n order for you to get an adequate understanding of Ethernet duplexing, you need to understand how Ethernet works at the Physical layer. This section helps familiarize you with Ethernet and its many layers, its addressing, and its protocols.
Critical Information
Ethernet is popular because it is easy to implement, easy to troubleshoot, and easy to add new technologies (like Fast Ethernet and Gigabit Ethernet) to existing network infrastructures. Ethernet uses the Data Link and Physical layer specifications, and this section of the book will give you both the Data Link and Physical-layer information you need to effectively implement, troubleshoot, and maintain an Ethernet network.
Ethernet networking uses a protocol called Carrier Sense Multiple Access with Collision Detection.
(CSMA/CD), which helps devices share bandwidth evenly without having two devices transmit at the same time on the network media.
Address Resolution Protocol
Address Resolution Protocol (ARP) is used by the IP to provide dynamic mapping of 32-bit IP addresses to 48-bit MAC addresses. The ARP cache is a table of these listings and is checked every time communication is initiated between machines in an IP network. If the required hardware address cannot be found in the cache, a broadcast is sent out to resolve it. The broadcast address of (FFFFFFFF) is already stored, but is not visible in the table. If the router receives a reply to an ARP broadcast, the previously unknown address is entered in the cache. The next time the IP address needs to be resolved to an IP address, no broadcast will be necessary. Every listing in the cache has a timestamp and a TTL (time to live). When this time expires, the entry is deleted from the table to make room for new address resolutions. Should the cache become full and no TTLs have expired yet, then the oldest entry or entries are purged.
Ethernet Frames
The Data Link layer is responsible for combining bits into bytes and bytes into frames. A frame is used at the Data Link layer to encapsulate packets. Packets are data from upper OSI layers that are encapsulated with
layer 3 information
Destination Address (DA)
Transmits a 48-bit value using the least significant bit first. Used by each receiving station to determine whether an incoming packet is addressed to its particular node. The destination address can be an individual address, a broadcast, or a multicast MAC address. Remember that a broadcast is all 1s, or Fs, in hex and is sent to all devices, where a multicast is sent only to a similar subset of nodes on a network.
Source Address (SA)
48-bit MAC address supplied by the transmitting device. It uses the least significant bit first. Broadcast and multicast address formats are illegal within the Source Address field.
Data
Packet sent down to the Data Link layer from the Network layer; 46 to 1500 bytes.
Frame Check Sequence (FCS)
A field at the end of the frame, used to store the CRC. Let’s take a look at some frames caught on our trusty Etherpeek network analyzer. The frame below has only three fields Destination, Source, and Type:
Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type:08-00 IP
This is an Ethernet frame. Notice the Type field is IP, or 08-00 in hexadecimal.
The next frame has the same fields, so it must also be an Ethernet frame. We put this one in so you could see that the frame can carry more than just IP. It can also
carry IPX, or 81-37h.
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type:81-37 NetWare
Notice that this frame was a broadcast: You can tell because the destination hardware address is all 1s in binary, or all Fs in hexadecimal.
Notice the length field in the next frame:
Flags: 0×80 802.3
Status: 0×00
Packet Length:64
Timestamp: 12:45:45.192000 06/26/1998
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 08:00:11:07:57:28
Length: 34
This must be an 802.3 frame. What protocol is this going to be handed to? It doesn’t say in the frame, so it must be IPX
SNAP Frame
The SNAP frame has its own Protocol field to identify the upper-layer protocol. This is really a way to allow an Ethernet frame to be used in an 802.3 frame. Even though the following network trace .shows a protocol field, it is really an Ethernet type (Ether-Type) field. Here is a SNAP frame:
Flags: 0×80 802.3
Status: 0×00
Packet Length:78
Timestamp: 09:32:48.264000 01/04/2000
802.3 Header
Destination: 09:00:07:FF:FF:FF AT Ph 2 Broadcast
Source: 00:00:86:10:C1:6F
LLC Length: 60
802.2 Logical Link Control (LLC) Header
Dest. SAP: 0xAA SNAP
Source SAP: 0xAA SNAP
Command: 0×03 Unnumbered Information
Protocol: 0×080007809B AppleTalk
You can identify a SNAP frame, because the DSAP and SSAP fields are always AA, plus the commandv field is always 3. The reason this frame type was created is because not all protocols worked well with the 802.3 Ethernet frame that didn’t have an Ether-Type field. To allow the proprietary protocols created by application developers to be used in the LLC frame, the IEEE then defined the SNAP format.It is not used that often, and it is mostly seen with AppleTalk and proprietary frames. Cisco uses a SNAP frame with their proprietary protocol CDP.
Full-Duplex Ethernet Design
Full-duplex Ethernet switch technology provides a point-to-point connection between the transmitter of the transmitting station and the receiver of the receiving station. With half-duplex circuitry, a standard Ethernet can usually provide only 50 to 60 percent of the bandwidth available. In contrast, full-duplex
Ethernets can provide a full 100 percent, because they can transmit and receive simultaneously and because collisions don’t occur.
In order to run a full-duplex Ethernet, you must have the following:
Two 10Mbps or 100Mbps paths
Full-duplex NICs
Loopback and collision detection disabled
Software drivers supporting two simultaneous data paths
Adherence to Ethernet distance standards
Half-Duplex Ethernet Design
Half-duplex Ethernet has been around a long time. Ethernet came out in 1984 and is still the most popular of all LAN topologies. When a station is sending to another station, the transmitting circuitry is active at the transmitting station. This circuitry uses a single cable similar to a narrow, one-way bridge.
Related posts:
- Full and Half-Duplex Ethernet
- Network Congestion Problem in Ethernet Networks
- LAN segmentation using bridges and switches
- Distance Limitations of Fast Ethernet
- Virtual LANs Exams Points
- Commands to Monitor Frame Relay Operation in the Router
- Function of the MAC Address
- Data link and Network Addresses
- Features and Benefits of Fast Ethernet
- Virtual LANs
