In this section of the CCNA course, we’ll learn a detail explanation about Ethernet Network and Gigabit Network.
So let’s start with Ethernet Network and its implementation at different Network layers like Data Link Layer, Physical Layer, etc.
If you want to know what are the layers of the OSI model, then click the link to learn more about them.
Ethernet is a contention media access method that allows all hosts on a network to share the same bandwidth of a link. Ethernet is popular because it’s really scalable, meaning that it’s comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet, into an existing network infrastructure.
It’s also relatively simple to implement in the first place, and with it, troubleshooting is reasonably straightforward. Ethernet uses both Data Link and Physical Layer specifications, and we are going to discuss both the Data link layer and physical layer information you need to effectively implement, troubleshoot, and maintain an Ethernet network.
Ethernet networking uses Carrier Sense Multiple Access with collision detection (CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium.
CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me good collision management is crucial because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network.
So, how does the CSMA/CD protocol work? Let’s start by taking a look at the figure below.
When a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it doesn’t stop there.
The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data (think busy signal).
The nodes respond to that jam signal by waiting a while before attempting to transmit again. Back off algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will then timeout. Pretty clean! When a collision occurs on an Ethernet LAN, the following happens:
- A jam signal informs all devices that a collision occurred.
- The collision invokes a random back off algorithm.
- Each device on the Ethernet segment stops transmitting for a short time until the timers expries.
- All hosts have equal priority to transmit after the timers have expired.
The following are the effects of having a CSMA/CD network sustaining heavy collisions:
- Low throughput
Half and Full-Duplex Ethernet
Half duplex Ethernet is defined in the original 802.3 ethernet; cisco says it uses only one wire pair with a digital signal running in both directions on the wire. Certainly, the IEEE specifications discuss the process of half duplex somewhat differently, but what cisco is talking about is a general sense of what is happening here with Ethernet.
It also uses the CSMA/CD protocol to help prevent collision and to permit retransmitting if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. Half-deplex Ethernet typically 10BaseTis only about 30 to 40 percent efficient as cisco sees it because a large 10BaseT network will usually only give you 3 to 4 Mbps, at most.
But full duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex. And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device. This means that with full duplex data transfer, you get a faster data transfer compared to half duplex. And because the transmitted data is sent on a different set of wires than the received data, no collision will occur.
The reason you don’t need to worry about collisions is because now it’s like a freeway with multiple lanes instead of the single lane road provided by half duplex. Full duplex Ethernet is supposed to offer 100 percent efficiency in both directions for example, you can get 20 Mbps with a 10 Mbps Ethernet running full duplex or 200 Mbps for fast Ethernet. But this rate is something known as an aggregate rate, which translates as “you’re supposed to get” 100 percent efficiency. No guarantees, in networking as in life.
Full duplex Ethernet can be used in three situations:
- With a connection from a switch to a host.
- With a connection from a switch to a switch.
- With a connection from a host to a host using a crossover cable.
Now, if it’s capable of all that speed, why wouldn’t it deliver? Well, when a full duplex Ethernet port is powered on, it first connects to the remote end and then negotiates with the other end of the fast Ethernet link. This is called an auto-detect mechanism. This mechanism first decides on the exchange capability, which means it checks to see if it can run at 10 or 100 Mbps. It then checks to see if it can run full duplex, and if it can’t, it will run half duplex.
Lastly, remember these important points:
- There are no collisions in full duplex mode.
- A dedicated switch port is required for each full duplex node.
- The host network card and the switch port must be capable of operating in full duplex mode.
Now let’s take a look at how Ethernet works at the Data Link Layer.
Ethernet at the Data Link Layer
Ethernet at the Data link layer is responsible for Ethernet addressing, commonly referred to as hardware addressing or MAC addressing. Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network through the Ethernet contention media access method.
Here’s where we get into how Ethernet addressing works. It uses the Media Access Control (MAC) address burned into each and every Ethernet network interface card (NIC). The MAC, or hardware, address is a 48-bit (6 byte) address written in a hexadecimal format.
The following Figure shows the 48-bit MAC addresses and how the bits are divided.
The organizationally unique identifier (OUI) is assigned by the IEEE to an organization. It’s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered address (24 bits, or 3 bytes) that is unique (supposedly, again no guarantees) to each and every adapter it manufactures. Look closely at figure. The high order bit is the individual/Group (I/G) bit. When it has a value of 0, we can assume that the address is the MAC address of a device and may well appear in the source portion of the MAC header. When it is a 1, we can assume that the address represents either a broadcast or multicast address in Ethernet or a broadcast or functional address in TR and FDDI (who really knows about FDDI?).
The next bit is the global/local bit, or just G/L bit (also known as U/L, where U means universal). When set to 0, this bit represents a globally administered address (as by the IEEE). When the bit is a 1, it represents a locally governed and administered address (as in what DECnet used to do). The low-order 24 bits of an Ethernet address represent a locally administered or manufacturer assigned code. This portion commonly starts with 24 0s for the first card made and continues in order until there are 24 1s for the last (16, 777, 216th) card made. You’ll find that many manufacturers use these same six hex digits as the last six characters of their serial number on the same card.
The Data link layer is responsible for combining bits into bytes and bytes into frames. Frames are used at the Data link layer to encapsulate packets handed down from the Network Layer for transmission on a type of Media access.
The function of Ethernet stations is to pass data frames between each other using a group of bits known as a MAC frame format. This provides error detection from a cyclic redundancy check (CRC). But remember this is error detection, not error correction. The 802.3 frames and Ethernet frames are shown in the figure below.
Following are the details of the different fields in the 802.3 and Ethernet frame types:
Preamble: an alternating 1,0 pattern provides a 5 MHz clock at the start of each packet, which allows the receiving devices to lock the incoming bit stream.
Start Frame Delimiter (SFD)/synch: the preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011, where the last pair of 1a allows the receiver to come into the alternating, 1,0 pattern somewhere in the middle and still sync up and detect the beginning of the data.
Destination address(DA): this transmits a 48-bit value using the least significant bit (LSB) first. The DA is used by receiving stations to determine whether an incoming packet is addressed to a particular node. The destination address can be an individual address or a broadcast or multicast MAC address. Remember that a broadcast is all 1s (or Fs in hex) and is sent to all devices but multicast is sent only to a similar subset of nodes on a network.
Source Address (SA): the SA is a 48-bit MAC address used to identify the transmitting device, and it uses the LSB first. Broadcast and multicast address formats are illegal within the SA field.
Length or type: 802.3 uses a length field, but the Ethernet frame uses a type field to identify the network layer protocol. 802.3 cannot identify the upper-layer protocol and must be used with a proprietary LANIPX, for example
Data: This is a packet sent down to the Data link layer from the Network layer. The size can vary from 64 to 1500 bytes.
Frame check sequence (FCS): FCS is a field at the end of the frame that’s used to store the CRC.
Let’s pause here for a minute and take a look at some frames caught on our trusty Omni Peek network analyzer. You can see that the frame below has only three fields: Destination, Source, and Type (shown as protocol type on this analyzer).
Destination: 00:60:f5:00:1f:27 Source: 00:60:f5:00:1f:2c Protocol Type: 08-00 IP
This is an Ethernet_II frame. Notice that the type field is IP, or 08-00 (mostly just referred to as 0x800) in hexadecimal.
The next frame has the same fields, so it must be an Ethernet_II frame too;
Destination: ff:ff:ff:ff:ff:ff Ethernet broadcast Source: 02:07:01:22:de:a4 Protocol Type: 08-00 IP
Did you notice that this frame was a broadcast? You can tell because the destination hardware address is all 1s is in binary, or all Fs in hexadecimal.
Let’s take a look at one more Ethernet-II frame. You can see that the Ethernet frame in IPv6 is the same Ethernet frame we use with the IPv4 routed protocol but the type field has 0x86dd when we are carrying IPv6 data, and when we have IPv4, we use 0x800 in the protocol field.
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03) Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd) Type: IPv6 (0x86dd)
This is the beauty of the Ethernet II frame. Because of the protocol field, we can run any Network layer routed protocol and it will carry the data because it can identify the Network Layer protocol.
Ethernet at the Physical Layer
Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). They created and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE 802.3 committee. This was a 10Mbps network that ran on coax and then eventually twisted pair and fiber physical media.
The IEEE extended the 802.3 committee to two new committees known as 802.3u (first Ethernet) and 802.3ab (Gigabit Ethernet on category 5) and then finally 802.3ae (10Gbps over fiber and coax).
The following Figure shows the IEEE 802.3 and the original Ethernet physical layer specifications.
When designing your LAN, it’s really important to understand the different types of etherent media available to you. Sure, it would be great to run Gibabit Ethernet to each desktop and 10Gbps between switches, and although this might happen one day, justifying the cost of that network today would be pretty difficult. But if you mix and match the different types of Ethernet media methods currently available, you can come up with a cost-effective network solution that works great.
The EIA/TIA (Electronic Industries Association and the newer Telecommunications Industry Alliance) is the standards body that creates the Physical layer specifications for Ethernet.
The EIA/TIA specifies that Ethernet uses a registered jack (RJ) connector with a 4 5 wiring sequence on unshielded twisted pair (UTP) cabling (RJ45). However, the industry is moving forward calling this just an 8-pin modular connector.
Each Ethernet cable type that is specified by the EIA/TIA has inherent attenuation, which is defined as the loss of signal strength as it travels the length of a cable and is measured in decibels (dB). The cabling used in corporate and home markets is measured in categories. A higher quality cable will have a higher rated category and lower attenuation.
For example, category 5 is better than category 3 because category 5 cables have more wire twists per foot and therefore less crosstalk. Crosstalk is the unwanted signal interference from adjacent pairs in the cable.
Here are the original IEEE 802.3 standards:
- 10Base2: 10Mbps, baseband technology, up to 185 meters in length known as thin net and can support up to 30 workstations on a single segment. Uses a physical and logical bus with AUI connectors. The 10 means 10Mbps, Base means baseband technology (which is a signaling method for communication on the network), and the 2 means almost 200 meters. 10Base2 ethernet cards use BNC (British Naval Connector, Bayonet Neill Concelman, or Bayonet Nut connector) and T-connectors to connect to a network.
- 10Base5: 10Mbps, baseband technology, up to 500 meters in length. Known as Thick Net. It uses a physical and logical bus with AUI connectors. It is up to 2500 meters with repeaters and 1024 users for all segments.
- 10BaseT: 10Mbps using category 3 UTP wiring. Unlike with the 10Base2 and 10Base5 networks, each device must connect into a hub or switch, and you can have only one host per segment or wire. Uses an RJ45 connector (8-pin modular connector) with a physical star topology and a logical bus.
Each of the 802.3 standards defines an Attachment Unit Interface (AUI), which allows a one-bit-at-a-time transfer to the physical layer from the data link media access method. This allows the MAC to remain constant but means the Physical layer can support any existing and new technologies. The original AUI interface was a 15-pin connector, which allowed a transceiver (transmitter/receiver) that provided a 15-pin-to-twisted-pair conversion.
The thing is, the AUI interface cannot support 100Mbps Ethernet because of the high frequencies involved. So 100BaseT needed a new interface, and the 802.3u specifications created one called the Media Independent Interface (MII), which provides 100Mbps throughput. The MII uses a nibble, defined as 4 bits. Gigabit Ethernet uses a Gigabit Media Independent Interface (GMII) and transmits 8 bits at a time.
802.3u (Fast Ethernet) is compatible with 802.3 Ethernet because they share the same physical characteristics. Fast Ethernet and Ethernet use the same maximum transmission unit (MTU), use the same MAC mechanisms, and preserve the frame format that is used by 10BaseT Ethernet.
Basically fast Ethernet is just based on an extension to the IEEE 802.3 specification, except that it offers a speed increase of 10 times that of 10BaseT.
Here are the expanded IEEE Ethernet 802.3 standards:
- 100BaseTX (IEEE 802.3u): EIA/TIA category 5, 6, or 7 UTP two pair wiring. One user per segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and a logical bus.
- 100BaseFX (IEEE 802.3u): uses fiber cabling 62.5/125 micron multimode fiber. Point-to-point topology; up to 412 meters long. It uses an ST or SC connector, which are media interface connectors.
- 1000BaseCX (IEEE 802.3z): copper twisted pair called Twinax (a balanced coaxial pair) that can only run up to 25 meters.
- 1000BaseT (IEEE 802.3ab): category 5, four pair UTP wiring up to 100 meters long.
- 1000BaseSX (IEEE 802.3z): MMF using 62.5 and 50-micron core; uses an 850-nanometer laser and can go up to 220 meters with 62.5 microns, 550 meters with 50 microns.
- 1000BaseLX (IEEE 802.3z): Single mode fiber that uses a 9-micron core and 1300 nanometer laser and can go from 3 kilometers to 10 kilometers.