Understanding the network.pdf - Back to Home

outer, the delivery path might be different for each datagram, depending on
network traffic conditions. The TCP/IP protocol suite only provides a connectionless
network layer transport, because TCP/IP was built on the premise that it would be
used in an internetworking environment where link reliability would be questionable
and so left the responsibility for actual delivery to Layer 2, the data link layer. The
IP protocol provides the connectionless network layer transport for the TCP/IP suite.
AppleTalk and IPX are also connectionless delivery services. AppleTalk uses
Datagram Delivery Service (DDS) and IPX uses Internetwork Packet Exchange
(IPX).
Layer 2: Data Link
Data link is the facility that controls the transport of the upper layer protocol (ULP)
data bits across the physical connection medium. ULP data is "enclosed" inside of a
Layer 2 protocol "envelope," called a frame, which is then transmitted. Layer 2 has
two data transport functions. The first (and most commonly associated) is Media
Access Control (MAC), which defines the logical representation of the ULP data and
access to transport medium. The second is link control (LC), which acts as the
interface between the Layer 3 protocol(s) and the MAC. Depending on the Layer 2
protocol and its application (such as WAN or LAN use), the LC function is handled
differently. WAN protocols define this process as part of their specification. The
majorities of LAN protocols (and some variations on WAN protocols) utilize the
Institute of Electrical and Electronics Engineers (IEEE) 802.2 LLC specification to
perform this function. Figure 1.15 illustrates the IEEE "enhanced" OSI-RM.
Advances in network speed, performance, and reliability for the most part, all occur
at the data link layer (Layer 2).