The Data Link Layer and IEEE
A D V E R T I S E M E N T
The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated �DLL� (though that abbreviation has other meanings as well in the computer world). The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (�wireless Ethernet� or �Wi-Fi�) are all sometimes called �data link layer technologies�. The set of devices connected at the data link layer is what is commonly considered a simple �network�, as opposed to an internetwork.
When we talk about Local Area Network (LAN) technology the IEEE 802
standard may be heard. This standard defines networking connections for
the interface card and the physical connections, describing how they are
done. The 802 standards were published by the Institute of Electrical
and Electronics Engineers (IEEE). The 802.3 standard is called ethernet,
but the IEEE standards do not define the exact original true ethernet
standard that is common today. There is a great deal of confusion
caused by this. There are several types of common ethernet frames. Many
network cards support more than one type.
The ethernet standard data encapsulation method is defined by RFC
894. RFC 1042 defines the IP to link layer data encapsulation for
networks using the IEEE 802 standards. The 802 standards define the two
lowest levels of the seven layer network model and primarily deal with
the control of access to the network media. The network media is the
physical means of carrying the data such as network cable. The control
of access to the media is called media access control (MAC). The 802
standards are listed below:
Data Link Layer Functions
The following are the key tasks performed at the data link layer:
Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.
Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.
Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.
Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.
Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.
- 802.1 - Internetworking
- 802.2 - Logical Link Control *
- 802.3 - Ethernet or CSMA/CD, Carrier-Sense Multiple Access with
Collision detection LAN *
- 802.4 - Token-Bus LAN *
- 802.5 - Token Ring LAN *
- 802.6 - Metropolitan Area Network (MAN)
- 802.7 - Broadband Technical Advisory Group
- 802.8 - Fiber-Optic Technical Advisory Group
- 802.9 - Integrated Voice/Data Networks
- 802.10 - Network Security
- 802.11 - Wireless Networks
- 802.12 - Demand Priority Access LAN, 100 Base VG-AnyLAN
*The Ones with stars should be remembered in order for network
certification testing. |
Network Access Methods
There are various methods of managing access to a network. If all network
stations tried to talk at once, the messages would become unintelligible, and no
communication could occur. Therefore a method of being sure that stations
coordinate the sending of messages must be achieved. There are several methods
listed below which have various advantages and disadvantages.
- Contention
- Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) -
Used by Ethernet
- Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)
- Token Passing - A token is passed from one computer to another, which
provides transmission permission.
- Demand Priority - Describes a method where intelligent hubs control data
transmission. A computer will send a demand signal to the hub indicating
that it wants to transmit. The hub sill respond with an acknowledgement that
will allow the computer to transmit. The hub will allow computers to
transmit in turn. An example of a demand priority network is 100VG-AnyLAN
(IEEE 802.12). It uses a star-bus topology.
- Polling - A central controller, also called the primary device will poll
computers, called secondary devices, to find out if they have data to
transmit. Of so the central controller will allow them to transmit for a
limited time, then the next device is polled.
Token passing performs better when the network has a lot of traffic, while
ethernet which uses CSMA/CD is generally faster but loses performance when the
network has a lot of traffic. CSMA/CD is basically a method that allows network
stations to transmit any time they want. They, however, sense the network line
and detect if another station has transmitted at the same time they did. This is
called a collision. If a collision happened, the stations involved will
retransmit at a later, randomly set time in hopes of avoiding another collision.
IP to link layer encapsulation
The requirements for IP to link layer encapsulation for hosts on a Ethernet
network are:
- All hosts must be able to send and receive packets defined by RFC 894.
- All hosts should be able to receive a mix of packets defined by RFC 894
and RFC 1042.
- All hosts may be able to send RDC 1042 defined packets.
Hosts that support both must provide a means to configure the type of packet
sent and the default must be packets defined by RFC 894.
Ethernet and IEEE 802 Encapsulation formats
Ethernet (RFC 894) message format consists of:
- 6 bytes of destination address.
- 6 bytes of source address.
- 2 bytes of message type which indicates the type of data being sent.
- 46 to 1500 bytes of data.
- 4 bytes of cyclic redundancy check (CRC) information.
IEEE 802 (RFC 1042) Message format consists of 3 sections plus data and CRC
as follows:
- 802.3 Media Access Control section used to coordinate the sending of
data between computers.
- 6 bytes of destination address.
- 6 bytes of source address.
- 2 bytes of length - The number of bytes that follow not including
the CRC.
- 802.2 Logical Link control establishes service access points (SAPs)
between computers.
- 1 byte destination service access point (DSAP).
- 1 byte source service access point (SSAP).
- 1 byte of control.
- Sub Network Access Protocol (SNAP).
- 3 bytes of org code.
- 2 bytes of message type which indicates the type of data being sent.
- 38 to 1492 bytes of data.
- 4 bytes of cyclic redundancy check (CRC) information.
Some ethernet message types include:
- 0800 - IP datagram with length of 38 to 1492 bytes.
- 0806 - ARP request or reply with 28 bytes and pad bytes that are used to
make the frame long enough for the minimum length.
- 8035 - RARP request or reply of 28 bytes and pad bytes that are used to
make the frame long enough for the minimum length.
These message types are the same for both formats above with the exception of
the pad bytes. The pad bytes for the RFC 894 and RFC 1042 datagrams are of
different lengths between the two message formats because the RFC 894 minimum
message length is 46 bytes and the RFC 1042 minimum message length is 38 bytes.
Also the two message formats above are distinguishable from each other. This is
because the RFC 894 possible length values are exclusive of RFC 1042 possible
type values.
Trailor Encapsulation
This is described in RFC 1122 and RFC 892, but this scheme is not used very
often today. The trailer protocol [LINK:1] is a link-layer encapsulation method
that rearranges the data contents of packets sent on the physical network. It
may be used but only after it is verified that both the sending and receiving
hosts support trailers. The verification is done for each host that is
communicated with.
RFC 1122 states: "Only packets with specific size attributes are encapsulated
using trailers, and typically only a small fraction of the packets being
exchanged have these attributes. Thus, if a system using trailers exchanges
packets with a system that does not, some packets disappear into a black hole
while others are delivered successfully."
Trailer negotiation is performed when ARP is used to discover the media
access control (MAC) address of the destination host. RFC 1122 states: "a host
that wants to speak trailers will send an additional "trailer ARP reply" packet,
i.e., an ARP reply that specifies the trailer encapsulation protocol type but
otherwise has the format of a normal ARP reply. If a host configured to use
trailers receives a trailer ARP reply message from a remote machine, it can add
that machine to the list of machines that understand trailers, e.g., by marking
the corresponding entry in the ARP cache."
