CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 30 potx

As this data propagates along the network medium, the NIC in each device on the network checks to see if its MAC address matches the physical des-tination address carried by the data fra

source device sends data out on a network, the data carries the MAC address of its

intended destination As this data propagates along the network medium, the NIC in

each device on the network checks to see if its MAC address matches the physical

des-tination address carried by the data frame If there is no match, the NIC discards the

data frame

If there is a match, the NIC verifies the destination address in the frame header to

determine whether the packet is properly addressed When the data passes its

destina-tion stadestina-tion, the NIC for that stadestina-tion makes a copy, takes the data out of the envelope,

and gives the data to the computer to be processed by upper-layer protocols such as IP

and TCP

Framing in General

Encoded bit streams on physical media represent a tremendous technological

accom-plishment, but they alone are not enough to make communication happen Framing

helps obtain essential information that could not otherwise be obtained with coded bit

streams alone Examples of such information are listed here:

■ Which computers are communicating with one another

■ When communication between individual computers begins and when it terminates

■ Recognition of errors that occur during the communication

■ Whose turn it is to “talk” in a computer “conversation”

■ Where the data is located within the frame

When you have a way to identify computers, you can move on to framing Framing is

the Layer 2 encapsulation process A frame is the Layer 2 protocol data unit Figure 5-5

illustrates the ideas of bits and frames

When you work with bits, the most accurate diagram you can use to visualize them is

a graph showing voltage versus time However, because you usually deal with larger

units of data and addressing and control information, this type of graph could become

ridiculously large and confusing Another type of diagram you could use is the frame

format diagram, which is based on voltage versus time graphs You read them from

left to right, just like an oscilloscope graph The frame format diagram shows different

groupings of bits (fields); certain functions are associated with the different fields

Figure 5-5 From Bits to Frame

Many different types of frames are described by various standards A single generic

frame has sections called fields, and each field is composed of bytes (see Figure 5-6) The names of the fields commonly found in a data link layer frame are as follows:

■ Frame Start field

■ Address field

■ Length/Type/Control field

■ Data field

■ Frame Check Sequence (FCS) field

Figure 5-6 A Generic Frame Format

The sections that follow describe the different frame fields in more detail

Bit V

t 1

Byte V

0 1 0 1 0 1 1

t

First Bit Sent Last Bit Sent

First Byte Sent Last Byte Sent

First Bytes Sent Last Bytes Sent

A, B, C, D, E, F Multiple, Often Many, Bytes

Bitstream

of Saved Bytes

Field Names

Start Frame Field

Address Field Length/

Type/

Control Field

Data Field

FCS Field

Frame Start Field

When computers are connected to a physical medium, there must be a way for them to

grab the attention of other computers to broadcast the message, as in, “Here comes a

frame!” Various technologies have different ways of doing this process, but, regardless

of technology, all frames have a beginning signaling sequence of bytes

Address Field

All frames contain identification information, such as the address of the source

com-puter (MAC address) and the address of the destination comcom-puter (MAC address)

Length and Type Fields

Most frames have some specialized fields In some technologies, a Length field specifies

the exact length of a frame Some have a Type field, which specifies the Layer 3

proto-col making the sending request There is also a set of technologies in which no such

fields are used

Data Field

The reason for sending frames is to get higher-layer data, ultimately the user

applica-tion data, from the source computer to the destinaapplica-tion computer The data package

that you want to deliver includes the message that you want to send (the data)

Pad-ding bytes are added sometimes so that the frames have a minimum length Logical

Link Control (LLC) bytes also are included with the data field in the IEEE standard

frames Remember that the LLC sublayer adds control information to the network

protocol data, a Layer 3 packet, to help deliver that packet to its destination Layer 2

communicates with upper layers through LLC

Frame Check Sequence Field

All frames (and the bits, bytes, and fields contained within them) are susceptible to

errors from a variety of sources The Frame Check Sequence (FCS) field contains a

number based on the data in the frame; this number is calculated by the source

com-puter When the destination computer receives the frame, it recalculates the FCS

num-ber and compares it with the FCS numnum-ber included in the frame If the two numnum-bers

are different, an error is assumed and the frame is discarded

The Frame Check Sequence number normally is calculated through the use of a cyclic

redundancy check (CRC), which performs polynomial calculations on the data

Ethernet Frame Structure

At the MAC sublayer, the frame structure is nearly identical for all speeds of Ethernet (10/100/1,000/10,000 Mbps) Half-duplex Gigabit Ethernet 1000BASE-T and the “W” versions of 10-Gb Ethernet have certain timing issues that require minor differences in how the interframe spacing is handled by the MAC sublayer, but these are otherwise the same as the other speeds However, at the physical layer, almost all versions of Ethernet are substantially different from one another, and each speed has its own set

of architecture design rules

IEEE 802.3 Ethernet Frame

In addition to the 802.2 frame type discussed previously, there is a simpler IEEE 802.3 frame type, which was the first developed by the IEEE As with 802.2, however, it is not used widely in today’s Ethernet LANs Figure 5-7 shows the basic IEEE 802.3 Ethernet frame format

Figure 5-7 IEEE 802.3 Ethernet Frame Structure

Table 5-2 lists the octet number and name for each 802.3 Ethernet frame field

Table 5-2 IEEE 802.3 Ethernet Frame Fields

Octets in Each Frame Field Frame Field

hexadecimal—otherwise, protocol Type)

Preamble 7

SFD 1

Destination 6

Source 6

Length/

Type 2

FCS 4

Data Pad

46 to 1500 FCS Calculation

Ethernet II Frame

In the DIX version of Ethernet that was developed before the adoption of the IEEE

802.3 version of Ethernet, the Preamble and Start Frame Delimiter (SFD) fields were

combined into a single field, although the binary pattern was identical The field labeled

Length/Type was listed only as Length in the early IEEE versions and only as Type in

the DIX version These two uses of the field officially were combined in a later IEEE

version because both uses of the field were common throughout the industry The early

DIX Ethernet frame format, also know as Ethernet Version2 or Ethernet II, is the most

commonly used frame type with TCP/IP–based Ethernet LANs Figure 5-8 illustrates

the Ethernet II frame format

Figure 5-8 Ethernet II Frame Format

Table 5-3 lists the octet number and name for each Ethernet II frame field

As indicated in Table 5-3, use of the Ethernet II Type field is incorporated into the

cur-rent 802.3 frame definition Upon receipt, a station must determine which higher-layer

protocol is present in an incoming frame This first is attempted by examining the

Length/Type field If the two-octet value is equal to or greater than 600 hex, the frame

is interpreted according to the Ethernet II type code indicated If it is less than 600 hex,

the frame is interpreted as an 802.3 frame and the length of the frame is indicated

Further investigation is required to determine how to proceed To proceed from here,

the first four octets of the 802.3 Data field are examined The value found in those first

Table 5-3 Ethernet II Frame Fields

Octets in Each Frame Field Frame Field

pad must be added to the end)

Preamble 8

Destination 6

Source 6

Type 2

FCS 4 Data Pad

46 to 1500

four octets usually is checked for two unique values; if they are not present, the frame

is assumed to be an 802.2 Logical Link Control (LLC) sublayer encapsulation and is decoded according to the 802.2 LLC encapsulation indicated One of the two values tested for is AAAA in hexadecimal, which indicates an 802.2/802 (Subnetwork Access Protocol [SNAP]) encapsulation The other value tested for is FFFF in hexadecimal, which might indicate an old Novell Internetwork Packet Exchange (IPX) “raw” encapsulation

Ethernet Frame Fields

The following list shows most of the fields permitted or required in an 802.3 Ethernet Frame Refer back to Figure 5-8 for an illustration of the 802.3 Ethernet frame:

■ Preamble—This field contains an alternating pattern of 1s and 0s that was used

for timing synchronization in the asynchronous 10-Mbps and slower implemen-tations of Ethernet Faster versions of Ethernet are synchronous, so this timing information is redundant but is retained for compatibility The preamble is seven octets in length and is represented by the following binary pattern:

10101010 10101010 10101010 10101010 10101010 10101010 10101010

■ Start Frame Delimiter (SFD)—This a one-octet field that marks the end of the

timing information It is represented by the binary pattern 10101011 In the early DIX form of Ethernet, this octet was the last in the eight-octet preamble Although the old DIX Ethernet described the first eight octets differently than the IEEE Ethernet, the pattern and usage is identical Also, the timing information represented by the preamble and SFD is discarded and is not counted toward the minimum and maximum frame sizes

■ Destination Address—This field contains the six-octet MAC destination address

The destination address can be a unicast (single node), multicast (group of nodes),

or broadcast address (all nodes)

■ Source Address—This field contains the six-octet MAC source address The source

address is supposed to be only the unicast address of the transmitting Ethernet station However, there are an increasing number of virtual protocols in use, which use and sometimes share a specific source MAC address to identify the virtual entity

In the early Ethernet specifications, MAC addresses were optionally two or six octets, as long as the size was constant throughout the broadcast domain Two-octet addressing first was excluded explicitly in paragraph 3.2.3 in the 1998 version

of 802.3 and no longer is supported in 802.3 Ethernet

■ Length/Type—If the value is less than 1536 decimal (0600 hexadecimal), value

indicates Length The Length interpretation is used where the LLC layer provides the protocol identification

— Type (Ethernet)—The Type specifies the upper-layer protocol to receive the

data after Ethernet processing is complete

— Length (IEEE 802.3)—The Length indicates the number of bytes of data

that follows this field If the value is equal to or greater than 1536 decimal (0600 hexadecimal), the value indicates Type, and the contents of the Data field are decoded per the protocol indicated A list of common Ethertype protocols is found in RFC 1700, beginning around page 168

■ Data and Pad—This field can be of any length that does not cause the frame to

exceed the maximum frame size The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size The content of this field is unspecified An unspecified pad is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length

The frame structure figures depict the Data field as being between 46 and 1500 octets In fact, Ethernet does not specify this The frame is required to be not less than 64 octets or more than 1518 octets, without actually specifying the size of the data field This left the user to calculate the size of the data field by subtracting all the other fields from the frame size If the currently required six octet MAC addresses are used, the data field size will be between 46 (padded, if necessary) and 1500 octets

— Data (IEEE 802.3)—After physical layer and data link layer processing is

complete, the data is sent to an upper-layer protocol, which must be defined within the data portion of the frame If data in the frame is insufficient

to fill the frame to its minimum 64-byte size, padding bytes are inserted to ensure at least a 64-byte frame

■ Frame Check Sequence (FCS)—This sequence contains a 4-byte CRC value that

is created by the sending device and is recalculated by the receiving device to check for damaged frames The mathematical result of a cyclic redundancy check (CRC) algorithm is placed in this four-octet field The sending station calculates the checksum for the transmitted frame, and the resulting four-octet value is appended following the Data/Pad Receiving station(s) perform the same calcula-tion and compare the new checksum against the checksum found at the end of the transmitted frame If the two match, the frame is good The fields used in the calculation include everything from the beginning of the destination address to the end of the Data/Pad as shown in Figure 5-7 The preamble, shown in Figure 5-8,

illustrates that SFD and Extension fields are not included in the calculation The FCS is the only Ethernet field transmitted in noncanonical order (MSB first) Because the corruption of a single bit anywhere from the beginning of the desti-nation address through the end of the FCS field causes the checksum to be differ-ent, the coverage of the FCS includes itself It is not possible to distinguish between corruption of the FCS itself and corruption of any preceding field used in the calculation

Ethernet Operation

When multiple stations (nodes) must access physical media and other networking devices, various media access control strategies are used This section briefly reviews the access-control strategies and focuses on Ethernet access access-control method—CSMA/CD

It should be noted that although CSMA/CD has immense historical importance and practical importance in original Ethernet, it is diminishing somewhat in implementa-tion for two reasons:

■ When four-pair UTP is used, separate wire pairs for transmission (Tx) and recep-tion (Rx) exist making, copper UTP potentially free from collisions and capable

of full-duplex operation, depending on whether it is deployed in a shared (hub)

or switched environment

■ Similar logic applies to optical fiber links, where separate optical paths—a trans-mission fiber and an reception fiber—are used

One new form of Ethernet—1000BASE-TX, Gigabit Ethernet over copper—uses all four wire pairs simultaneously in both directions, resulting in a permanent collision In older forms of Ethernet, such a permanent collision, preclude the system from working Yet in 1000BASE-TX, sophisticated circuitry can accommodate this permanent colli-sion, resulting from an attempt to get as much data as possible over UTP

Media Access Control

Media Access Control (MAC) refers to protocols that determine which computer on a shared-medium environment (collision domain) is allowed to transmit the data MAC, with LLC, comprises the IEEE version of Layer 2 MAC and LLC are both sublayers of Layer 2 Two broad categories of MAC exist:

■ Deterministic (taking turns)

■ Nondeterministic (first come, first served) Token Ring and FDDI are deterministic, and Ethernet/802.3 is nondeterministic (also called probabilistic)

Deterministic MAC Protocols

Deterministic MAC protocols use a form of taking turns Token passing is example

of the deterministic MAC protocol Some Native American tribes used the custom of

passing a “talking stick” during gatherings Whoever held the talking stick was allowed

to speak When that person finished, he or she passed it to another person

In this analogy, the shared medium is the air, the data is the words of the speaker, and

the protocol is possession of the talking stick The stick might even be called a token

This situation is similar to a data link protocol called a Token Ring In a Token Ring

network, individual hosts are arranged in a ring, as shown in Figure 5-9 A special data

token circulates around the ring When a host wants to transmit, it seizes the token,

transmits the data for a limited time, and then places the token back in the ring, where

it can be passed along, or seized, by another host

Figure 5-9 A Token Ring Network

Nondeterministic MAC Protocols

Nondeterministic MAC protocols use a first-come, first-served (FCFS) approach

CSMA/CD is an example of a nondeterministic MAC protocol

To use this shared-medium technology, Ethernet allows the networking devices to

arbi-trate for the right to transmit Stations on a CSMA/CD network listen for quiet, at which

time it’s okay to transmit However, if two stations transmit at the same time, a collision

occurs and neither station’s transmission succeeds All other stations on the network

also hear the collision and wait for silence The transmitting stations, in turn, each wait a

random period of time (a backoff period) before retransmitting, thus minimizing the

probability of a second collision

Token Ring

Three Specific Topological Implementations and Their MACs Three well-known Layer 2 technologies are Token Ring, FDDI, and Ethernet Of these, Ethernet is by far the most common; however, they all serve to illustrate a different approach to LAN requirements All three specify Layer 2 elements (for example, LLC, naming, framing, and MAC), as well as Layer 1 signaling components and media issues The specific technologies for each are as follows:

■ Ethernet—Logical bus topology (information flow on a linear bus) and physical

star or extended star (wired as a star)

■ Token Ring—Logical ring topology (in other words, information flow controlled

in a ring) and a physical star topology (in other words, wired as a star)

■ FDDI—Logical ring topology (information flow controlled in a ring) and physical dual-ring topology (wired as a dual-ring)

Ethernet MAC

Ethernet is a shared-media broadcast technology The access method CSMA/CD used

in Ethernet performs three functions:

■ Transmitting and receiving data packets

■ Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model

■ Detecting errors within data packets or on the network

In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode (CS = carrier sense) With shared Ethernet, this means that when a device wants to send data, it first must check

to see whether the networking medium is busy The device must check whether there are any signals on the networking media After the device determines that the network-ing media is not busy, the device begins to transmit its data While transmittnetwork-ing its data

in the form of signals, the device also listens, to ensure that no other stations are trans-mitting data to the networking medium at the same time If two stations send data at the same time, a collision occurs, as shown in the upper half of Figure 5-10 When it completes transmitting its data, the device returns to listening mode With traditional shared Ethernet, only one device can transmit at a time This is not true with switched Ethernet, which is covered in Chapter 6