Data Link Layers 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:
Point-to-Point Protocol (PPP)
High-Level Data Link Control (HDLC)
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.
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.
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.
Difference in bandwidth normally results in the use of different protocols for LANs and WANs.
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.
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.