Ethernet Networking
Ethernet is a contention media access method that allows all hosts on a
network to share the bandwidth of a link, is specified at the Data Link layer,
and uses specific physical layer cabling and signaling techniques.
Ethernet networking uses Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) to share the bandwidth without having two devices transmit at the same
time on the network medium. When a node transmits in a CSMA/CD
environment, all the other nodes receive and examine the packet to see if it is
destined for them, bridges and routers prevent the transmission from propagating
throughout the internetwork.
CSMA/CD
- Carrier Sense – a workstation listens to the network to ensure that there
aren't any other stations transmitting when it wants to transmit.
- If the network is quiet for a period of time called IFG (InterFrame Gap),
the station may transmit.
- The network is continuously monitored if it is determined to be busy. Once
the IFG is observed, the station may transmit.
- When two or more stations are listening at the same time and determine the
network is free and then try to transmit at the same time, a collision will
occur and destroy both packets. The network is monitored during transmissions to
detect collisions.
- Transmission is stopped immediately if the station detects a collision on
the network during its transmission. A signal is sent on the network to indicate
that a collision has occurred and informs all stations to disregard all
corrupted packets they may have been receiving.
- A random backoff algorithm is applied to determine how long each station
that had a collision during its transmission will have to wait to retransmit.
- Starts over at step one to repeat the transmitting process.
Since Ethernet networks are connected with repeaters to extend the network,
whenever a collision occurs it is repeated on each adjoining segment.
How some LAN Hardware treats collisions:
- A repeater receives and immediately retransmits each bit; it doesn’t
depend on any particular protocol, it simply duplicates everything,
including the collisions.
- A bridge receives the entire message into memory. If the message was
damaged by a collision or noise, it is discarded. Otherwise, the
message is queued up and will be retransmitted onto another Ethernet cable.
The bridge has no address. Its actions are transparent to the client and
server workstations.
- A router acts as an agent to receive and forward messages. The router has
an address and is known to the client or server machines. Typically,
machines directly send messages to each other when they are on the same
cable, and they send the router messages addressed to another zone,
department, or sub-network.
IEEE Specifications for Ethernet
- 802.3 - Ethernet LAN Specifications
Two new specifications, both run on twisted pair and fiber optic.
- 802.3u - Fast Ethernet
- 802.3z - Gigabit Ethernet
Ethernet Speeds at Cisco Hierarchical Layers
- Access - 10Mbps switches for clients, 100Mbps for Servers.
- Access to Distribution - use 100Mbps.
- Distribution to Core - use 100Mbps or 1000Mbps, with redundant links.
|
Cable Specifications |
| Name |
Class |
Length |
Host per Segment |
Topology |
| 10base2 |
50ohm Coax |
185m |
30 |
Phys. + logical Bus |
| 10base5 |
50ohm Coax |
500m |
208 |
Phys. + logical Bus |
| 10baseT |
Cat 3,4,5 UTP |
100m |
1 |
Phys.. star + logical bus |
| 100baseTx |
Cat 5,6,7 UTP |
100m |
1 |
Phys. star + logical bus |
| 100baseFX |
Multimode Fiber |
400m |
1 |
point to point |
| 1000baseCX |
STP |
25m |
1 |
Point to point |
| 1000baseT |
Cat 5 UTP |
100m |
1 |
Phys. star + logical bus |
| 1000baseSX |
Multimode Fiber |
260m |
|
|
| 1000baseLX |
Singlemode Fiber |
3 - 10K |
|
|
802.3u Fast Ethernet Features and Benefits
- 100baseT was adopted by IEEE as the 802.3u standard in 1995.
- 100baseT is ethernet operating at 10 times the speed of regular ethernet.
- Just like 10baseT, it can be used in a shared or switched environment.
- Can operate in full-duplex without collisions.
- Use twisted pair or fiber.
- The 100BaseT maximum network diameter is 205 meters, which is approximately
10 times less than 10-Mbps Ethernet.
- 100BaseT networks support an optional feature, called autonegotiation, that
enables a device and a hub to exchange information (using 100BaseT FLPs) about
their capabilities, thereby creating an optimal communications environment.
Autonegotiaton supports a number of capabilities, including speed matching for
devices that support both 10-and 100-Mbps operation, full-duplex mode of
operation for devices that support such communications, and an automatic
signaling configuration for 100BaseT4 and 100BaseTX stations.
- The IEEE 802.3u specification for 100BaseTX networks allows a maximum of two
repeater (hub) networks and a total network diameter of approximately 200
meters. A link segment, which is defined as a point-to-point connection
between two Medium Independent Interface (MII) devices, can be up to 100 meters.
100BaseT supports three media types at the OSI physical layer (Layer 1):
100BaseTX, 100BaseFX, and 100BaseT4.
Characteristics of 100BaseT Media Types
| Characteristics |
100BaseTX |
100BaseFX |
100BaseT4 |
|---|
| Cable
| Category 5 UTP, or Type 1 and 2 STP
| 62.5/125 micron multi-mode fiber
| Category 3, 4, or 5 UTP
|
| Number of pairs or strands
| 2 pairs
| 2 strands
| 4 pairs
|
| Connector
| ISO 8877 (RJ-45) connector
| Duplex SCmedia-interface connector (MIC) ST
| ISO 8877 (RJ-45) connector
|
| Maximum segment length
| 100 meters
| 400 meters
| 100 meters
|
| Maximum network diameter
| 200 meters
| 400 meters
| 200 meters
|
Half-Duplex Ethernet
- Defined in 802.3 ethernet, usually runs on 10baseT.
- Uses only one wire pair with signals running in both directions on the wire,
stations either transmit or receive not both.
- Uses CSMA/CD protocol to detect collisions and retransmit if they occur.
- If you attach a hub to a switch, the switch must run in half-duplex so the
end stations can detect collisions.
Full-Duplex Ethernet
Full-duplex provides the means of transmitting and receiving simultaneously
on a single wire. Full-duplex is typically used between two endpoints, such as
between switches, between switches and servers, between switches and routers,
and so on. Full-duplex has allowed bandwidth on Ethernet and Fast Ethernet
networks to be easily and cost-effectively doubled from 10 Mbps to 20 Mbps and
100 Mbps to 200 Mbps, respectively.
- Data can be simultaneously transmitted and received, doubling the nominal
throughput.
- Uses two pairs of wire, one pair sends and the other receives data.
- Can't run on COAX, must be twisted pair or fiber.
- Uses a point-to-point connection between transmitting and receiving devices.
- When powered on, it negotiates with the other end of the link (auto-detect
mechanism), it first checks for available speed 10 or 100mbps, it then checks to
see if it can run in full-duplex. If it can't, it will run in half-duplex.
UTP
- Uses RJ-45 connector with up to four pairs of twisted wire.
- Twisted wires eliminate crosstalk between the wire and the more twists in
the wire the higher the category rating and the quality.
Straight Wired
- The wires are in the same order at both ends of the RJ-45s.
Used For
- Router to hub or switch.
- Server to hub or switch.
- Workstation to hub or switch.
Crossover
- Two pairs of the wires are crossed at one end of the cable.
- Switch the first and third and second and sixth wires on one end.
Used For
- Uplinks between hubs and switches.
- Hubs to switches.
- Router to router.
- Connecting two PCs without a hub or switch.
CAT 5 UTP Wiring Order for Patch and Crossover Cables
- To make your own CAT 5 (patch/crossover) cable you will have to first cut
the wire to the needed length and strip about 3/4 of an inch of the outer
jacket off both ends.
- Next arrange the eight individual wires in the following order:
| Patch Cable Wiring Order [ Left to Right
] |
| Wire Color |
orange/white |
orange |
green/white |
blue |
blue/white |
green |
brown/white |
brown |
| Wire Number |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
- Next, holding the wires together, trim them so the ends are all flush.
Wire number 1 will stay on the left side.
- Next turn an RJ-45 jack upside down {locking tab down} and slide the
wires in until they align below the brass connectors. Make sure they are
still in the same order as above.
- Now, holding the wires firmly in the jack, place the jack in the pair of
crimpers and proceed to firmly crimp them in place.
- Once done crimping, pull gently on the wires to verify that they were
securely crimped; if they aren't secure, cut the bad end off and go back to
step 1.
- If that side is done, you can now proceed to step 8 to make a crossover
cable (the other side is done the same except wires 1 and 3 and 2 and 6 are
switched) or if you want to make a patch cable (connects PC to hub) follow
steps 1 through 6 for both sides of the cable.
- Only one end of a crossover cable is switched, and is in
the following order:
| Crossover Cable Wiring Order [1 + 3 and 2
+ 6 switched from patch cable order] |
| Wire Color |
green/white |
green |
orange/white |
blue |
blue/white |
orange |
brown/white |
brown |
| Wire Number |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
- Now follow steps 3 - 6 to create the crossover cable.
|