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Multiple Access
The upper sublayer of Datalink layer, that is responsible for flow and error control is called the logical link
control (LLC) layer; the lower sublayer that is mostly responsible for multiple- access resolution is called the
media access control (MAC) layer.
According to CSMA/CD, a node should not send a packet unless the network is clear of traffic. If two nodes
send packets at the same time, a collision occurs and the packets are lost. Then both nodes send a jam signal,
wait for a random amount of time, and retransmit their packets. Any part of the network where packets from two
or more nodes can interfere with each other is considered a collision domain. A network with a larger number of
nodes on the same segment has a larger collision domain and typically has more traffic. As the amount of traffic
in the network increases, the likelihood of collisions increases.
CSMA/CD Algorithm:
1. If the medium is idle, transmit; otherwise, go to step 2.
2. If the medium is busy, continue to listen until the channel is idle, then transmit immediately.
3. If a collision is detected during transmission, transmit a brief jamming signal to assure that all stations
know that there has been a collision and then cease transmission.
4. After transmitting the jamming signal, wait a random amount of time, then attempt to transmit again.
(Repeat from step 1.
Traditional Ethernet uses CSMA/CD.
A networking component used either to extend or to segment networks. Bridges work at the OSI data-link layer.
They can be used both to join dissimilar media such as unshielded twisted-pair (UTP) cabling and fiber-optic
cabling, and to join different network architectures such as Token Ring and Ethernet. Bridges regenerate signals
but do not perform any protocol conversion, so the same networking protocol (such as TCP/IP) must be running
on both network segments connected to the bridge. Bridges can also support Simple Network Management
Protocol (SNMP), and they can have other diagnostic features.
How it works?
Bridges operate by sensing the source MAC addresses of the transmitting nodes on the network and
automatically building an internal routing table. This table is used to determine which connected segment to
route packets to, and it provides the filtering capability that bridges are known for. If the bridge knows which
segment a packet is intended for, it forwards the packet directly to that segment. If the bridge doesn’t recognize
the packet’s destination address, it forwards the packet to all connected segments except the one it originated on.
And if the destination address is in the same segment as the source address, the bridge drops the packet. Bridges
also forward broadcast packets to all segments except the originating one.
The basic networking component used in traditional 10-Mbps Ethernet networks to connect network
stations to form a local area network (LAN). Hubs can be used for
• Connecting about a dozen computers to form a workgroup or departmental LAN
• Connecting other hubs in a cascaded star topology to form a larger LAN of up to roughly a hundred
How It Works
Hubs are the foundation of traditional 10BaseT
Ethernet networks. The hub receives signals
from each station and repeats the signals to all
other stations connected to the hub. In active
hubs (which all of today’s hubs are), the signal
received from one port is regenerated
(amplified) and retransmitted to the other ports
on the hub. Hubs thus perform the function of a
repeater and are sometimes called multiport
repeaters. From a logical cabling point of view,
stations wired into a hub form a star topology.
Hubs generally have RJ-45 ports for unshielded
twisted-pair (UTP) cabling, and they range in
size from 4 to 24 or more ports for connecting
stations to the hub, plus one or more uplink
ports for connecting the hub to other hubs in a
cascaded star topology. Hubs generally have various light-emitting diode (LED) indicator lights to indicate the
status of each port, link status, collisions, and so on.
Switch is essentially a multi-port bridge. Switches allow the segmentation of the LAN into separate collision
domains. Each port of the switch represents a separate collision domain and provides the full media bandwidth
to the node or nodes connected on that port. With fewer nodes in each collision domain, there is an increase in
the average bandwidth available to each node, and collisions are reduced.
Why Switches:
In a LAN where all nodes are connected directly to the switch, the throughput of the network increases
dramatically. The three primary reasons for this increase are:
• Dedicated bandwidth to each port
• Collision-free environment
• Full-duplex operation
Hub VS Switch:
Works on physical layer
Works on Datalink layer
Full Duplex
Hub extends the collision domain
Switch splits the collision domain (Each
port of the switch acts as a collision
Multiport Repeater
Multiport Bridge
Overall Bandwidth is shared
Each port receives its own bandwidth.
Not used in todays market due to degraded Mostly used today.
There are three forwarding methods a switch can use:
• Cut through (cut-through switching is a switching method for packet switching systems, wherein the
switch starts forwarding that frame (or packet) before the whole frame has been received, normally as
soon as the destination address is processed. This technique reduces latency through the switch, but
decreases reliability.)
• Store and forward - the switch, unlike cut through, buffers and typically, performs a checksum on each
frame before forwarding it on.
• Fragment free (Fragment-free switching is suitable for backbone applications in a congested network, or
when connections are allocated to a number of users. The packets are sent through the switch as a
continuous flow of data--the transmit and receive rates are always the same. Because of this, fragmentfree switching cannot pass packets to higher speed networks, for example, to forward packets from a 10
Mbit/s to a 100 Mbit/s Ethernet network. Therefore, if you opt for fragment-free switching, you cannot
make direct connections to higher speed networks from that port.)
The data link layer, needs to pack bits into frames, so that each frame is distinguishable from another. The Data
Link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to
create a frame.
The Data Link layer frame includes:
• Data - The packet from the Network layer
• Header - Contains control information, such as addressing, and is located at the beginning of the PDU
• Trailer - Contains control information added to the end of the PDU
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