Category: OSI Model

OSI Model Tutorials

Frame Relay

Frame Relay is a packet-switching technology that exists at the Data Link layer of the OSI model and is one that has become increasingly common as a WAN solution since the early 1990s. Unlike with leased lines and circuit-switched networks, the available bandwidth on a provider’s Frame Relay network is shared amongst many subscribers. This sharing of resources leads to significantly lower costs than traditional leased lines.

Many people tend to be confused by packet-switching technologies like Frame Relay. Mostly this is a result of trying to understand how data actually gets from one location to another. On packet-switched networks (like Frame Relay), data streams are separated through the use of “virtual” rather than dedicated hardware circuits. In other words, a logical path is defined between endpoints, through a provider’s packet-switched network. Many virtual circuits will be defined for different customers, and will be multiplexed over the shared physical links of the network. As an example, consider the figure below. It shows two different companies, each connecting two offices over a provider’s Frame Relay network. Notice that between Frame Relay switches X and Y, both of their virtual circuits traverse a common physical link. The data that one company passes between their own offices is completely separate from the data of the other company – all data stays within each company’s dedicated virtual circuit only.

Figure: PVCs of two different companies traveling over the same Frame Relay network, and at times, common links.

Two main types of virtual circuits can be defined on a Frame Relay networks – permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). A PVC functions somewhat similar to a leased line, in that a service provider defines a path through the packet switched network to each customer location. In cases where companies wish to have “always-on” connectivity between locations using Frame Relay, PVCs are usually defined.

A switched virtual circuit (SVC) functions somewhat differently, almost like a circuit-switched connection. SVCs are not permanent, and can instead be created as required across a packet-switched network. For example, an SVC could be created between a company’s head office and a remote location. For the duration of the connection, data would travel across the path defined by the SVC. However, if the circuit was terminated and a new SVC established at a later time, data might travel over a completely different path.

Frame Relay networks are referred to as being non-broadcast multi-access (NBMA). What this means is that, by default, broadcast traffic will normally not be passed over a virtual circuit without explicit configuration. This is an important consideration when dealing with the use of broadcast-based routing protocols like RIP or IGRP in a Frame Relay environment. You’ll look at how broadcast traffic can be handled on Frame Relay networks later in this section.

ISDN and the OSI Model

ISDN maps to the three lower layers of the OSI model – Network, Data Link, and Physical. However, different protocols exist at the Data Link and Network layers for B and D channels, as illustrated in the figure below. The functions handled at each OSI layer are described below.

  • Physical Layer. The ISDN Physical layer is concerned with the actual sending and receiving of bits over variety of interfaces. For example, the I.430 standard is responsible for providing communication over S/T reference points, while the ANSI T1.601 standard defines communication over U interfaces in North America.
  • Data Link Layer. ISDN D channels use the Link Access Procedure for D Channels (LAPD – Q.921) to frame signaling and control data at Layer 2. On the B channel, data can be framed in a variety of ways, including via PPP and HDLC.
  • Network Layer. ISDN D channels handle call setup, termination, and maintenance at the OSI Network layer using the Q.931 protocol, which implements common signaling standards. On B channels, ISDN uses common Network layer protocols like IP, IPX, AppleTalk, and so forth.

Figure: ISDN protocols and their relationship to the OSI model for B and D channels.

WAN Technologies and the OSI Model

WAN technologies are considered to exist and function at the three lower layers of the OSI model – Physical, Data Link, and Network. While not all WAN technologies have elements that function at the Network Layer, some (like X.25 and ISDN) do. The figure below provides an overview of how the WAN technologies that you’ll look at in this chapter map to the OSI model.

Figure: A high-level overview of how various WAN technologies map to the Physical, Data Link, and Network layers of the OSI model.

TCP/IP and the OSI Model – The Application (Process) Layer

The Application/Process Layer is where TCP/IP applications and services reside. You’re more than likely familiar with many of these, since you probably interact with many TCP/IP applications on a daily basis – a web browser using HTTP, or your email client connecting to a POP3 server are but two simple examples.

The list below outlines some of the more common Application layer protocols that you should be familiar with.

  • Telnet. Telnet is used to create a terminal session with a remote host, providing command-line access to the target system running a telnet server (daemon).
  • FTP. The File Transfer Protocol is used to reliably transfer files between an FTP client and server using TCP.
  • SMTP. The Simple Mail Transfer Protocol is used for the exchange of email between systems.
  • DNS. The Domain Name Service is a distributed database that is queried to resolve (or translate) names such as www.2000trainers.com to an IP address.
  • SNMP. The Simple Network Management Protocol is a lightweight network protocol that allows information to be gathered about network devices. Examples include information about utilization, hardware configuration, and so forth.
  • TFTP. The Trivial File Transfer Protocol is used to transfer files between a client and a TFTP server over UDP. You’ll learn more about TFTP later, since it’s the protocol used to transfer files to and from a Cisco router.

TCP/IP and the OSI Model – The Network (Internet) Layer

If you recall, the Internet layer’s primary responsibilities are determining a path between networks (routing), as well as network addressing. The addressing that takes place at the Internet layer is often referred to as logical addressing. These addresses aren’t “burned-in” like Ethernet MAC addresses, but instead are assigned by an administrator. The addressing protocol of the TCP/IP stack is the Internet Protocol (IP).

Note that TCP/IP routing protocols such as RIP, OSPF, and others also exist at the Internet layer. These will be looked at in Chapter 8, when routing is covered in detail.

The 4-Layer TCP/IP Network Model

The Department of Defence TCP/IP model is a 4-layer model that defines areas of responsibility much like the OSI, while providing insight into the functions of the different protocols that make up the TCP/IP suite. The model provides an excellent point of reference when compared to the OSI. We won’t look at all the details of the TCP/IP model just yet – the majority will be covered in Chapter 4. My feeling is that the data encapsulation process is much better explained using a popular protocol suite.

To begin, let’s take a look at how the TCP/IP model maps to the OSI model. While the names of the TCP/IP layers are different, they generally encompass the same responsibilities as one or more OSI layers. Consider the diagram below.

Figure: Comparing the OSI and TCP/IP network models.

Tip: Although the layers of the TCP/IP model technically use different names, Cisco will still refer to protocols by their associated OSI layer name. For example, Cisco will describe TCP as being a Transport layer protocol.

For the sake of illustration, I’ve included some of the key protocols that make up the TCP/IP suite in the figure below. Be aware that the terms data, segment, packet, and frame still apply as data is encapsulated in the TCP/IP model.

Figure: TCP/IP protocol stack including common protocols and network technologies.

Physical Layer of the OSI Model

The Physical layer of the OSI model is concerned with the electrical, optical, and mechanical properties of the network, including elements such as voltage, media, connector types, signal regeneration, and so forth. The physical layer doesn’t actually alter packets, but rather acts as the transmission facility over which the actual bits (1’s and 0’s) are sent. This isn’t limited to plain old copper wire – it can include optical signals, radio waves, and infrared transmissions to name but a few. Examples of equipment found at the Physical layer include network cabling, hubs, and repeaters. A number of popular Physical layer standards are listed below.

Examples of Physical layer standards:

  • High Speed Serial Interface (HSSI): High speed serial communications
  • EIA/TIA-232: Low speed serial transmissions
  • V.35: ITU-T serial transmission standard

Data Link Layer of the OSI Model

The Data Link Layer of the OSI model acts as an interface between the Network and Physical layers. The main responsibilities of the Data Link layer include:

  • Data framing and physical addressing. When data is passed to the Data Link layer, it is framed for transmission using various LAN and WAN protocols. This allows network protocols to be transmitted over different network technologies including Ethernet, Token Ring, and Frame Relay as examples. Hardware or Media Access Control (MAC) addressing is used to uniquely identify hosts at the Data Link layer. Since they make forwarding decisions based on MAC addresses, bridges and switches are examples of equipment found at this layer.
  • Flow control, error checking, and frame sequencing. Data Link layer devices are capable of transmitting flow control codes that identify whether upper layer protocols are capable of receiving data at the current rate. Error checking is provided in the form of a Cyclic Redundancy Check (CRC), a simple mathematical calculation performed on each frame to ensure it hasn’t been corrupted in transit. Frame sequencing reorders frames that were received in a different order than they were sent.

Interacting with Network layer protocols. When a host receives a frame, the frame header contains information on which Network layer protocol the data will be passed to. The Data Link layer helps to make network technologies independent of the upper layer protocols in use.

Examples of Data Link layer protocols:

  • Ethernet (802.3): Contention-based LAN technology
  • Token Ring (802.5): Token-passing LAN technology
  • Wireless LAN (802.11): Wireless LANs
  • Frame Relay: Packet-switched WAN technology
  • ISDN: Digital dial-up connections

Tip: Remember that the protocol data unit (PDU) of the Data Link layer is referred to as a frame.

The Data Link layer is actually comprised of two sub-layers (defined by the Institute of Electronics and Electrical Engineers – the IEEE), called Media Access Control (MAC) and Logical Link Control (LLC).

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Network Layer of the OSI Model

The Network layer of the OSI model is commonly referred to as Layer 3, and has the following responsibilities:

  • Routing. When a host on one network wishes to exchange data with a host on another, packets will be sent to a router interface. After determining where the packet should be forwarded next using information found in its routing table, a router will switch the packets out of the optimal interface. This process will take place at each router encountered on a packet’s journey to the destination host. Routing protocols are used to allow routers to exchange information with one another.
  • Network Addressing. Each host on a routed internetwork will have at least one network address. A network address is made up of two parts – the first part identifies the network, while the second identifies a unique host on that network. These addresses have different formats depending on the routed protocol in use – we’ll look at examples shortly.

Examples of Network-layer protocols:

Internet Protocol (IP): TCP/IP addressing and routing

Internetwork Packet Exchange (IPX): IPX/SPX addressing and routing

Internet Control Message Protocol (ICMP): Diagnostics and error notification

Internet Group Management Protocol (IGMP): Multicast group management

Tip: Remember that the protocol data unit (PDU) of the Network layer is referred to as a packet. In some cases, you may also see this PDU referred to as a datagram.

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