411 6b

ICSA 411: Week 6: 

ICSA 411: Week 6 Local Area Networks

Hubs and Switches: 

Hubs and Switches Shared medium hubs Switched LAN hubs x

Advantages of Switched Hubs: 

Advantages of Switched Hubs No modifications needed to workstations when replacing shared-medium hub Each device has a dedicated capacity equivalent to entire LAN Easy to attach additional devices to the network

Types of Switched Hubs: 

Types of Switched Hubs Store and forward switch Accepts a frame on input line Buffers it briefly Routes it to appropriate output line Cut-through switch Begins repeating the frame as soon as it recognizes the destination MAC address Higher throughput, increased chance of error

Token Ring LANs (802.5):Medium Access Control: 

Token Ring LANs (802.5): Medium Access Control Token “seized” by changing a bit on the circulating frame to indicate start of frame rather than token Default configuration requires sender to complete transmission and begin receiving transmitted frame before releasing the token “Early token release” allows release of token after transmission but before receipt of frame

802.5 Transmission Media: 

802.5 Transmission Media Original specified shielded twisted pair with data rates of 4 and 16mbps New addition to standard allows use of UTP for 4mbps Utilizes differential Manchester encoding

High-Speed & Wireless LANs: 

High-Speed & Wireless LANs Why? Extraordinary growth in speed, power, and storage capacity of PCs Increasing use of LANs as computing platforms Examples Server farms Workgroups with “power” requirements High-speed backbones

Fast Ethernet: 

Fast Ethernet Easy to integrate with existing systems Can use UTP (-TX) or fiber (-FX) Uses star-wired topology, using a central multiport repeater (broadcast method) If NICs support full-duplex mode, switched hub must be used

Gigabit Ethernet: 

Gigabit Ethernet Still under development Retains CSMA/CD protocol and Ethernet format, ensuring smooth upgrade path Uses optical fiber over short distances 1-gbps switching hub provides backbone connectivity

Internetworking: 

Internetworking When diverse networks are interconnected, the collection is an internet, and the individual networks are subnets Networks are interconnected using bridges and routers Internet (“big I”) is the largest and best known example of an internet (“little I”)

Internetworking Devices: 

Internetworking Devices Repeaters Bridges Routers Brouters Gateways

Repeater: 

Repeater Accepts a signal, regenerates or amplifies it, and passes it along at full strength Permits the originally-transmitted signal to go further than the attenuation limits Operate at Layer 1 (Physical Layer) of the OSI reference model Least expensive internetworking devices Used to join exactly identical LANs

Bridges: 

Bridges Allow connections between LANs and to WANs Operates at Layer 2 (Data Link Layer) of OSI Used between networks using identical physical and link layer protocols Provide a number of advantages Reliability: Creates self-contained units Performance: Less contention Security: Not all data broadcast to all users Geography: Allows long-distance links

Bridge Functions: 

Bridge Functions Read all frames from each network Accept frames from sender on one network that are addressed to a receiver on the other network Retransmit frames from sender using MAC protocol for receiver Must have some routing information stored in order to know which frames to pass

Transparent Bridges: 

Transparent Bridges Also called spanning tree bridge Source stations don’t need to know receiver’s locations No changes are required to install the bridge Intelligence necessary to make the relaying decision exists in the bridge itself

Transparent Bridge Operation: 

Transparent Bridge Operation Operates in promiscuous mode: accepts every frame transmitted on all LANs Decision to forward or to discard is made based on a destination table lists each possible destination determines LAN for source address Table created by listening to all transmissions (learning) Learns destinations from source addresses

Source Routing Bridges: 

Source Routing Bridges Assumes that the sender of each frame knows whether or not the destination is on its own LAN Source determines the frame route, includes exact path in the frame header Bridge itself requires less processing power Developed by IBM, and preferred for use in token ring networks

Source Routing Transparent Bridges: 

Source Routing Transparent Bridges Used to connect a transparent bridge based network (CDMA/CD or Token bus) to a source routing based network (Token ring) Combines both methods Source routing is performed only when the frame received has the routing information

Routers: 

Routers Used to connect two networks that may or may not be similar Three primary functions Provide a link between networks Provide for routing and delivery of data between nodes on different networks Provide services in a way that does not require modification of the subnets Operate at Layer 3 (Network Layer) of OSI model

Router Characteristics: 

Router Characteristics Protocol-based device; most routers use TCP/IP TCP provides end-to-end reliability at higher layers, so routers need not be concerned with error recovery or flow control Network security and management is much more robust than with bridges If the network goes down, the problem can be easily isolated to one port of a router, making it possible to restore the network quickly

Gateways: 

Gateways Operate at the network layer or above Connect two dissimilar networks or systems by providing conversion from one network to another (protocol converters) Must be able to understand the protocols of both nodes being connected