Bab 7 Data Link Layer
BAB 7
DATA LINK
LAYER
DATA
LINK LAYER – ACCESSING THE MEDIA
1.
Data
Link Layer – Supporting & Connecting To Upper Layer Services
The
Data Link layer provides a means for exchanging data over a common local media.
The
Data Link layer performs two basic services:
-
Allows the
upper layers to access the media using techniques such as framing
-
Controls how
data is placed onto the media and is received from the media using techniques
such as media access control and error detection
As
with each of the OSI layers, there are terms specific to this layer:
Frame
- The Data Link layer PDU
Node
- The Layer 2 notation for network devices connected to a common medium
Media/medium
(physical)* - The physical means for the transfer of information between two
nodes
Network
(physical)** - Two or more nodes connected to a common medium
The
Data Link layer is responsible for the exchange of frames between nodes over
the media of a physical network.
*
It is important to understand the meaning of the words medium and media within
the context of this chapter. Here, these words refer to the material that
actually carries the signals representing the transmitted data. Media is the
physical copper cable, optical fiber, or atmosphere through which the signals
travel. In this chapter media does not refer to content programming such as
audio, animation, television, and video as used when referring to digital
content and multimedia.
**
A physical network is different from a logical network. Logical networks are
defined at the Network layer by the arrangement of the hierarchical addressing
scheme. Physical networks represent the interconnection of devices on a common
media. Sometimes, a physical network is also referred to as a network segment.
Upper Layer
Access to Media
As
we have discussed, a network model allows each layer to function with minimal
concern for the roles of the other layers. The Data Link layer relieves the
upper layers from the responsibility of putting data on the network and
receiving data from the network. This layer provides services to support the
communication processes for each medium over which data is to be transmitted.
In
any given exchange of Network layer packets, there may be numerous Data Link
layer and media transitions. At each hop along the path, an intermediary device
- usually a router - accepts frames from a medium, decapsulates the frame, and
then forwards the packet in a new frame appropriate to the medium of that
segment of the physical network.
Imagine
a data conversation between two distant hosts, such as a PC in Paris with an
Internet server in Japan. Although the two hosts may be communicating with
their peer Network layer protocols (IP for example), it is likely that numerous
Data Link layer protocols are being used to transport the IP packets over
various types of LANs and WANs. This packet exchange between two hosts requires
a diversity of protocols that must exist at the Data Link layer. Each
transition at a router could require a different Data Link layer protocol for
transport on a new medium.
Notice
in the figure that each link between devices uses a different medium. Between
the PC and the router may be an Ethernet link. The routers are connected
through a satellite link, and the laptop is connected through a wireless link
to the last router. In this example, as an IP packet travels from the PC to the
laptop, it will be encapsulated into Ethernet frame, decapsulated, processed,
and then encapsulated into a new data link frame to cross the satellite link.
For the final link, the packet will use a wireless data link frame from the
router to the laptop.
The
Data Link layer effectively insulates the communication processes at the higher
layers from the media transitions that may occur end-to-end. A packet is
received from and directed to an upper layer protocol, in this case IPv4 or
IPv6, that does not need to be aware of which media the communication will use.
Without
the Data Link layer, a Network layer protocol, such as IP, would have to make
provisions for connecting to every type of media that could exist along a
delivery path. Moreover, IP would have to adapt every time a new network
technology or medium was developed. This process would hamper protocol and
network media innovation and development. This is a key reason for using a
layered approach to networking.
The
range of Data Link layer services has to include all of the currently used
types of media and the methods for accessing them. Because of the number of
communication services provided by the Data Link layer, it is difficult to
generalize their role and provide examples of a generic set of services. For
that reason, please note that any given protocol may or may not support all
these Data Link layer services.
2.
Data
Link Layer – Controlling Transfer across Local Media
Layer
2 protocols specify the encapsulation of a packet into a frame and the
techniques for getting the encapsulated packet on and off each medium. The
technique used for getting the frame on and off media is called the media
access control method. For the data to be transferred across a number of
different media, different media access control methods may be required during
the course of a single communication.
Each
network environment that packets encounter as they travel from a local host to
a remote host can have different characteristics. For example, one network
environment may consist of many hosts contending to access the network medium
on an ad hoc basis. Another environment may consist of a direct connection
between only two devices over which data flows sequentially as bits in an
orderly way.
The
media access control methods described by the Data Link layer protocols define
the processes by which network devices can access the network media and
transmit frames in diverse network environments. A node that is an end device
uses an adapter to make the connection to the network. For example, to connect
to a LAN, the device would use the appropriate Network Interface Card (NIC) to
connect to the LAN media. The adapter manages the framing and media access
control.
At
intermediary devices such as a router, where the media type could change for
each connected network, different physical interfaces on the router are used to
encapsulate the packet into the appropriate frame, and a suitable media access
control method is used to access each link. The router in the figure has an
Ethernet interface to connect to the LAN and a serial interface to connect to
the WAN. As the router processes frames, it will use Data Link layer services
to receive the frame from one medium, decapsulate it to the Layer 3 PDU,
re-encapsulate the PDU into a new frame, and place the frame on the medium of
the next link of the network.
3.
Data
Link Layer – Creating a Frame
The
description of a frame is a key element of each Data Link layer protocol. Data
Link layer protocols require control information to enable the protocols to
function. Control information may tell:
-
Which nodes
are in communication with each other
-
When
communication between individual nodes begins and when it ends
-
Which errors
occurred while the nodes communicated
-
Which nodes
will communicate next
The
Data Link layer prepares a packet for transport across the local media by
encapsulating it with a header and a trailer to create a frame.
Unlike
the other PDUs that have been discussed in this course, the Data Link layer
frame includes:
-
Data - The
packet from the Network layer
-
Header -
Contains control information, such addressing, and is located at the beginning
of the PDU
-
Trailer -
Contains control information added to the end of the PDU
Formatting
Data for Transmission
When
data travels on the media, it is converted into a stream of bits, or 1s and 0s.
If a node is receiving long streams of bits, how does it determine where a
frame starts and stops or which bits represent the address?
Framing
breaks the stream into decipherable groupings, with control information
inserted in the header and trailer as values in different fields. This format
gives the physical signals a structure that can be received by nodes and
decoded into packets at the destination. Typical field types include:
-
Start and stop
indicator fields - The beginning and end limits of the frame
-
Naming or
addressing fields
-
Type field -
The type of PDU contained in the frame
-
Quality -
control fields
-
A data field
-The frame payload (Network layer packet)
4.
Data
Link Layer – Connecting Upper Layer Services to the Media
The
Data Link layer exists as a connecting layer between the software processes of
the layers above it and the Physical layer below it. As such, it prepares the
Network layer packets for transmission across some form of media, be it copper,
fiber, or the atmosphere.
In
many cases, the Data Link layer is embodied as a physical entity, such as an
Ethernet network interface card (NIC), which inserts into the system bus of a
computer and makes the connection between running software processes on the
computer and physical media. The NIC is not solely a physical entity, however.
Software associated with the NIC enables the NIC to perform its intermediary
functions of preparing data for transmission and encoding the data as signals
to be sent on the associated media.
Data Link Sublayers
To
support a wide variety of network functions, the Data Link layer is often
divided into two sublayers: an upper sublayer and an lower sublayer. The upper
sublayer defines the software processes that provide services to the Network
layer protocols. The lower sublayer defines the media access processes
performed by the hardware.
Separating
the Data Link layer into sublayers allows for one type of frame defined by the
upper layer to access different types of media defined by the lower layer. Such
is the case in many LAN technologies, including Ethernet.
The
two common LAN sublayers are:
-
Logical Link
Control
Logical
Link Control (LLC) places information in the frame that identifies which
Network layer protocol is being used for the frame. This information allows
multiple Layer 3 protocols, such as IP and IPX, to utilize the same network
interface and media.
-
Media Access
Control
Media
Access Control (MAC) provides Data Link layer addressing and delimiting of data
according to the physical signaling requirements of the medium and the type of
Data Link layer protocol in use.
5.
Data
Link Layer – Standards
Unlike
the protocols of the upper layers of the TCP/IP suite, Data Link layer
protocols are generally not defined by Request for Comments (RFCs). Although
the Internet Engineering Task Force (IETF) maintains the functional protocols
and services for the TCP/IP protocol suite in the upper layers, the IETF does
not define the functions and operation of that model's Network access layer.
The TCP/IP Network Access layer is the equivalent of the OSI Data Link and
Physical layers. These two layer will be discussed in separate chapters for
closer examination..
The
functional protocols and services at the Data Link layer are described by
engineering organizations (such as IEEE, ANSI, and ITU) and communications
companies. Engineering organizations set public and open standards and
protocols. Communications companies may set and use proprietary protocols to
take advantage of new advances in technology or market opportunities. Data Link
layer services and specifications are defined by multiple standards based on a
variety of technologies and media to which the protocols are applied. Some of
these standards integrate both Layer 2 and Layer 1 services.
Engineering
organizations that define open standards and protocols that apply to the Data
Link layer include:
Unlike
the upper layer protocols, which are implemented mostly in software such as the
host operating system or specific applications, Data Link layer processes occur
both in software and hardware. The protocols at this layer are implemented
within the electronics of the network adapters with which the device connects
to the physical network.
MEDIA
ACCESS CONTROL TECHNIQUES
1. Placing Daat
on the Media
Regulating
the placement of data frames onto the media is known as media access control.
Among the different implementations of the Data Link layer protocols, there are
different methods of controlling access to the media. These media access
control techniques define if and how the nodes share the media.
Media
access control is the equivalent of traffic rules that regulate the entrance of
motor vehicles onto a roadway. The absence of any media access control would be
the equivalent of vehicles ignoring all other traffic and entering the road
without regard to the other vehicles.
However,
not all roads and entrances are the same. Traffic can enter the road by
merging, by waiting for its turn at a stop sign, or by obeying signal lights. A
driver follows a different set of rules for each type of entrance.
In
the same way, there are different ways to regulate the placing of frames onto
the media. The protocols at the Data Link layer define the rules for access to
different media. Some media access control methods use highly-controlled
processes to ensure that frames are safely placed on the media. These methods
are defined by sophisticated protocols, which require mechanisms that introduce
overhead onto the network.
The
method of media access control used depends on:
-
Media sharing
- If and how the nodes share the media
-
Topology - How
the connection between the nodes appears to the Data Link layer
2.
Media
Access Control for Shared Media
Some
network topologies share a common medium with multiple nodes. At any one time,
there may be a number of devices attempting to send and receive data using the
network media. There are rules that govern how these devices share the media.
There
are two basic media access control methods for shared media:
Controlled
- Each node has its own time to use the medium
Contention-based
- All nodes compete for the use of the medium
Controlled
Access for Shared Media
When
using the controlled access method, network devices take turns, in sequence, to
access the medium. This method is also known as scheduled access or
deterministic. If a device does not need to access the medium, the opportunity
to use the medium passes to the next device in line. When one device places a
frame on the media, no other device can do so until the frame has arrived at
the destination and has been processed by the destination.
Although
controlled access is well-ordered and provides predictable throughput,
deterministic methods can be inefficient because a device has to wait for its
turn before it can use the medium.
Contention-based
Access for Shared Media
Also
referred to as non-deterministic, contention-based methods allow any device to
try to access the medium whenever it has data to send. To prevent complete
chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA)
process to first detect if the media is carrying a signal. If a carrier signal
on the media from another node is detected, it means that another device is
transmitting. When the device attempting to transmit sees that the media is
busy, it will wait and try again after a short time period. If no carrier
signal is detected, the device transmits its data. Ethernet and wireless
networks use contention-based media access control.
It
is possible that the CSMA process will fail and two devices will transmit at
the same time. This is called a data collision. If this occurs, the data sent
by both devices will be corrupted and will need to be resent.
Contention-based
media access control methods do not have the overhead of controlled access
methods. A mechanism for tracking whose turn it is to access the media is not
required. However, the contention-based systems do not scale well under heavy
media use. As use and the number of nodes increases, the probability of
successful media access without a collision decreases. Additionally, The
recovery mechanisms required to correct errors due to these collisions further
diminishes the throughput.
CSMA
is usually implemented in conjunction with a method for resolving the media
contention. The two commonly used methods are:
-
CSMA/Collision
Detection
In
CSMA/Collision Detection (CSMA/CD), the device monitors the media for the
presence of a data signal. If a data signal is absent, indicating that the
media is free, the device transmits the data. If signals are then detected that
show another device was transmitting at the same time, all devices stop sending
and try again later. Traditional forms of Ethernet use this method.
-
CSMA/Collision
Avoidance
In
CSMA/Collision Avoidance (CSMA/CA), the device examines the media for the
presence of a data signal. If the media is free, the device sends a
notification across the media of its intent to use it. The device then sends
the data. This method is used by 802.11 wireless networking technologies.
3. Media Access
Control for Non-Shared Media
Media
access control protocols for non-shared media require little or no control
before placing frames onto the media. These protocols have simpler rules and
procedures for media access control. Such is the case for point-to-point
topologies.
In
point-to-point topologies, the media interconnects just two nodes. In this
arrangement, the nodes do not have to share the media with other hosts or
determine if a frame is destined for that node. Therefore, Data Link layer
protocols have little to do for controlling non-shared media access.
Full Duplex
and Half Duplex
In
point-to-point connections, the Data Link layer has to consider whether the
communication is half-duplex or full-duplex.
Half-duplex
communication means that the devices can both transmit and receive on the media
but cannot do so simultaneously. Ethernet has established arbitration rules for
resolving conflicts arising from instances when more than one station attempts
to transmit at the same time.
In
full-duplex communication, both devices can transmit and receive on the media
at the same time. The Data Link layer assumes that the media is available for
transmission for both nodes at any time. Therefore, there is no media
arbitration necessary in the Data Link layer.
4. Logical Topology
vs Physical Topology
The
topology of a network is the arrangement or relationship of the network devices
and the interconnections between them. Network topologies can be viewed at the
physical level and the logical level. The physical topology is an arrangement
of the nodes and the physical connections between them. The representation of
how the media is used to interconnect the devices is the physical topology.
These will be covered in later chapters of this course.
A
logical topology is the way a network transfers frames from one node to the
next. This arrangement consists of virtual connections between the nodes of a
network independent of their physical layout. These logical signal paths are
defined by Data Link layer protocols. The Data Link layer "sees" the
logical topology of a network when controlling data access to the media. It is
the logical topology that influences the type of network framing and media
access control used.
The
physical or cabled topology of a network will most likely not be the same as
the logical topology. Logical topology of a network is closely related to the
mechanism used to manage network access. Access methods provide the procedures
to manage network access so that all stations have access. When several
entities share the same media, some mechanism must be in place to control
access. Access methods are applied to networks regulate this media access.
Access methods will be discussed in more detail later.
Logical
and physical topologies typically used in networks are:
-
Point-to-Point
-
Multi-Access
-
Ring
5. Point-to-Point
Topology
In
point-to-point networks, if data can only flow in one direction at a time, it
is operating as a half-duplex link. If data can successfully flow across the
link from each node simultaneously, it is a full-duplex link.
Data
Link layer protocols could provide more sophisticated media access control
processes for logical point-to-point topologies, but this would only add
unnecessary protocol overhead.
Logical
Point-to-Point Networks
The
end nodes communicating in a point-to-point network can be physically connected
via a number of intermediate devices. However the use of physical devices in
the network does not affect the logical topology. As shown in the figure, the
source and destination node may be indirectly connected to each other over some
geographical distance. In some cases, the logical connection between nodes
forms what is called a virtual circuit. A virtual circuit is a logical
connection created within a network between two network devices. The two nodes
on either end of the virtual circuit exchange the frames with each other. This occurs
even if the frames are directed through intermediary devices. Virtual circuits
are important logical communication constructs used by some Layer 2
technologies.
The
media access method used by the Data Link protocol is determined by the logical
point-to-point topology, not the physical topology. This means that the logical
point-to-point connection between two nodes may not necessarily be between two
physical nodes at each end of a single physical link.
6. Multi-Access
Topology
A
logical multi-access topology enables a number of nodes to communicate by using
the same shared media. Data from only one node can be placed on the medium at
any one time. Every node sees all the frames that are on the medium, but only
the node to which the frame is addressed processes the contents of the frame.
Having
many nodes share access to the medium requires a Data Link media access control
method to regulate the transmission of data and thereby reduce collisions
between different signals. The media access control methods used by logical
multi-access topologies are typically CSMA/CD or CSMA/CA. However, token
passing methods can also be used.
A
number of media access control techniques are available for this type of
logical topology. The Data Link layer protocol specifies the media access
control method that will provide the appropriate balance between frame control,
frame protection, and network overhead.
7. Ring Topology
In
a logical ring topology, each node in turn receives a frame. If the frame is
not addressed to the node, the node passes the frame to the next node. This
allows a ring to use a controlled media access control technique called token
passing.
Nodes
in a logical ring topology remove the frame from the ring, examine the address,
and send it on if it is not addressed for that node. In a ring, all nodes
around the ring between the source and destination node examine the frame.
There
are multiple media access control techniques that could be used with a logical
ring, depending on the level of control required. For example, only one frame
at a time is usually carried by the media. If there is no data being
transmitted, a signal (known as a token) may be placed on the media and a node
can only place a data frame on the media when it has the token.
Remember
that the Data Link layer "sees" a logical ring topology. The actual
physical cabling topology could be another topology.
MEDIA
ACCESS CONTROL ADDRESSING AND FRAMING DATA
1. Data Link
Layer Protocols – The Frame
-
Header
-
Data
-
Trailer
All
Data Link layer protocols encapsulate the Layer 3 PDU within the data field of
the frame. However, the structure of the frame and the fields contained in the
header and trailer vary according to the protocol.
The
Data Link layer protocol describes the features required for the transport of
packets across different media. These features of the protocol are integrated
into the encapsulation of the frame. When the frame arrives at its destination
and the Data Link protocol takes the frame off the media, the framing
information is read and discarded.
There
is no one frame structure that meets the needs of all data transportation
across all types of media. As shown in the figure, depending on the
environment, the amount of control information needed in the frame varies to
match the media access control requirements of the media and logical topology.
2. Framing – Role
of the Header
As
shown in the figure, the frame header contains the control information
specified by the Data Link layer protocol for the specific logical topology and
media used.
Frame
control information is unique to each type of protocol. It is used by the Layer
2 protocol to provide features demanded by the communication environment.
Typical
frame header fields include:
-
Start Frame
field - Indicates the beginning of the frame
-
Source and
Destination address fields - Indicates the source and destination nodes on the
media
-
Priority/Quality
of Service field - Indicates a particular type of communication service for
processing
-
Type field -
Indicates the upper layer service contained in the frame
-
Logical
connection control field - Used to establish a logical connection between nodes
-
Physical link
control field - Used to establish the media link
-
Flow control
field - Used to start and stop traffic over the media
-
Congestion
control field - Indicates congestion in the media
The
field names above are nonspecific fields listed as examples. Different Data
Link layer protocols may use different fields from those mentioned. Because the
purposes and functions of Data Link layer protocols are related to the specific
topologies and media, each protocol has to be examined to gain a detailed
understanding of its frame structure.
3. Addresing –
Where the Frame Goes
The
data Link layer provides addressing that is used in transporting the frame
across the shared local media. Device addresses at this layer are referred to
as physical addresses. Data Link layer addressing is contained within the frame
header and specifies the frame destination node on the local network. The frame
header may also contain the source address of the frame.
Unlike
Layer 3 logical addresses that are hierarchical, physical addresses do not
indicate on what network the device is located. If the device is moved to
another network or subnet, it will still function with the same Layer 2
physical address.
Because
the frame is only used to transport data between nodes across the local media,
the Data Link layer address is only used for local delivery. Addresses at this
layer have no meaning beyond the local network. Compare this to Layer 3, where
addresses in the packet header are carried from source host to destination host
regardless of the number of network hops along the route.
If
the packet in the frame must pass onto another network segment, the
intermediate device - a router - will decapsulate the original frame, create a
new frame for the packet, and send it onto the new segment. The new frame will
use source and destination addressing as necessary to transport the packet
across the new media.
Addressing Requirements
The
need for Data Link layer addressing at this layer depends on the logical
topology. Point-to-point topologies, with just two interconnected nodes, do not
require addressing. Once on the medium, the frame has only one place it can go.
Because
ring and multi-access topologies can connect many nodes on a common medium,
addressing is required for these typologies. When a frame reaches each node in
the topology, the node examines the destination address in the header to
determine if it is the destination of the frame.
4. Framing – Role
of the Trailer
Data
Link layer protocols add a trailer to the end of each frame. The trailer is
used to determine if the frame arrived without error. This process is called
error detection. Note that this is different from error correction. Error
detection is accomplished by placing a logical or mathematical summary of the
bits that comprise the frame in the trailer.
Frame Check
Sequence
The
Frame Check Sequence (FCS) field is used to determine if errors occurred in the
transmission and reception of the frame. Error detection is added at the Data
Link layer because this is where data is transferred across the media. The
media is a potentially unsafe environment for data. The signals on the media
could be subject to interference, distortion, or loss that would substantially
change the bit values that those signals represent. The error detection
mechanism provided by the use of the FCS field discovers most errors caused on
the media.
To
ensure that the content of the received frame at the destination matches that
of the frame that left the source node, a transmitting node creates a logical
summary of the contents of the frame. This is known as the cyclic redundancy
check (CRC) value. This value is placed in the Frame Check Sequence (FCS) field
of the frame to represent the contents of the frame.
When
the frame arrives at the destination node, the receiving node calculates its
own logical summary, or CRC, of the frame. The receiving node compares the two
CRC values. If the two values are the same, the frame is considered to have
arrived as transmitted. If the CRC value in the FCS differs from the CRC
calculated at the receiving node, the frame is discarded. There is always the
small possibility that a frame with a good CRC result is actually corrupt.
Errors in bits may cancel each other out when the CRC is calculated. Upper
layer protocols would then be required to detect and correct this data loss.
The
protocol used in the Data Link layer, will determine if error correction will
take place. The FCS is used to detect the error, but not every protocol will
support correcting the error.
5. Data Link
Layer Protocols – the Frame
In
a TCP/IP network, all OSI Layer 2 protocols work with the Internet Protocol at
OSI Layer 3. However, the actual Layer 2 protocol used depends on the logical
topology of the network and the implementation of the Physical layer. Given the
wide range of physical media used across the range of topologies in networking,
there are a correspondingly high number of Layer 2 protocols in use. Protocols that will be covered in CCNA courses
include:
-
Ethernet
-
Point-to-Point
Protocol (PPP)
-
High-Level
Data Link Control (HDLC)
-
Frame Relay
-
Asynchronous
Transfer Mode (ATM)
Each
protocol performs media access control for specified Layer 2 logical
topologies. This means that a number of different network devices can act as
nodes that operate at the Data Link layer when implementing these protocols.
These devices include the network adapter or network interface cards (NICs) on
computers as well as the interfaces on routers and Layer 2 switches.
The
Layer 2 protocol used for a particular network topology is determined by the
technology used to implement that topology. The technology is, in turn,
determined by the size of the network - in terms of the number of hosts and the
geographic scope - and the services to be provided over the network.
LAN Technology
A
Local Area Network typically uses a high bandwidth technology that is capable
of supporting large numbers of hosts. A LAN's relatively small geographic area
(a single building or a multi-building campus) and its high density of users
make this technology cost effective.
WAN Technology
However,
using a high bandwidth technology is usually not cost-effective for Wide Area
Networks that cover large geographic areas (cities or multiple cities, for
example). The cost of the long distance physical links and the technology used
to carry the signals over those distances typically results in lower bandwidth
capacity.
Ethernet Protocol for LANs
Ethernet
is a family of networking technologies that are defined in the IEEE 802.2 and
802.3 standards. Ethernet standards define both the Layer 2 protocols and the
Layer 1 technologies. Ethernet is the most widely used LAN technology and
supports data bandwidths of 10, 100, 1000, or 10,000 Mbps. The basic frame
format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across
all forms of Ethernet. However, the methods for detecting and placing data on
the media vary with different implementations.
Ethernet
provides unacknowledged connectionless service over a shared media using
CSMA/CD as the media access methods. Shared media requires that the Ethernet
packet header use a Data Link layer address to identify the source and
destination nodes. As with most LAN protocols, this address is referred to as
the MAC address of the node. An Ethernet MAC address is 48 bits and is
generally represented in hexadecimal format.
The
Ethernet frame has many fields, as shown in the figure. At the Data Link layer,
the frame structure is nearly identical for all speeds of Ethernet. However, at
the Physical layer, different versions of Ethernet place the bits onto the
media differently. Ethernet II is the Ethernet frame format used in TCP/IP
networks.
Point-to-Point
Protocol for WANs
Point-to-Point
Protocol (PPP) is a protocol used to deliver frames between two nodes. Unlike
many Data Link layer protocols that are defined by electrical engineering
organizations, the PPP standard is defined by RFCs. PPP was developed as a WAN
protocol and remains the protocol of choice to implement many serial WANs. PPP
can be used on various physical media, including twisted pair, fiber optic
lines, and satellite transmission, as well as for virtual connections.
PPP
uses a layered architecture. To accommodate the different types of media, PPP
establishes logical connections, called sessions, between two nodes. The PPP
session hides the underlying physical media from the upper PPP protocol. These
sessions also provide PPP with a method for encapsulating multiple protocols
over a point-to-point link. Each protocol encapsulated over the link
establishes its own PPP session.
PPP
also allows the two nodes to negotiate options within the PPP session. This
includes authentication, compression, and multilink (the use of multiple
physical connections).
Wireless Protocol for LANs
802.11
is an extension of the IEEE 802 standards. It uses the same 802.2 LLC and
48-bit addressing scheme as other 802 LANs, However there are many differences
at the MAC sublayer and Physical layer. In a wireless environment, the
environment requires special considerations. There is no definable physical
connectivity; therefore, external factors may interfere with data transfer and
it is difficult to control access. To meet these challenges, wireless standards
have additional controls.
The
Standard IEEE 802.11, commonly referred to as Wi-Fi, is a contention-based
system using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
media access process. CSMA/CA specifies a random backoff procedure for all
nodes that are waiting to transmit. The most likely opportunity for medium
contention is just after the medium becomes available. Making the nodes back
off for a random period greatly reduces the likelihood of a collision.
802.11
networks also use Data Link acknowledgements to confirm that a frame is
received successfully. If the sending station does not detect the
acknowledgement frame, either because the original data frame or the
acknowledgment was not received intact, the frame is retransmitted. This
explicit acknowledgement overcomes interference and other radio-related
problems.
Other
services supported by 802.11 are authentication, association (connectivity to a
wireless device), and privacy (encryption).
An
802.11 frame is shown in the figure. It contains these fields:
-
Protocol
Version field - Version of 802.11 frame in use
-
Type and
Subtype fields - Identifies one of three functions and sub functions of the
frame: control, data, and management
-
To DS field -
Set to 1 in data frames destined for the distribution system (devices in the
wireless structure)
-
From DS field
- Set to 1 in data frames exiting the distribution system
-
More Fragments
field - Set to 1 for frames that have another fragment
-
Retry field -
Set to 1 if the frame is a retransmission of an earlier frame
-
Power
Management field - Set to 1 to indicate that a node will be in power-save mode
-
More Data
field - Set to 1 to indicate to a node in power-save mode that more frames are
buffered for that node
-
Wired
Equivalent Privacy (WEP) field - Set to 1 if the frame contains WEP encrypted
information for security
-
Order field -
Set to 1 in a data type frame that uses Strictly Ordered service class (does
not need reordering)
-
Duration/ID
field - Depending on the type of frame, represents either the time, in
microseconds, required to transmit the frame or an association identity (AID)
for the station that transmitted the frame
-
Destination
Address (DA) field - MAC address of the final destination node in the network
-
Source Address
(SA) field - MAC address of the node the initiated the frame
-
Receiver
Address (RA) field - MAC address that identifies the wireless device that is
the immediate recipient of the frame
-
Transmitter
Address (TA) field - MAC address that identifies the wireless device that
transmitted the frame
-
Sequence
Number field - Indicates the sequence number assigned to the frame;
retransmitted frames are identified by duplicate sequence numbers
-
Fragment
Number field - Indicates the number for each fragment of a frame
-
Frame Body
field - Contains the information being transported; for data frames, typically
an IP packet
-
FCS field -
Contains a 32-bit cyclic redundancy check (CRC) of the frame
PUTTING IT ALL TOGETHER
1. Follow Data
Throught an Internetwork
The
figure on the next page presents a simple data transfer between two hosts
across an internetwork. We highlight the function of each layer during the
communication. For this example we will depict an HTTP request between a client
and a server.
To
focus on the data transfer process, we are omitting many elements that may
occur in a real transaction. In each step we are only bringing attention to the
major elements. Many parts of the headers are ignored, for example.
We
are assuming that all routing tables are converged and ARP tables are complete.
Additionally, we are assuming that a TCP session is already established between
the client and server. We will also assume that the DNS lookup for the WWW
server is already cached at the client. In the WAN connection between the two
routers, we are assuming that PPP has already established a physical circuit
and has established a PPP session.
A
simple data transfer between two hosts across an internetwork
Step
through the communication
·
Step 1
· Step 2
· Step 3
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Step 4
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Step 6
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Step 8
·
Step 9
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Step 10
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Step 11
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Step 12
· Step 13
· Step 13
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Step 14
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Step 15
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Step 16
The
Data Link layer refers to the ARP cache of RouterA to determine the MAC address
that is associated with the interface of Web Server. It then uses this MAC
address to build an Ethernet II frame to transport the IPv4 packet across the
local media to the server. The MAC address of the fa0/0 interface of RouterA is
used as the source MAC address, and the MAC address of the server is used as
the destination MAC address in the frame. The frame also indicates the upper
layer protocol of IPv4 with a value of 0800 in the Type field. The frame begins
with a Preamble and Start of Frame (SOF) indicator and ends with a cyclic
redundancy check in the Frame Check Sequence at the end of the frame for the
error detection. It then uses CSMA/CD to control the placing of the frame onto
the media.
·
Step 17
·
Step 18
·
Step 19
·
Step 20
·
Step 21
·
Step 22
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