CHAPTER 13Packets, Frames & Topologies

CECS 474 Computer Network Interoperability

Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

Tracy Bradley Maples, Ph.D.Computer Engineering & Computer ScienceCalifornia State University, Long Beach

Local and Wide Area Packet Networks

Recall: Packet switching technologies are commonly classified according to the distances they span.

Standards for Packet Format and ID

• Each packet sent across a packet network must contain the ID of the intended recipient.

• All senders must agree on the exact details thus standards have been created by various organizations. The most famous: Institute for Electrical and Electronic Engineers (IEEE)

• In 1980, IEEE organized the Project 802 LAN/MAN Standards Committee to produce standards for networking.

IEEE 802 Model and Standards  IEEE 802 divides Layer 2 of the protocol stack into two conceptual sub-layers:

The Logical Link Control (LLC) The LLC sublayer specifies addressing and the use of addresses for multiplexing/demultiplexing. 

The Media Access Control (MAC) The MAC sublayer specifies how multiple computers share the underlying medium.

Figure 13.6: Examples of the identifiers IEEE has assigned to various LAN standards

LAN Topologies

Each network is classified into a category according to its topology (or general shape) and MAC protocol.

Examples:

• Ethernet Bus• Ethernet Star (hub or switch)• Token Passing Ring• Token Passing Bus

LAN Topologies: Bus• Bus topology usually consists of a single cable to

which computers attach• The ends of a bus network must be terminated to

prevent electrical signals from reflecting back along the bus

• Any computer attached to a bus can send on the cable and all computers receive the signal

• The computers attached to a bus network must coordinate to prevent or manage collisions

 LAN Topologies: Ring• In a ring topology computers are connected in a

closed loop (with possible gateways to other networks).

• Ring topologies usually have a direction associated with the transmission (i.e, clockwise or counterclockwise).

• In ring topologies, an acknowledgement of successful transmission can be sent for free.

 

LAN Topologies: Mesh• Mesh networks provide a direct connection

between every pair of computers.• Main disadvantage of a mesh: Cost!

Mesh networks are not scalable. The number of connections needed for

a mesh network grows faster than the number of computers.

• Because connections are expensive, few LANs employ a mesh topology  

LAN Topologies: Star• In star topologies, all computers attach to a

central point (or hub).• In practice, star networks seldom have a

symmetric shape (i.e., the hub is not located an equal distance from all computers). Instead, a hub often resides in a

location separate from the computers attached to it.

Packet Identification, Demultiplexing, MAC Addresses

IEEE 802 standards include: packet addressing

Consider packets traversing a shared medium as in the figure:  

• Each computer is assigned a unique address and each packet contains the address of the intended recipient. 

• In the IEEE 802 addressing scheme, each address consists of 48 bits.• IEEE 802 uses the term Media Access Control address (also called the MAC

address, Ethernet address or physical address).• Each Network Interface Card (NIC) contains a unique IEEE 802 address assigned

when the device was manufactured.• IEEE assigns a block of addresses to each vendor and allows the vendor to assign a

unique value to each device.

Unicast, Broadcast, and Multicast Addresses The IEEE addressing supports three types of addresses that correspond to three types of packet delivery:  

• The standard specifies that a MAC broadcast address consists of 48 bits that are all 1s (i.e., FF:FF:FF:FF:FF:FF).

• MAC broadcast can be viewed as a special form of multicast.• Each multicast address corresponds to a group of computers.• The Broadcast address corresponds to a group that includes all computers on the

network.

Unicast, Broadcast, and Multicast Addresses

Frames and Framing

Defn: Framing refers to the structure added to a sequence of bits or bytes that allows a sender and receiver to specify and recognize the exact format of the message.

In a packet-switched network, each frame corresponds to a packet at Layer 2. A frame consists of two conceptual parts:

1. A header that contains information used to process the frame.2. A payload that contains the message being sent.

Notes: • We say the the message is opaque because the network only examines the frame

header. • The payload can contain an arbitrary sequence of bytes that are only meaningful

to the sender and receiver.• Some technologies delineate each frame by sending a short prelude before the

frame and a short postlude after it.

Frames and Framing  Frames depend on the network standard to specify:

• a minimum/maximum size• Format for the message

 Defn: A hardware packet (or Layer 2 packet) is called a frame. Case 1: Byte Stuffing with RS-232 as an Example

Frame Format Used with RS-232

RS-232 is character-oriented (i.e., RS-232 sends characters not individual bits).

Two special characters are used to delineate the packet:• Start of header (soh)• End of text (eot)

RS-232 is a serial binary standard for transmitting data. It was a tremendously popular standard for sending serial data in computers over short (<50 feet) distances. It was used with the serial ports in PCs to connect keyboards, mice, etc. Although replaced in newer PCs by USB and Bluetooth, it is still used for some equipment.

Case 1: Byte Stuffing with RS-232 as an Example (Cont’d)

Q: What happens when the special characters are used in the message being transmitted? A: We must translate the special characters into alternative form to avoid confusion. This is called byte stuffing.  Illustration of Frame with Byte Stuffing: Figure (a) shows the message the sending computer wishes to transmit.Figure (b) shows the Bytes that are actually transmitted.

RS-232 Substitution Table

Case 2: Bit Stuffing with PPP as an Example

What if the transmission scheme is bit-oriented rather than byte oriented? Two Real-world examples of bit stuffed networks are:

• HDLC (High-Level Data Link Control), ISO standard• PPP (Point-to-Point Protocol)

Bit-oriented protocols view the a frame as a collection of bits. A special bit sequence 01111110 is used at both the beginning and end of frames.

PPP is a popular data link layer protocol for communication between two end-points. It provides authentication, encryption, and compression. PPPoE (Point-to-Point Protocol over Ethernet) is often used to provide a link from a modem to an ISP (Internet Service Provider).

01111110 111111111 control protocol check 01111110

FLAG Information (variable) FLAG

The PPP Frame Format

Case 2: Bit Stuffing with PPP as an Example (Cont’d)

Problem: This bit sequence (01111110) may occur naturally in the bit stream. Solution: Use bit stuffing. Any time five consecutive 1 bits of the data message need to be transmitted, an additional 0 is “stuffed” in following the five 1s. At the receiving end, after five ones a 0 is always removed.

Note: Using this scheme it is not possible to make all frames the same size.

TRAILERHEADER 01111110 01111110

Data for transmission (passed down from upper Layer 3 at Source):

010111111011111010111111111010101

Bit-stuffed frame ready to send by Layer 2:

HEADER01111110 TRAILER 01111110

Data delivered (ready to passed up to upper Layer 3 at Destination):

Bit-stuffed frame received at destination Layer 2:

---------- DATA SENT OVER NETWORK FROM SOURCE TO DESTINATION ----------