Published on December 28, 2007
Networking Devices and Internetworking: Networking Devices and Internetworking CT303 – Nets & Comms Content: Content Networking Devices Repeaters Hubs Bridges Switches Routers Gateways Internetworking Spanning tree bridges Remote bridges Virtual LANs Networking devices: Networking devices An internet (or internetwork) is a collection of individual networks (do not confuse with Internet, that is a worldwide network) To create an internet we need internetworking devices called repeaters, bridges, routers and/or gateways Those devices are acting at different layers of the OSI model. a) Which device is in which layer b) Packets, frames and headers Repeaters (1): Repeaters (1) Receives the signal before it becomes to weak or corrupted, regenerates the original bit pattern and puts the refreshed copy back onto the link Allows the extension of the physical length of a network Doesn’t change the functionality of the network in any way Act only upon the electrical components of a signal and are therefore active only at the physical layer Repeaters (2): Repeaters (2) Station A sends a frame to station B that will be received by station C and D as well The repeater is not an amplifier An amplifier cant discriminate between the intended signal and noise; it amplifies both A repeater does amplify only the signal; it regenerates it Repeaters (3): Repeaters (3) The location of the repeater on the link is vital: it must be placed so that a signal reaches it before any noise changes the meaning of the carried information A little noise can alter the precision of a bit voltage without destroying its identity; if the corrupted bit travels much further, the accumulated noise can change its meaning completely; at that point the original voltage is unrecoverable A repeater placed on the line before this complete loss takes place, will still read the signal well enough to determine the intended voltages and replicate them in their original form Hubs: Hubs They have a number of incoming lines that are joined electrically. Frames coming on one line are sent on all the others. It forms a single collision domain If two frames are arriving at the same time, they will collide in same way as on coaxial cable They don’t examine nor use in any way the 802 addresses They operates at the physical layer Bridges (1): Bridges (1) Connect two or more LANs. When frames arrive, software in the bridge extracts the destination address from the frame and looks it up on a table to see where to send it. For Ethernet this is 48 bit address Bridges (2): Bridges (2) Operate in both physical and data link layer of OSI Can divide a large network into smaller segments Can relay frames between two originally separate LANs Keep the traffic for each segment separated, filtering the traffic. They are useful to keep the congestion low Bridges (3): Bridges (3) The bridge, beside regenerating the signal, will check the physical address of the destination and forwards the new copy only to the segment to which the address belongs It reads the address contained in the frame and it compares it against an internal table with all the addresses of the stations on both segments. When it finds a first match, it discovers to which segment the station belongs and relays the packet only to that segment. Bridges (4): Bridges (4) The bridge will block a packet from station A addressed to station D from crossing into the lower segment The bridge will allow a packet from station A to station G to cross into the lower segment and relays it to the entire lower segment where it is received by station G Bridges (5): Bridges (5) Simple Bridge Connects two LANs Table of stations is maintained manually Multiport Bridge Connects more than two LANs Transparent Bridge Learning bridge Bridges Connecting Different Types of LANs Connects different type of 802 LANs Simple bridge: Simple bridge Links two segments and contains a table that lists he addresses of all the stations included in each of them Addresses must be entered manually (before a simple bridge can be used, an operator has to sit down and enter the addresses of every station) Whenever a new station has been added, the table has to be modified; when a station is removed, the table has to be modified, the newly invalid address has to be deleted The bridge is simple to build and inexpensive to manufacture but installation and maintenance are time consuming, probably more expensive than the price saving resulted out of the cheap manufacturing cost Multiport bridge: Multiport bridge Connects more than two LANs For this bridge, there are three tables, each holding the physical addresses of stations reachable through the corresponding port Transparent bridge: Transparent bridge This learning bridge builds its table of station addresses on its own as it performs bridge functions When first installed, its table is empty; as it encounters each packet, it looks at both the destination and the source addresses. It checks the destination and decides where to send the packet; if it doesn’t recognize yet the destination address, it sends the packet on all of the ports It uses the source address to build its table; with the first packet transmitted by each station, it learns the segment associated with that station Continuing this process even after each station has been learned, it assures that it is self-updating Bridges Connecting Different LANs (1): Bridges Connecting Different LANs (1) Frame format – frames from different LANs have different formats (i.e. Ethernet frame and Token Ring frame) Payload size – the size of the data that can be encapsulated in a frame varies from protocol to protocol (i.e. Ethernet has 1500 + headers while Token Ring has 4500 + headers) Data rate – different protocols use different data rates (i.e. 10 Mb/s for Ethernet and 16Mb/s for Token Ring) Address bit order – the bit order of addresses in different types of LANs is not the same (i.e. a bridge should reverse an address if it is connecting an Ethernet LAN to a Token Ring LAN) Other issues: collision, acknowledgements, priority, etc… Bridges Connecting Different LANs (2): Bridges Connecting Different LANs (2) Bridge from 802.11 to 802.3 Switches: Switches Similar to bridges in that both route on frame addresses. The main difference is that switches interconnect individual computers The switch must actively forward the frame from a station to another (i.e. A to B) Each port is its own collision domain They never loose frames due to collisions They may loose frames due to lack of buffer space (when frames came faster than they can transmit) Routers (1): Routers (1) Relay packets among multiple interconnected networks and operate at physical, data link and network layer of OSI model Have access to network layer addresses and have two or more networks at the same time Routers (2): Routers (2) A packet sent from a station on one network to a station on a neighboring network, goes first to the jointly held router, which forwards it to the destination network If the router is not connected to both source and destination network, then the sending router transfers the packet across one of its connected networks to the next router, in the direction of the destination, and so on, until the destination is reached Transport Gateways and Gateways: Transport Gateways and Gateways Transport Gateways Operate at Transport Layer Connect two or more computers that use different connection-oriented transport layer protocols i.e. a computer using the TCP/IP protocol needs to a computer using connection oriented ATM protocol. Gateways Operate at Application Layer Understand the format and contents of the data and translate one format into another i.e. an e-mail gateway may translate Internet E-mails into SMS messages or mobile phones Internetworking (1): Internetworking (1) To extend a LAN, ideally, it should be possible to go out and buy bridges, plug the connectors into the bridges and everything should work perfectly No hardware changes should be required No software changes should be required No setting of address switches No downloading of bridging tables All of this is actually possible … Internetworking (2): Internetworking (2) A configuration with four LANs and two bridges. Spanning Tree Bridges (1): Spanning Tree Bridges (1) Two parallel transparent bridges. Spanning Tree Bridges (2): Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree. Remote Bridges (1): Remote Bridges (1) A company may have factories in several cities, each of them with its own LANs Ideally, all the LANs should be interconnected so the complete system acts like one large LAN This goal can be achieved by installing a bridge on every LAN and connecting the bridges pairwise with point to point links Point to point links could be phone lines leased from a phone company Remote Bridges: Remote Bridges Remote bridges can be used to interconnect distant LANs. Virtual LANs (1): Virtual LANs (1) LANs should be configurable logically rather than physically If we want k LANs, we can get k hubs and by connecting the appropriate machines to each hub, we obtain k networks Problems arise when we need to connect machines that are not geographically in the same coverage area as the hub i.e. two people from the same department work in different buildings Does matter who is in which LAN? Security Any net card can be put in promiscuity mode, so it can copy all the traffic that comes down on the wire Load Some LANs are more heavily used than others and it may be desirable o separate them (research people may run experiments that may run out of hand and saturate their LAN) Broadcasting Virtual LANs (2): Virtual LANs (2) A building with centralized wiring using hubs and a switch. Virtual LANs (3): Virtual LANs (3) (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches. Virtual LANs (4): Virtual LANs (4) To associate what VLAN (color) an incoming frame is, three methods are possible Every port is assigned a VLAN id (color) this works if all the machines connected via one port belong to one VLAN (not the case with previous example) Every MAC address is assigned a VLAN id (color) The bridge or switch has a table that lists the 48 bit MAC address of each machine connected to it along with VLAN id that machine is on When a frame arrives, all a switch has to do is to extract the MAC address and look it up in the table to see which VLAN the incoming frame came from Every layer 3 protocol or IP address is assigned a VLAN id (color) The bridge or switch examines the payload field of the frame For IP protocol, the IP address can be used to associate with a VLAN This approach violates the most fundamental rule of networking: independence of the layers IEEE 802.1Q Standard (1): IEEE 802.1Q Standard (1) Some more thinking reveals that what matters is the actual VLAN of the frame itself, not of the sending machine The way to go is to identify the VLAN in the frame header, then the need to inspect the payload would vanish What to do about Ethernet? Does not have any spare fields IEEE committee changed the frame format, the new format was published in IEEE 802.1Q, in 1998 New format contains a VLAN tag IEEE 802.1Q Standard (2): IEEE 802.1Q Standard (2) Need to throw away several hundred of million existing Ethernet cards? The answer is no and the key idea is that the VLAN field are only used by bridges and switches and not by the end user machines The originator of the frame (if it is not 802.1Q aware) will not generate the VLAN field into the frame Who generates the new fields? The first bridge or switch (802.1Q compliant) will attach the VLAN field to the frames and the last bridge or switch will remove it. What happens to frames that are at maximum size? 1518 bytes (maximum size of classical frames) is raised to 1522 bytes. However, the idea is that a large frame will never reach an old card. The IEEE 802.1Q Standard (3): The IEEE 802.1Q Standard (3) Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not. The IEEE 802.1Q Standard (4): The IEEE 802.1Q Standard (4) The 802.3 (legacy) and 802.1Q Ethernet frame formats. VLAN Protocol ID (0x8100) – since it is larger than 1500, all cards should interpret it as a type rather than a length. However, what is a legacy network card doing with such of frame doesn’t matter, since they should never receive such frame Pri 3 bit field has nothing to do with VLANs … but since such change in frame format is rare, it has been added to distinguish real-time traffic from insensitive time traffic. Supports quality of service over Ethernet It presence indicates that the frame contains a 802.5 frame that is hoping to find a 802.5 LAN at the destination VLAN ID – it is used as an index into a table in the switch, to find out which ports to send it on (which outgoing lines) References: References Behrouz A. Forouzan – Data Communications and Networking, ISBN: 0-07-118160-1 Andrew S. Tanenbaum – Computer Networks, ISBN: 0-13066102-3
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