ch07.htm

this describes managing multivendor networks

	<LI><I>Destination Service Access Point (DSAP).</I> The DSAP is a seven-bit address
	with an eighth bit to indicate if it is a specific address (0) or a group (broadcast)
	address (1). The DSAP is not a station or device address; rather it designates the
	service control point where the message should be routed.<BR>
	<BR>
	
	<LI><I>Source Service Access Point (SSAP).</I> The SSAP is also a seven-bit address,
	but in this case the eighth bit is used to indicate if the message is a command (0)
	or a response (1). Like the DSAP, the SSAP designates a control point and not a station
	address. In the case of the SSAP, this is the control point from which the message
	originated.<BR>
	<BR>
	
	<LI><I>Control.</I> The control field is either 8 or 16 bits long, with the length
	indicated by the first two bits. The 16-bit fields are used to exchange sequence
	numbers, while the 8-bit variation is used for unsequenced information.
</UL>

<P>Below the 802.2 LLC are the MACs for the various physical LAN implementations.
These standards are known as 802.3 for CSMA/CD, 802.4 for token passing bus, and
802.5 for token passing ring.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig02.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig02.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig02.gif"><B>FIG. 7.2</B></A> <I>IEEE LLC and MAC Layers</I>
<H3><A NAME="Heading8"></A><FONT COLOR="#000077">IEEE 802.3</FONT></H3>
<P>The IEEE 802.3 standard specifies a CSMA&#160;/CD bus network that supports 10-Mbps
transmission over baseband, broadband, and twisted pair cable. This networking standard
closely resembles Ethernet. Both HP and IBM (and others) support the IEEE 802.3 networking
standards (HP for their native NS networking product, and IBM for their TCP/IP products).</P>
<P>The 802.3 header (see Figure 7.4) includes the following:

<UL>
	<LI><I>Preamble.</I> An 8-byte pattern of binary 1s and 0s used to establish synchronization.
	The last bit of the preamble is always 0.<BR>
	<BR>
	
	<LI><I>Start Frame Delimiter.</I> An 8-bit pattern indicating the formal start of
	the frame.<BR>
	<BR>
	
	<LI><I>Destination Address.</I> An address specifying a specific destination station,
	a group of stations, or all stations in the LAN. This address can be 16 bits or 48
	bits in length, but all stations in the LAN must adhere to one format or the other.<BR>
	<BR>
	
	<LI><I>Source Address.</I> The address of the originating station. This address has
	the same length requirements as the Destination Address.<BR>
	<BR>
	
	<LI><I>Length.</I> The length, measured in bytes, of the actual data, including the
	802.2 header. This is a 16-bit field.
</UL>

<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig03.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig03.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig03.gif"><B>FIG. 7.3</B></A> <I>IEEE 802.2 LLC Header</I> </P>

<P>Following the header is the 802.2 header and the actual data. At the end of the
data is the 802.3 trailer, which includes:

<UL>
	<LI><I>Padding.</I> Extra, nondata bytes can be inserted into the frame to make the
	overall frame length more palatable to the physical network.<BR>
	<BR>
	
	<LI><I>Frame Check Sequence.</I> At the end of the frame is a 32-bit Cyclic Redundancy
	Check (CRC) on the data starting with the destination address and terminating at
	the end of the data (not including any padding).
</UL>

<H3><A NAME="Heading9"></A><FONT COLOR="#000077">IEEE 802.4</FONT></H3>
<P>The IEEE 802.4 specification defines a token passing bus that can operate at speeds
of 1, 5, or 10 Mbps. The 802.4 standard is, in many ways, a marriage of Ethernet
and token ring technologies. The physical topology for 802.4 is a bus, much like
in Ethernet, but the MAC-level discipline is a token-passing logical ring (as opposed
to a token-passing physical ring). Although the 802.4 specification does not have
as many active supporters as the 802.3 and 802.5 standards, its popularity is rapidly
growing. The format for 802.4 transmissions (see Figure 7.5) is as follows:

<UL>
	<LI><I>Preamble.</I> One or more bytes used for synchronization patterns.<BR>
	<BR>
	
	<LI><I>Start Frame Delimiter.</I> An 8-bit pattern signaling the start of the frame.<BR>
	<BR>
	
	<LI><I>Frame Control.</I> A 1-byte field used to indicate if the frame contains actual
	data or if it is a control message.<BR>
	<BR>
	
	<LI><I>Destination Address.</I> An address specifying a specific destination station,
	a group of stations, or all stations in the LAN. This address can be 16 bits or 48
	bits in length, but all stations in the LAN must adhere to one format or the other.<BR>
	<BR>
	
	<LI><I>Source Address.</I> The address of the originating station. This address has
	the same length requirements as the destination address.
</UL>

<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig04.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig04.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig04.gif"><B>FIG. 7.4</B></A><B> </B><I>IEEE 802.3 CSMA/CD Frame</I>
</P>

<P>Following this header is the 802.2 header and the actual data. At the end of the
data is the 802.4 trailer, which includes the following:

<UL>
	<LI><I>Frame Check Sequence.</I> At the end of the frame is a 32-bit Cyclic Redundancy
	Check (CRC) on the data starting with the Frame Control field and terminating at
	the end of the data.<BR>
	<BR>
	
	<LI><I>End Delimiter.</I> The 8-bit pattern signaling the end of the frame. The last
	two bits of this field signal if the frame is the last frame to be transmitted and
	whether any station has detected an error in the frame.
</UL>

<H3><A NAME="Heading10"></A><FONT COLOR="#000077">IEEE 802.5</FONT></H3>
<P>The IEEE 802.5 standard specifies a token passing ring operating over shielded
twisted pair cables at speeds of 1, 4, or 16 Mbps. This standard is supported by
IBM in its Token Ring implementation. The 802.5 construction (see Figure 7.6) is
defined as follows:

<UL>
	<LI><I>Start Frame Delimiter.</I> An 8 bit-pattern signaling the start of the frame.<BR>
	<BR>
	
	<LI><I>Access Control.</I> An 8-bit field used for priority and maintenance control.
	Most important, one bit of this field is the token bit. If set to 1, the frame contains
	data. If set to 0, the frame is actually a token that can be seized by a station
	waiting to transmit. Also note that when the token bit is set to 0, the entire frame
	consists only of the start frame delimiter, the access control byte and the end delimiter
	byte.<BR>
	<BR>
	
	<LI><I>Frame Control.</I> A 1-byte field used to indicate if the frame contains actual
	data or a control message.<BR>
	<BR>
	
	<LI><I>Destination Address.</I> An address specifying a specific destination station,
	a group of stations, or all stations in the LAN. This address can be 16 bits or 48
	bits in length, but all stations in the LAN must adhere to one format or the other.<BR>
	<BR>
	
	<LI><I>Source Address.</I> The address of the originating station. This address has
	the same length requirements as the destination address.
</UL>

<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig05.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig05.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig05.gif"><B>FIG. 7.5</B></A> <I>IEEE 802.4 Token Bus Frame</I></P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig06.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig06.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig06.gif"><B>FIG. 7.6</B></A> <I>IEEE 802.5 Token-Ring Frame</I></P>
<P>Following this header is the 802.2 header and the actual data. At the end of the
data is the 802.5 trailer that includes the following:

<UL>
	<LI><I>Frame Check Sequence.</I> At the end of the frame is a 32-bit Cyclic Redundancy
	Check (CRC) on the data starting with the Frame Control field and terminating at
	the end of the data.<BR>
	<BR>
	
	<LI><I>End delimiter.</I> The 8-bit pattern signaling the end of the frame. The last
	two bits of this field signal if the frame is the last frame to be transmitted and
	whether any station has detected an error in the frame.<BR>
	<BR>
	
	<LI><I>Frame Status. </I>An 8-bit pattern indicating whether a station has recognized
	the frame and also if the frame has been copied (received).
</UL>

<H3><A NAME="Heading11"></A><FONT COLOR="#000077">802.3 Versus Ethernet</FONT></H3>
<P>The implementations of Ethernet and IEEE 802.3 are so compatible that computer
systems using each can coexist on the same network. The most significant difference
between the two is the way information is formatted into frames. Although both specifications
define the destination and origin of the information, the 802.3 frame includes significantly
more detail.</P>
<P>The Ethernet frame begins in the same fashion as the 802.3 frame with a preamble,
start delimiter, and then the destination and source addresses (see Figure 7.7&#160;).
The similarity stops here, because in Ethernet these addresses are followed by a
type field, which identifies which Ethernet service the frame applies to. However,
because the headers are so similar, these frames can coexist on the same LAN without
interfering with one another (&#160;providing that the 802.3 frame uses 48-bit addresses
as does Ethernet).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig07.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch07/07fig07.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig07.gif"><B>FIG. 7.7</B></A> <I>Comparison of Ethernet and 802.2/802.3
Frame Formats</I>


<BLOCKQUOTE>
	<P>
<HR>
<B><font color=#000077>NOTE:</font> </B>Ethernet Version 1.0 does not have the same level of compatibility
	with IEEE 802.3 as Ethernet II. Specifically, the primary difference is that Ethernet
	II and IEEE 802.3 both include a &quot;heartbeat&quot; function performed by the
	transceivers (units that attach computer and computer equipment to the physical LAN)
	to signal their ongoing operation (the absence of a heartbeat signals a failed or
	failing transceiver). 
<HR>


</BLOCKQUOTE>

<H2><A NAME="Heading12"></A><FONT COLOR="#000077">Wide Area Networks</FONT></H2>
<P>At a basic level, a WAN can be created by tying a series of simple, point-to-point
links together. On the other end of the spectrum, a WAN might comprise many different
systems and LANs, all interconnected using a variety of techniques, including standard
telephone lines, packet-switching networks and ISDN links. Between the two extremes
are networks that are superficially simple but technically complex, and those that
are superficially complex but technically simple.</P>