HDLC and MAC

[alanchia]

Hi all,

some basic questions,

what is the differences between HDLC and MAC ?

thanks,
alan

 

[rwullems]

some explanation about HDLC, mayby it helps:

HDLC is the default encapsulation used by Cisco routers over synchronous serial links. HDLC is a bit-oriented
ISO standard Data Link layer protocol. It specifies a method to encapsulate data over
synchronous serial links using frame characters and checksums. HDLC is a point-to-point protocol
used on leased lines between Cisco devices. If you need to establish a link between a Cisco device
and a non Cisco device, you must use PPP encapsulation instead of HDLC. No authentication can be
used with HDLC. The reason each vendor has a proprietary encapsulation of HDLC is that they each
have a different way for the HDLC protocol to communicate with the Network layer protocols, and the
ISO standard doesn't allow for multiple protocols on a single link. They needed a way for HDLC to
communicate with layer-3 protocols so HDLC can carry more than one type of protocol.
Example of Configuring HDLC Encapsulation
Although HDLC is the default encapsulation for serial interfaces, you might have to change it back to
HDLC from another encapsulation method; to do that use the following CLI commands:
Router3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router3(config)#int s0
Router3(config-if)#encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
Router3(config-if)#encapsulation hdlc

Robert A.H. Wullems
Sniffer University Instructor
SCM / CNX / MCP
Citee Education
the Netherlands

 

[ElijahBaley]

But if 802.3 & 802.5 are (Media Access Control) MAC protocols that specify the datalink encapsulations and the physical transmission methods, then isn't HDLC also a MAC protocol?

EB

 

[alanchia]

hi all,

i have checked it out,

LLC - logical link control a subset of HDLC
and MAC coexists in the OSI 2nd layer (data link).

MAC is used to translate the MAC address whereas LLC is used to provide a link for the 2nd layer and 3rd layer. Correct me if i am wrong.

cheers,
alan

 

[rwullems]

Yes, your right.
the DLC layer of the OSI model consist of several sublayers, where LLC is on top of MAC. MAC is for addressing, creating and calculating CRC's and some more. LLC comes in differency types (I, II and III, were III is the one i have never seen in the wild).

See the following text comming from my website:

Data Link Layer
The Data Link layer provides reliable transit of data across a physical network link. Different Data Link
layer specifications define different network and protocol characteristics, including physical
addressing, network topology, error notification, sequencing of frames, and flow control. The
Data Link layer translates messages from the Network layer into bits for the Physical layer to transmit.
It formats messages into data frames and adds a customized header containing the source and
destination hardware addresses. Data Link layer is responsible for uniquely identifying each device on
a local network.

•Physical addressing (as opposed to network addressing) defines how devices are
addressed at the data link layer.
•Network topology consists of the data link layer specifications that often define how devices
are to be physically connected, such as in a bus or a ring topology.
•Error notification alerts upper-layer protocols that a transmission error has occurred, and the
sequencing of data frames reorders frames that are transmitted out of sequence.
•Flow control moderates the transmission of data so that the receiving device is not
overwhelmed with more traffic than it can handle at one time.

-When a packet is sent between routers, it is framed with control information at the Data Link layer.
The information is removed at the destination router and only the original packet remains. If the
packet is to go to another router, the framing process is repeated until it gets to the receiving host.
The packet is never altered, only encapsulated with control information to be passed on to the different
media type.
The IEEE has subdivided the data link layer into two sub layers: Logical Link Control (LLC) and Media
Access Control (MAC).
MAC (Media Access Control)
The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the
physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple
devices to uniquely identify one another at the data link layer.
--The MAC describes how a station schedules, transmits and receives data on a shared media
environment.
--Ensures reliable transfer of information across the link, synchronizes data transmission, recognizes
errors (doesn't correct them), and controls the flow of data.
--Defines how packets are placed on the media.
--Physical addressing is defined here as well as local topologies.
--MAC example is Ethernet/802.3 and Token Ring/802.5
--Line discipline, error notification, ordered delivery of frames, and optional flow control can be used at
this layer.
--In General, MACs are only important in shared medium environments where multiple nodes can
connect to the same transmission medium.

LLC (Logical Link Control)

The Logical Link Control (LLC) sub layer of the data link layer manages communications between
devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports
both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2
defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share
a single physical data link.
--Responsible for identifying Network layer protocols and encapsulating them.
--A LLC header tells the Data Link layer what to do with a packet once it is received.


Regards,
Robert

Robert A.H. Wullems
Sniffer University Instructor
SCM / CNX / MCP
Citee Education
the Netherlands

 

[alanchia]

Thank you Mr Wullems for your help, can u send me a link for your website ?

cheers,
alan

 

[rwullems]

Hi Alan,

Yes i can, but it is officialy stil under construction.
You can access the site by the following link on a temporary domain name:

http://www.robertwullems.com/index9999.html

Regards,
Robert

Robert A.H. Wullems
Sniffer University Instructor
SCM / CNX / MCP
Citee Education
the Netherlands