1 K. Salah Module 4.3: Repeaters, Bridges, & Switches Repeater Hub NIC Bridges Switches VLANs GbE

2 K. Salah Hardware Components Repeater –Layer 1 device that provides physical and electrical connections. –It receives signals from one cable segment, regenerates, retimes, and amplifies them, and then transmits these “revitalized” signals to another cable segment. –Transmits in both directions –Joins two segments of cable –No buffering –No logical isolation of segments Hub –Used to describe a repeater –Can be “repeater hub”, “switching hub”, bridging hub”. NIC –Network Interface Card –Performs layer-2 functions: framing, error detection, and flow control. –Performs layer-1 functions by converting the bits into electrical signals using appropriate coding scheme.

3 K. Salah Bridges Layer 2 devices Interconnects two or more individual LANs or LAN segments. Desirable for separating traffic among segments. A segment is part of a LAN in which traffic is common to all nodes, i.e. it is a single continuous conductor, though it may include repeaters. Split the segment with bridges/switches, if link utilization is more than 30%. Store-and-forward devices. They capture the entire frame before deciding whether to filter or forward it. Frames with bad CRC are not forwarded. Minimal buffering to meet peak demand

4 K. Salah Bridges Standards Transparent Bridges –Operate in promiscuous mode. –Bridging is transparent to stations, as if they are on one single LAN. –“plug and play” unit, learns addresses connecting to ports by examining source and destination addresses. –examines the destination address to forward or filter frames. –All broadcast and multicast frames are forwarded. Source Routing Bridges –Sender provides routing information for frames. –Routing information includes local or remote destination address, and list of intermediate route designators. –A route designator contain 12-bit LAN number and 4-bit bridge number. –Every station has a map of the network (different routes to get to different destinations). –Routes can be configured manually or by performing route discovery.

5 K. Salah Bridges Standards (cont.) Network loops can happen. Can cause broadcast storms that can bring the LAN down. Spanning Tree algorithm resolves network loops. Algorithm is based on graph theory. –Any connected graph, consisting of nodes and edges connecting pairs of nodes, there is a spanning tree of edges that maintains the connectivity of the graph but contains no closed loops. –Each LAN is a node and each bridge is an edge. –Specified in IEEE It involves a brief exchange of messages among all bridges to discover the minimum-cost spanning tree. Whenever there is a change in topology, the bridges automatically recalculate the spanning tree. Disabling B3-LAN4 port will result in a spanning tree. If B4 fails, the algorithm should enable this port again. Spanning Tree

6 K. Salah Switches Switches can operate at different layers: layer 2, 3, 4, and 7.. Basically a switch is hardware based, not software based. Three types of layer 2 switches: –Store-and-Forward Switch  Similar to store-and-forward bridge. Store entire frame, check for errors, and then switch to the other ports, based on the destination MAC address. –Cut-Through Switch  The transmission of frame begins as soon as it reads the destination MAC address. Two switch fabric/matrix designs:  Crossbar  Backplane with bus speed > aggregate port speeds –Hybrid Switch  Reliability: store-and-forward. Turn ON when errors are high.  Low latency: cut-through. Turn ON when errors are low.

7 K. Salah VLANs VLAN is a logical grouping of nodes using Ethernet switches. Nodes don’t need to be connected physically to the same switch. A broadcast frame will be heard by all nodes within VLAN. Benefits: –Isolates broadcasts –Frees up network from physical locations –Easily shares resources. A server can be part of multiple VLANs. –Performance. Easily can be enhanced by creating new VLANs. –Security. By containing who can listen to broadcast. VLAN Membership (implicit tagging) –Port-based –MAC-based –Layer 3/IP –Combination of the above

8 K. Salah Gigabit Ethernet With GbE CSMA/CD –Network diameter shrinks to 25 meter. This is not a good option. –Therefore, minimum frame time was increased to 512 bytes. This give network diameter of 200 meter, but waste in bandwidth especially for small size data. Most common use is point-to- point fame switching. –No CSMA/CD We have now 10GbE that can go more than 50 km over SMF.