Another way to think about transmission time is
the time it takes a frame to be transmitted. Small frames take
a shorter amount of time. Large frames take a longer amount of
time. Each 10 Mbps Ethernet bit has a 100 ns transmission
window. This is the bit time. A byte equals 8 bits. Therefore,
1 byte takes a minimum of 800 ns to transmit. A 64-byte frame,
the smallest 10BASE-T frame allowing CSMA/CD to function
properly, takes 51,200 ns ( 51.2 microseconds). Transmission of
an entire 1000-byte frame from the source station requires 800
microseconds just to complete the frame. The time at which the
frame actually arrives at the destination station depends on
the additional latency introduced by the network. This latency
can be due to a variety of delays including all of the
following: - NIC delays
- Propagation delays
- Layer 1, Layer 2, or Layer 3 device delays
Interactive Media Activity Drag and Drop:
10BASE-T Transmission Times After completing this activity,
students will be able to identify the transmission times of
10BASE-T.
Content 4.1 Introduction to
Ethernet/802.3 LANs 4.1.8 The benefits of using
repeaters The distance that a LAN can cover is limited due to
attenuation. Attenuation means that the signal weakens as it
travels through the network. The resistance in the cable or
medium through which the signal travels causes the loss of
signal strength. An Ethernet repeater is a physical layer
device on the network that boosts or regenerates the signal on
an Ethernet LAN. When a repeater is used to extend the distance
of a LAN, a single network can cover a greater distance and
more users can share that same network. However, the use of
repeaters and hubs compounds problems associated with
broadcasts and collisions. It also has a negative effect on the
overall performance of the shared media LAN.
Interactive Media Activity PhotoZoom: Cisco 1503 Micro Hub
In this PhotoZoom, the student will view the Cisco 1503 Micro
Hub.
Content 4.1 Introduction to
Ethernet/802.3 LANs 4.1.9 Full-duplex transmitting
Full-duplex Ethernet allows the transmission of a packet and
the reception of a different packet at the same time. This
simultaneous transmission and reception requires the use of two
pairs of wires in the cable and a switched connection between
each node. This connection is considered point-to-point and is
collision free. Because both nodes can transmit and receive at
the same time, there are no negotiations for bandwidth.
Full-duplex Ethernet can use an existing cable infrastructure
as long as the medium meets the minimum Ethernet standards. To
transmit and receive simultaneously, a dedicated switch port is
required for each node. Full-duplex connections can use
10BASE-T, 100BASE-TX, or 100BASE-FX media to create
point-to-point connections. The NICs on all connected devices
must have full-duplex capabilities. The full-duplex Ethernet
switch takes advantage of the two pairs of wires in the cable
by creating a direct connection between the transmit (TX) at
one end of the circuit and the receive (RX) at the other end.
With the two stations connected in this manner a collision free
environment is created as the transmission and receipt of data
occurs on separate non-competitive circuits. Ethernet usually
can only use 50%-60% of the available 10 Mbps of bandwidth
because of collisions and latency. Full-duplex Ethernet offers
100% of the bandwidth in both directions. This produces a
potential 20 Mbps throughput, which results from 10 Mbps TX and
10 Mbps RX. Interactive Media Activity Drag and Drop:
Full Duplex Ethernet After completing this activity, students
will be able to identify the requirements for full duplex
Ethernet.
Content 4.2 Introduction to LAN
Switching 4.2.1 LAN segmentation A network can be
divided into smaller units called segments. Figure shows an
example of a segmented Ethernet network. The entire network has
fifteen computers. Of these fifteen computers, six are servers
and nine are workstations. Each segment uses the CSMA/CD
access method and maintains traffic between users on the
segment. Each segment is its own collision domain. Segmentation
allows network congestion to be significantly reduced within
each segment. When transmitting data within a segment, the
devices within that segment share the total available
bandwidth. Data passed between segments is transmitted over the
backbone of the network using a bridge, router, or switch.
Content 4.2 Introduction to LAN Switching
4.2.2 LAN segmentation with bridges Bridges are Layer 2
devices that forward data frames according to the MAC address.
Bridges read the sender's MAC address of the data packets that
are received on the incoming ports to discover which devices
are on each segment. The MAC addresses are then used to build a
bridging table. This will allow the bridge to block packets
that do not need to be forwarded from the local segment.
Although the operation of a bridge is transparent to other
network devices, the latency on a network is increased by ten
to thirty percent when a bridge is used. This latency is a
result of the decision making process prior to the forwarding
of a packet. A bridge is considered a store-and-forward device.
The bridge must examine the destination address field and
calculate the cyclic redundancy check (CRC) in the frame check
sequence field before forwarding the frame. If the destination
port is busy, the bridge can temporarily store the frame until
that port is available.
Content 4.2 Introduction to
LAN Switching 4.2.3 LAN segmentation with routers
Routers provide segmentation of networks, adding a latency
factor of 20% to 30% over a switched network. This increased
latency is because a router operates at the network layer and
uses the IP address to determine the best path to the
destination node. Figure shows a Cisco router. Bridges and
switches provide segmentation within a single network or
subnetwork. Routers provide connectivity between networks and
subnetworks. Routers also do not forward broadcasts while
switches and bridges must forward broadcast frames.
Interactive Media Activity PhotoZoom: Cisco 2621 Router In
this PhotoZoom, the student will view a Cisco 2621 router.
Interactive Media Activity PhotoZoom: Cisco 3640 Router In
this PhotoZoom, the student will view a Cisco 3640 router.
Content 4.2 Introduction to LAN Switching
4.2.4 LAN segmentation with switches LAN switching
decreases bandwidth shortages and network bottlenecks, such as
those between several workstations and a remote file server.
Figure shows a Cisco switch. A switch will segment a LAN into
microsegments which decreases the size of collision domains.
However all hosts connected to the switch are still in the same
broadcast domain. In a pure switched Ethernet LAN, the sending
and receiving nodes function as if they are the only nodes on
the network. When these two nodes establish a link, or virtual
circuit, they have access to the maximum available bandwidth.
These links provide significantly more throughput than Ethernet
LANs connected by bridges or hubs. This virtual network circuit
is established within the switch and exists only when the nodes
need to communicate.
Content 4.2 Introduction to
LAN Switching 4.2.5 Basic operations of a switch
Switching is a technology that decreases congestion in
Ethernet, Token Ring, and Fiber Distributed Data Interface
(FDDI) LANs. Switching accomplishes this by reducing traffic
and increasing bandwidth. LAN switches are often used to
replace shared hubs and are designed to work with existing
cable infrastructures. Switching equipment performs the
following two basic operations: - Switching data frames
- Maintaining switching operations
Figures
- show the basic operations of a switch.
Content
4.2 Introduction to LAN Switching 4.2.6 Ethernet
switch latency Latency is the period of time from when the
beginning of a frame enters to when the end of the frame exits