Data Link Layer

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Cisco Hierarchical Model:
For more information about this, please read our separate tutorial titled "The Cisco Hierarchical Model".

OSI Model:
The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.

The OSI Model consists of 7 layers as follows:

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes.	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers.	Switch, bridge and brouter	None
Physical	Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector.	Multiplexer and repeater	None

Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.

Class	Range	Explanation
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid. The later two classes aren’t used for networks.
B	128-191
C	192-223
D	224-239 (Multicasting)
E	240-255 (Experimental)

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows:

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the Net ID.

3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR.

IPX/SPX:
IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal.

Sequenced Packet Exchange(SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol(SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.

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 sublayers: 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) sublayer 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.

Switches and Bridges

Work at Data Link layer and filter network using MAC addresses. Layer 2 switching is hardware based switching because it uses an ASIC (Application Specific Integrated Circuit).
Switches and Bridges read each frame as it passes through, it then puts the source address in a filter table and keeps track of which port it was received on. This tells the switch where that device is located.
After a filter table is built, the device will only forward frames to the segment where the destination address is located. If the destination device is on the same segment as the frame, the layer 2 device will block it from being forwarded. If the destination is on another segment, the frame will only be forwarded to that segment (transparent bridging).
When a layer 2 device receives a frame and the destination is unknown to the device's filter table, it will forward the frame to all connected segments. If the unknown device replies, the filter table is updated with that device's location.

-Layer 2 devices (Switches / Bridges) propagate broadcast storms and the only way to prevent them is with a router.

-Each port on a switch is in its own collision domain.

-Switches allow all segments to transmit simultaneously.

-Switches can't translate different media types.

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Data Link Layer