Critical applications 4Video 5Voice 6Internetwork control 7Network control (highest priority)
Routers • Router - device that connects a LAN to a WAN or a WAN to a WAN • Router: • Accepts outgoing packet • Removes any LAN headers and trailers • Encapsulates necessary WAN headers and trailers • Because router has to make wide area network routing decisions Ú router has to dig down into the network layer of the packet to retrieve network destination address • Routers are often called “layer 3 devices” • Operate at the third layer, or OSI network layer, of the packet • Often incorporate firewall functions
