Sending Data in Ethernet Networks Although physical layer standards vary quite bit, other parts of the Ethernet Networks standards work the same way, no matter the sort of physical Ethernet link.Next, this final major section of this chapter looks at several protocols and rules that Ethernet uses no matter the sort of link. especially , this section examines the details of the Ethernet Networks link layer protocol, plus how Ethernet nodes, switches, and hubs forward Ethernet frames through an Ethernet LAN.
Ethernet Data-Link Protocols
One of the foremost significant strengths of the Ethernet family of protocols is that these protocols use the same data-link standard. In fact, the core parts of the data-link standard date back to the original Ethernet standards Sending Data in Ethernet Networks.
The Ethernet data-link protocol defines the Ethernet frame: an Ethernet header at the front, the encapsulated data within the middle, and an Ethernet trailer at the end. Ethernet actually defines a couple of alternate formats for the header, with the frame format shown in Figure 2-14 being commonly used today.
While all the fields in the frame matter, some matter more to the topics discussed in this book. Table 2-4 lists the fields in the header and trailer and a brief description for reference, with the upcoming pages including more detail about a few of these fields.Table 2-4 IEEE 802.3 Ethernet Header and Trailer Fields.
The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of 1500 bytes. The term maximum transmission unit (MTU) defines the maximum Layer 3 packet which will be sent over a medium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is that the largest IP MTU allowed over an Ethernet .
Ethernet Addressing
The source and destination Ethernet address fields play an enormous role in how Ethernet LANs work. the overall idea for every is relatively simple: The sending node puts its own address within the source address field and therefore the intended Ethernet destination device’s address within the destination address field. The sender transmits the frame, expecting that the Ethernet LAN, as an entire , will deliver the frame thereto correct destination. Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long (48-bitlong) binary numbers. For convenience, most computers list MAC addresses as 12-digit hexadecimal numbers. Cisco devices typically add some periods to the number for easier readability as well; for instance , a Cisco switch might list a MAC address as 0000.0C12.3456. Most MAC addresses represent one NIC or other Ethernet port, so these addresses are often called a unicast Ethernet address. The term unicast is just a formal way to refer to the fact that the address represents one interface to the Ethernet LAN. (This term also contrasts with two other sorts of Ethernet addresses, broadcast and multicast, which can be defined later during this section.)
The entire idea of sending data to a destination unicast MAC address works well, but it works only if all the unicast MAC addresses are unique. If two NICs tried to use the same MAC address, there might be confusion. (The problem would be just like the confusion caused to the postal service if you and that i both tried to use the same mailing address—would the mail deliver mail to your house or mine?) If two PCs on the same Ethernet tried to use the same MAC address, to which PC should frames sent thereto MAC address be delivered? Ethernet solves this problem using an administrative process in order that , at the time of manufacture, all Ethernet devices are assigned a universally unique MAC address. Before a manufacturer can build Ethernet products, it must ask the IEEE to assign the manufacturer a universally unique 3-byte code, called the organizationally unique identifier (OUI). The manufacturer agrees to give all NICs (and other Ethernet products) a MAC address that begins with its assigned 3-byte OUI. The manufacturer also assigns a unique value for the last 3 bytes, variety that manufacturer has never used thereupon OUI. As a result, the MAC address of each device within the universe is unique.
Also Read:- Ethernet LAN Overview and Types of Ethernet LAN
Ethernet addresses go by many names: LAN address, Ethernet address, hardware address, burned-in address, physical address, universal address, or MAC address. for instance , the term burned-in address (BIA) refers to the thought that a permanent MAC address has been encoded (burned into) the ROM chip on the NIC. As another example, the IEEE uses the term universal address to emphasize the very fact that the address assigned to a NIC by a manufacturer should be unique among all MAC addresses within the universe. additionally to unicast addresses, Ethernet also uses group addresses. Group addresses identify more than one LAN interface card. A frame sent to a group address could be delivered to a small set of devices on the LAN, or maybe to all or any devices on the LAN. In fact, the IEEE defines two general categories of group addresses for Ethernet:
Broadcast address: Frames sent to this address should be delivered to all or any devices on the
Ethernet LAN. it’s a value of FFFF.FFFF.FFFF.
Multicast addresses: Frames sent to a multicast Ethernet address will be copied and forwarded to a subset of the devices on the LAN that volunteers to receive frames sent to a specific multicast address.
Table 2-5 summarizes most of the details about MAC addresses.
Identifying Network Layer Protocols with the Ethernet Type Field
While the Ethernet header’s address fields play an important and more obvious role in Ethernet LANs, the Ethernet Type field plays a much less obvious role. The Ethernet Type field, or EtherType, sits within the Ethernet Networks data link layer header, but its purpose is to directly help the network processing on routers and hosts. Basically, the sort field identifies the sort of network layer (Layer 3) packet that sits inside the Ethernet frame. First, think about what sits inside the data a part of the Ethernet frame shown earlier in Figure 2-14. Typically, it holds the network layer packet created by the network layer protocol on some device within the network. Over the years, those protocols have included IBM Systems network architecture (SNA), Novell NetWare, Digital Equipment Corporation’s DECnet, and Apple Computer’s AppleTalk. Today, the foremost common network layer protocols are both from TCP/IP: IP version 4 (IPv4) and IP version 6 (IPv6). the original host has a place to insert a worth (a hexadecimal number) to spot the sort of packet encapsulated inside the Ethernet Networks frame. However, what number should the sender put within the header to spot an IPv4 packet because the type? Or an IPv6 packet? because it turns out, the IEEE manages a list of EtherType values, so that every network layer protocol that needs a unique EtherType value can have variety . The sender just has got to know the list. (Anyone can view the list; just go to www.ieee.org and search for EtherType.) for instance , a host can send one Ethernet frame with an IPv4 packet and the next Ethernet frame with an IPv6 packet. Each frame would have a different Ethernet Type field value, using the values reserved by the IEEE, as shown in Figure 2-16.
Error Detection with FCS
Ethernet Networks also defines a way for nodes to find out whether a frame’s bits changed while crossing over an Ethernet link. (Usually, the bits could change due to some kind of electrical interference, or a nasty NIC.) Ethernet, like most data-link protocols, uses a field within the data-link trailer for the purpose of error detection.
The Ethernet Frame Check Sequence (FCS) field within the Ethernet trailer—the only field within the Ethernet trailer—gives the receiving node a way to compare results with the sender, to discover whether errors occurred within the frame. The sender applies a complex math formula to the frame before sending it, storing the results of the formula within the FCS field. The receiver applies the same math formula to the received frame. The receiver then compares its own results with the sender’s results. If the results are the same, the frame didn’t change; otherwise, an error occurred and therefore the receiver discards the frame. Note that error detection doesn’t also mean error recovery. Ethernet Networks defines that the error frame should be discarded, but Ethernet doesn’t attempt to recover the lost frame. Other protocols, notably TCP, recover the lost data by noticing that it’s lost and sending the data again Sending Data in Ethernet Networks.
Questions related to this topic
- Which type of address is used in an Ethernet frame header?
- What protocols are in the Ethernet frame?
- What is the MAC address that is used to send a frame to every local Ethernet device?
- What does the Preamble field of a frame header contain?
- What is Sending Data in Ethernet Networks ?
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