Protocols and Architectures
Protocols are key enablers for all types of communications including Internet access from cellular devices. (See Chapter 9 for cellular service.) New protocols have been developed for lower cost wireless LANs for homes and businesses that provide convenience previously only affordable to large corporations. (See the next section for wireless LANs for homes.)
Protocols—A Common Set of Rules
Protocols enable like devices to communicate with each other by providing a common set of rules. Devices communicate over the Internet using a suite of protocols called TCP/IP. For example, the IP, or Internet protocol portion of TCP/IP, enables portions of messages called datagrams to take different routes through the Internet. The datagrams are assembled into one message at the receiving end of the route. Other protocols, such as Bluetooth, enable wireless communications among devices located within 33 feet of each other.
Examples of protocol functions are:
Who transmits first?
Is the other end ready to receive?
In a network with many devices, how is it decided whose turn it is to send data?
What is the structure of the addresses of devices such as computers?
How is it determined if an error has occurred?
How are errors fixed?
If no one transmits, how long is the wait before disconnecting?
If there is an error, does the entire transmission have to be resent or just the portion with the error?
How is data packaged to be sent—one bit at a time or one block of bits at a time? How many bits are in each block? Should data be put into envelopes called packets?
Protocol structures have implications on speed and efficiency. The following protocols illustrate this point:
Secure Sockets Layer (SSL)— Encrypts communications between a user's browser and the Web site server in electronic transactions so that only the authorized server can read credit card information.
Signaling System 7 Protocol (SS7)— A way for carriers to bill, track and provide enhanced services such as caller ID over their networks. It also enables carriers to manage traffic sent to them from other carriers. The signaling system it supplanted, Signaling System 6 (SS6) did not support enhanced services and used network capacity less efficiently than SS7. (See Chapter 5 for SS7.)
Architectures—A Framework for Multiple Networks to Communicate
The main goal of architectures is to enable dissimilar protocols and computer networks to communicate. Standards bodies and dominant companies such as IBM develop architectures. During the 1970s, the International Standards Organization (ISO) developed an architecture, Open System Interconnection (OSI), to provide the means for devices from multiple vendors to interoperate.
While OSI was not widely implemented because of its complexity, it has had a profound influence on telecommunications and has become a reference model. It laid the foundation for the concept of open communications among multiple manufacturers' devices. The basic concept of OSI is that of layering (see Table 1.3). Groups of functions are broken up into seven layers, which can be changed and developed without having to change any other layer. LANs, public networks and the Internet's TCP/IP suite of protocols are based on concepts for a layered architecture.
The Internet suite of protocols, TCP/IP, corresponds to the functions in Layers 3 and 4 of the OSI model. These functions are addressing, error control and access to the network. The TCP/IP suite of protocols provides a uniform way for diverse devices to communicate with each other from all over the world. It was developed in the 1970s by the U.S. Department of Defense and was provided at no charge to end users in its basic format. Having a readily available, standard protocol is a key ingredient in the spread of the Internet.
| Layer Name and Number | Layer Function |
|---|---|
| Layer 1 physical layer | Layer 1 is the most basic layer. It defines the electrical interface (RS 232 plugs) that connects modems to computers. Layer 1 defines, for example, which pins are used for sending, which for receiving and which for requests to send on the pins in the connectors. Layer 1 also defines type of media—for example, copper, wireless and fiber optics. SONET, discussed in Chapter 6, is a Layer 1 protocol used to define how information is transmitted on fiber optic networks. |
| Layer 2 data link layer | Ethernet, used in local area networks (LANs) within corporations and carrier networks, corresponds to Layer 2 of the OSI model. It provides rules for error correction and access to LANs. Layer 2 devices also have addressing information analogous to the postal system's routing mail all the way to an end user's residence. |
| Layer 3 network layer | Layer 3, also known as the routing layer, is responsible for routing traffic between networks using IP (Internet protocol). It also has error control functions. Layer 3 is analogous to a local post office routing an out-of-town letter by ZIP Code, not looking at the street address. For example, once an email message is received at the distant network, a layer 2 device looks at the device address and delivers it to the correct computer. |
| Layer 4 transport | Layer 4 protocols enable networks to differentiate between types of applications. Layer 4 devices route by content. For example, video or voice transmissions over data networks might receive a higher priority or quality of service than email, which can tolerate delay. TCP (Transmission Control Protocol) is a Layer 4 protocol. Filters in routers that check for computer viruses by looking at a message's entire IP address perform a Layer 4 function. |
| Layer 5 session | Layer 5 manages the actual dialog of sessions. Encryption that scrambles signals to ensure privacy occurs in Layer 5. Other Layer 5 functions include determining the full- or half-duplex nature of the transmission. For example, can both ends send at the same time (full duplex)? Can transmissions be half duplex, with one-way-at-a-time sending? |
| Layer 6 presentation | Layer 6 controls the format or how the information looks on the user's screen. |
| Layer 7 application | Layer 7 includes the application itself plus specialized services such as file transfers or print services. Hypertext transfer protocol (HTTP) is a Layer 7 protocol that advanced switches use to transfer Internet users to the server containing the requested content. |
In layered architectures or protocol suites, when transmitting, layers communicate with the layer immediately below them. Only Layer 1 actually transmits to the network. On the receiving end, Layer 1 receives the data and sends it to Layer 2, which then reads the Layer 2 protocol before sending the message to the next higher layer and so on to the application layer.