Lec # 25 Computer Network Muhammad Waseem Iqbal

Learn about the Internetworking Devices – Repeaters – Hubs – Switches – Bridges – Routers

Connecting Devices

Connecting devices

A Repeater in the OSI Model

Layer 1: Repeaters (1) Operates only in the physical layer. It receives the (weak) signal and regenerates the original bit pattern. A repeater is a regenerator not an amplifier regenerate and propagate signals. Used in Ethernet/ LANs to extend the length of the LAN. Length of Ethernet/ LANs has restrictions. – Restriction in signal quality. – Segment exceeds max. Length  signal quality begins to deteriorate.

Layer 1: Repeaters (2) Transmits in both directions No buffering It connects segments of a single LAN. It cannot connect two LANs using different protocols. It is used within a single LAN. A repeater forwards every frame, it has no filtering capability. Can be used to overcome the 10base5 Ethernet length restriction.

Layer 1: Repeaters

A repeater does not filter frames, e.g A’s Frame to B is also received by C& D

Function of a Repeater

Layer 1: Hubs (1) Unintelligent device works at the physical layer Used in star topology of LAN e.g. Used in 10baseT Can be called a multi-port repeater All devices in the same collision domain All devices in the same broadcast domain Devices share the same bandwidth With hubs, normally CSMA/CD is used

Layer 1: Hubs (2) Total network bandwidth is limited to the speed of the hub, i.e. a 10Base-T hub provides 10Mb bandwidth max, no matter how many ports it has. Supports half duplex communications limiting the connection speed to that of the port, i.e. 10Mb port provides a 10Mb link. Hop count rules limit the number of hubs that can be interconnected between two computers. Hub retransmits incoming signal to all outgoing lines Only one station can transmit at a time With a 10mbps LAN, total capacity is 10mbps

Layer 1: Hubs (3).

Hubs

Network Domains Domain is referred to a specific area. – In Networks those computers which come in a single domain can by default communicate with each other. Broadcast Domain – A range in which if a broadcast is done, all hear it. – A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain. Collision Domain – A collision domain is a set of network interface cards (NICs) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

Network Domains Any device with more broadcast domains is better, as it can segment the network into multiple network segments, reducing the broadcasts and thus improving the efficiency in terms of time and load. Hubs –Network becomes Half Duplex with Hubs –Hubs supports a single collision domain (assume that at the back plane/motherboard, there is a single road available for the frames to travel) Switch –Network becomes Full Duplex with Switches –Switch supports multiple collision domains (assume that at the back plane/motherboard, there are as many roads are available for the frames to travel as many devices are attached to the switch)

Hubs

A Bridge in the OSI Model

Bridges A bridge (or bridge-like device) can be used to connect two similar LANs, such as two CSMA/CD LANs. A bridge can also be used to connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN. The bridge examines the destination address in a frame and either forwards this frame onto the next LAN or does not. The bridge examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.

Layer 2: Bridges Connect separate networks together – Make use of the network’s frame header, therefore supports only networks that have exactly the same format for addresses Mac-layer bridges: connect networks of the same type (Ethernet to Ethernet) Link-layer bridges: connect different network types (e.G. Ethernet to rings) Maps Ethernet addresses of the nodes residing on each network segment Filtering device  allows only necessary traffic to pass through the bridge

Bridge From to 802.3

Bridge From to ~ ~ ~ ~

Layer 2: Bridges

Most widely adopted types of bridges Transparent bridges – Makes all routing decisions – Used in Ethernet/802.3 networks – Operate in promiscuous mode accepting every transmitted frame – Easy to install Source routing bridges – End stations perform major route-finding – Used in token ring networks Main difference between them: routing algorithm

Why Bridges? Reliability – Large LAN can be partitioned: fault will only affect small segment Performance – Traffic can be distributed: decrease amount of contention Security – Different security needs on physically separate media

Function of a Bridge

Bridge Operation: Learning example Suppose C sends frame to D and D replies back with frame to C. addressport A1 B1 E2 H3 J3 C1 C sends frame, bridge has no info about D, so floods both LANs – bridge notes that C is on port 1 – frame ignored in upper LAN frame received by D A B C D E F H I J bridge

Bridge Operation: Learning Example addressport A1 B1 E2 H3 J3 C1 D2 A B C D E F H I J bridge D generates reply to C, sends – bridge sees frame from D – bridge notes that D is on interface 2 – bridge knows C on interface 1, so selectively forwards frame out via interface 1

Transparent Bridges A transparent bridge does not need programming but observes all traffic and builds routing tables from this observation. This observation is called backward learning. Each bridge has two connections (ports) and there is a routing table associated with each port. A bridge 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. A transparent bridge is found with CSMA/CD LANs.

Layer (2): Switch (1) A switch is a combination of a hub and a bridge. It 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.

Layer (2): Switch (2) Total network bandwidth is determined by the number of ports on the switch. i.e. an 8 port 100Mb switch can support up to 800Mb/s bandwidth. Supports full duplex communications. This allows devices to both transmit and receive data at the same time, thereby doubling traffic capacity and avoiding packet collisions. Switch allows users to greatly expand networks; there are no limits to the number of switches that can be interconnected between two computers. Switch hub's increased performance is worth the slightly higher price.

MAC address Table Switches reduce network overhead by forwarding traffic from one segment to another only when necessary. To decide whether to forward a frame, the switch uses a dynamically built table called a bridge table or MAC address table. The switch examines the address table to decide whether it should forward a frame.

Bridges and Switches Similarities Both are called Layer 2 devices (Data Link Layer) Both filters the traffic on the basis of MAC address Each segment has its own collision domain All segments are in the same broadcast domain.

Bridges and Switches The basic 5 operations: –Learning –Forwarding –Flooding –Filtering –Aging

Switches / Bridges Forwarding Decisions: –If Source and Destination LANs are same, discard the frame –If Source and Destination LANs are different, forward the frame –If Destination LAN is unknown, use flooding.

Bridges/Switches Operations Learning – The switch learns MAC addresses by examining the source MAC address of each frame it receives. – Switches build the MAC address table by listening to incoming frames and examining the frame’s source MAC address. – If a frame enters the switch, and the source MAC address is not in the address table, the switch creates an entry in the table. – The MAC address is placed in the table, along with the interface in which the frame arrived. – By learning, the switch can make good forwarding choices in the future.

Bridges/Switches Operations Flooding –Bridges forward LAN broadcast frames, and unknown unicast frames, out all ports. –The switch floods the frame with the hope that the unknown device will be on some other Ethernet segment, it will reply, and the switch will build a correct entry in the address table.

Bridges/Switches Operations Forwarding –The switch decides when to forward a frame based on the destination MAC address. –The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.

Bridges/Switches Operations Filtering – A switch generally does filtering along with forwarding – There are two cases When the source and destination devices exist on the same interface (port) of bridge/switch, then the bridge/switch will filter the frame and doesn’t pass it to any other interface If source and destination are on different interfaces, and the destination port is known, then the frame is forwarded to only the destination interface and it will be filtered or not sent on the remaining interfaces.

Bridges/Switches Operations Aging –By default, every switch keeps the entry of a computer in its table for 5 minutes. After 5 minutes or aging time, the entry is deleted from the table, assuming that the device doesn’t exit on the network. –(This is useful, if a device is disconnected from the network, or moved from one segment to another.)

Switches and Bridges

Switch Forwarding Techniques Cut-through switching. Store and forward switching. Fragment free switching. – This technology combines the features of the above two switching modes. – In essence, this technology lies between the above two switching technologies. Most switches today use one of the following: – Cut-through switching—or. – Store-and-forward switching.

Cut-Through Switches begin transmitting a frame to destination node as soon as switch identifies the destination node's address delay is less than store-and-forward propagate errors because no integrity checks are done beforehand also available a combination of store-and-forward and cut-through  hybrid switch

HUB & Switch.

The main difference between hubs and switches is the bandwidth available to network users.

Routers A Router Is Layer 3 i.e. Network Layer Device It Doesn’t Support Broadcasting Each Segment Has Separate Collusion and Broadcast Domains. Use Layer 3 Addresses : IP Addresses Main Function Performed Is Routing Using the IP Addresses Maintains Routing Tables, ARP Caches. Exchanges Information With Other Routers Responsible for Routing the Packet From Source Network to the Destination Network.

Router Functionality Like bridges, filtering and forwarding rates are often used as router performance measures Unlike bridges, routers only process packets that are addressed to them – Also unlike bridges, forwarding decisions are based on destination addresses in network layer packet headers Routers can also be used to limit access to a network; many have firewall capabilities Multiprotocol routers are capable of forwarding messages using more than one network layer protocols Encapsulation may be used to enable non-routable data link protocols, such as SDLC, to be routed over TCP/IP networks

Network Device Domains