1. Physical and Data Link Layers
Surasak Sanguanpong
Last updated: May 24,1999

2. Networks Devices
LANs
ATM
Switch
Bridge
Hub
Switch
Router
WANs
X.25 or
Frame Relay
Switch
Modem
CSU/DSU
TA/NT1
Comm.
Server
Most of the network administrator’s tasks deal with local area networks (LANs). Major characteristics of LANs follow:
The network operates within a building or floor of a building. The geographic scope for ever more powerful LAN desktop devices running more powerful applications is for less area per LAN.
LANs provide multiple connected desktop devices (usually personal computers) with access to high-bandwidth media.
An enterprise purchases the media and connections used in the LAN; the enterprise can privately control the LAN as it chooses.
LANs rarely shut down or restrict access to connected workstations; local services are usually always available.
By definitions, the LAN connects physically adjacent devices on the media. LAN devices include
Bridges that connect LAN segments and help filter traffic
Hubs that concentrate LAN connection and allow use of twisted pair copper media
Ethernet switches that offer full-duplex, dedicated bandwidth to segments or desktops
Routers that offer many services including internetworking and broadcast control
ATM switches that provide high-speed cell switching
Multiplexer
stat
mux S

3. Common Data Links
Token
Ring
FDDI
Dual Ring
Point-to-point
The data link layer carries data across a physical link in frames. The link directly connects devices. The graphic shows the following common topologies for popular local-area network (LAN) and wide-area network (WAN) technologies :
Ethernet bus
Token Ring
Fiber Distributed Data Interface (FDDI) dual ring
Point-to-point serial
In this chapter, material is presented in the order of the bullet points shown above.

4. LAN/WAN layers comparison
Data Link
(frames)
Physical
(bits,
signals,
clocking) E t h e r n
802.2 LLC
Dial on
Demand
SDLC
HDLC
X.25
Link
Frame
Relay
ISDN
PPP 8 2 . 3 8 2 . 4 8 2 . 5 F D I
V.24
EIA/TIA-232 G.703
V.35
EIA/TIA-499 EIA-530
HSSI
The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link between end systems. The physical layer specifies characteristics such as voltage levels, data rates, maximum transmission distances, and physical connectors.
The data link layer provides data transport across a physical link. The data link layer handles physical addressing, network topology, error notification, orderly delivery of frames, and optional flow control.
Separate physical and data link layer for LAN and WAN

5. Medium Access Control
Protocol for controlling access to transmission medium
Defined as part of Data Link layer
The protocol performs:
perform functions related to medium access (MAC sublayer)
concerned with the transmission of a link-level between two nodes (LLC sublayer)
Network
The role of Data Link layer is to permit the transfer of data between the stations and detect transmission errors. IEEE divides this layer into separated sublayer : MAC (Medium Access Control) and LLC (Logical Link Layer)
The MAC sublayer control the access of medium using access method. The LLC provides interface to next upper layer.
LLC
Logical Link Control sublayer
Data Link
MAC
Medium Access Control sublayer
Physical

6. Ethernet and IEEE 802.3
10Base2 - Thin Ethernet
hub
bridge
10Base5 - Thick Ethernet
hub
The Ethernet and IEEE standards define a bus-topology LAN that operates at a baseband signaling rate of 10 Mbps. The graphic illustrates the three defined wiring standards:
10Base2 - known as thin Ethernet - allows network segments up to 185 meters on coaxial cable.
10Base5 - known as thick Ethernet - allows network segments up to 500 meters on coaxial cable.
10BaseT - carries Ethernet frames on twisted pair wiring
The 10Base5 and 10Base2 standards provide access for several stations on the same segment. Stations are attached to the segment by a cable that runs from an attachment unit interface (AUI) in the station to a transceiver that is directly attached to the Ethernet coaxial cable. In some interfaces, the AUI and the transceiver are built in to the network interface card and no cable is required.
Because the 10BaseT standard provides access for a single station only, stations attached to an Ethernet by 10BaseT are connected to a hub. The hub is analogous to an Ethernet segment, and the twisted-pair cable is analogous to the cable running between the AUI and the transceiver.
router
server
10BaseT-Twisted pair

7. Ethernet/802.3 Operation
Every node can receive a transmission by all other nodes
need addressing scheme to identify a destination
only destination copies frame to it, all other nodes have to discarded the frame B C A B C A
terminator A
C finds the bus is free
C transmits frame addressed to A A B C A B C
A packet in an Ethernet network traverses the entire network and is received and examined by every node. When the signal reaches the ends of a segment, it is absorbed by terminators to prevent it from going back onto the segment. If two or more station try to transmit packets at the same time, a collision is occurred and the packet can not be used. A A
A copies frame
B ignores frame
signal is absorbed by the terminators

8. Token Ring and IEEE 802.5
Token
Ring
Token Ring was developed originally by IBM in the 1970s. It is still IBM’s primary LAN technology, and is second only to Ethernet/IEEE in popularity. The IEEE specification is almost identical to, and completely compatible with, IBM’s Token Ring. Both Token Ring specifications are now administered by the IEEE committee. The term Token Ring is generally used to refer to IBM’s Token Ring network and IEEE networks.
Access to a Token Ring is granted by a token frame, that is passed from station to station sequentially. Media control in predictable (deterministic) delays in accessing the network. When a station has information to transmit, it seizes the token.
IBM’s token ring is equivalent to IEEE 802.5

9. Token Ring/802.5 Operation T
T = 0 A
T = 0 A T
T = 1 A
Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a station receiving the token has no information to send, it simply passes the token to the next station.
If a station possessing the token has information to transmit, it claims the token by altering one bit of the frame, the T bit. The station then appends the information it wishes to transmit and sends the information frame to the next station on the Token Ring.
The information frame circulates the ring until it reaches the destination station where the frame is copied by the station and tagged as having been copied. The information frame continues around the ring until it returns to the station that originated it, and is removed. T
Data
Token Ring LANs continuously pass a token or a Token Ring frame

10. Fiber Distributed Data Interface (FDDI)
Dual Ring
100 Mbps
FDDI is an American National Standards Institute (ANSI) standard that defines a dual Token Ring LAN operating at 100 Mbps over an optical fiber medium. The FDDI standards were published in 1987 in the ANSI X3T9.5 standards.
_____________________________________________________________
Note Work is currently underway at ANSI to define a copper-based medium for 100-Mbps dual Token Ring LANs. The new standard will be known as Copper Distributed Data Interface (CDDI).
___________________________________________________________________________________________________________________________
FDDI specifies communication over fiber-optic cable, so it is well suited for operations where nodes are separated by large distances or where networks must operate in electronically hostile environments such as factory floors.
FDDI has high speeds that make it suitable for network applications requiring large bandwidth - for example, video and graphics applications.
Devices on FDDI maintain connectivity on dual counter-rotating rings

11. FDDI FDDI standards describe the physical layer and MAC sublayer.
Dual-Homed
SAS
DAC
DAC
SAS
FDDI standards describe the physical layer and MAC sublayer.
FDDI uses a token-passing protocol that operates on dual counter-rotating rings, as shown in the graphic. Under normal operation, data flows on a primary ring, while the secondary ring is idle. Some stations known as dual attachment stations (DAS) attach to both rings. Single-attachment stations (SASs) have only a single physical medium dependent (PMD) connection to the primary ring by way of a dual attachment concentrator (DAC).
Mission-critical stations such as routers or mainframe hosts can use a technique called dual homing to provide additional fault-tolerance and help guarantee operation. With dual homing, a station I single-attached to two DACs, thereby providing an active primary link and a backup path to the FDDI LAN.
DAS
Devices attached to FDDI use token passing

12. Common WAN Technologies
SDLC
HDLC
LAPB
PPP
WAN physical layer protocols describe how to provide electrical, mechanical operational, and functional connections for wide-area networking services. These services are most often obtained from WAN service providers like Regional Bell operating Companies (RBOCs), alternate carriers and Public Telephone and Telegraph (PTT).
WAN data link protocols describe how frames are carried between systems on a single data link. They include protocols designed to operate over dedicated point-to-point facilities, multipoint facilities based on dedicated facilities, and multi-access switched services such as Frame Relay.
WAN standards are defined and managed by a number of recognized authorities including the following agencies:
International Telecommunications Union--Telecommunication Standardization Sector (ITU/T), formerly the Consultative Committee for International Telegraph and Telephone (CCITT)
International Organization for Standardization (ISO)
Internet Engineering Task Force (IETF)
Electronic Industries Association (EIA)
WAN standards typically describe both physical layer and data link layer requirements. The graphic identifies several of the popular WAN services used in internetworks today.
X.25
Frame Relay
ISDN

13. Data Circuit-terminating Equipment Data Terminal Equipment
Physical Layer: WAN
RS-232
V.35
X.21
HSSI
others
DSU/CSU
(Modem)
DCE
Data Circuit-terminating Equipment
DTE
Data Terminal Equipment
The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DCE is the service provider, and the DTE is the attached device. In this model, the services offered to the DTE are made available through a modem or a data service unit/channel service unit (DSU/CSU).
Several physical layer standards specify this interface:
EIA/TIA-232
EIA/TIA-449
V.24
V.35
X.21
G.703
EIA-530
High-Speed Serial Interface (HSSI)
End of the WAN providers
side of the communication facility
End of the users device
on the WAN link

14. Data Link Layer: WAN protocols
(Modem)
(Modem)
DSU/CSU
DSU/CSU
SDLC--Synchronous Data Link Control
For IBM SNA networks; primary and secondary roles on link
HDLC--High-level Data Link Control
Common WAN data link
LAPB--Link Access Protocol, Balanced
DTE-to-DCE data link for X.25; either side initiates a link
Frame Relay--Simplified version of HDLC framing for
higher speed, unacknowledged data communications
PPP--Point-to-Point Protocol
Part of TCP/IP stack for WAN links; can support ISDN
The common data link encapsulations associated with synchronous serial lines are listed in the graphic:
Synchronous Data Link Control (SDLC)--A bit-oriented protocol developed by IBM. SDLC defines a multipoint WAN environment that allows several stations to connect to a dedicated facility. SDLC defines a primary station and one or more secondary stations. Communication is always between the primary station and one of its secondary stations. Secondary stations cannot communicate with each other directly.
High-Level Data Link Control (HDLC)--An ISO standard. HDLC might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations.
Link Access Protocol, Balanced (LAPB)--Primarily used with X.25, but can also be used as a simple data link transport. LAPB includes capabilities for detecting out-of-sequence or missing frames as well as for exchanging, retransmitting and acknowledging frames.
Frame Relay--Uses high-quality digital facilities where the error checking of LAPB is unnecessary. By using a simplified framing with no error correction mechanisms, Frame Relay can send Layer 2 information very rapidly compared to these other WAN protocols
Point-to-Point Protocol (PPP)--Described by RFC 1331 and 1334, two standards developed by the Internet Engineering Task Force (IETF). PPP contains a protocol field to identify the network layer protocol.

15. SLIP : Serial Line IP SLIP
Method for encapsulation IP datagrams on serial line
RFC 1005 de facto standard
Popular for connecting home computer to Internet, via modem
SLIP with Remote Connection
Host
Workstation
SLIP
modem
modem
RS-232
RS-232
Dialup or Leased Line
SLIP with Direct Connection
RS-232
RS-232

16. SLIP frame format
Simple : no header, just a framing character around data
Use 0xC0 (SLIP END) to terminate datagrams
SLIP ESC is 0xDB
if found 0xC0, substitute with 0xDB 0xDC
if found 0xDB, substitute with 0xDB 0xDD
IP datagrams
C DB
DB DC DB DD C0
SLIP encapsulation

17. SLIP deficiencies
Need manual configuration of IP address both sides (no negotiating mechanism)
Only one protocol can be used because there is no field to specify type of protocol
No checksum; bad for protocols that depend on CRC!
Slow line make inefficient to carry only 1 byte date with 40 bytes overhead (IP+ TCP header)
CSLIP (Compress SLIP) reduces 40 byte headers to 3-5 bytes
known as Van Jacobson Compression (see RFC 1144)
smaller headers greatly improve the interactive response time

18. PPP: Point-to-Point Protocol
Method for encapsulation IP datagrams on serial line, correct all deficiencies in SLIP
Support either an asynchronous link with 8N1 or bit-oriented synchronous link
Two parts:
LCP (Link Control Protocol) to establish, configure and test connection
NCP (Network Control Protocol) support different network layer protocols
RFC 1548 specifies encapsulation method
RFC 1322 NCP

19. PPP frame format Each frame begins and ends with a flag 0x7E.
Followed by an address byte whose value is always 0xFF, and then a control byte, with a value of 0x03.
To solve flag 0x7E in information field:
On a synchronous link is done by the hardware bit stuffing.
On asynchronous links the 0x7D is used
up to 1500 bytes
Flag 7E
Addr FF
Control 03
Protocol
LCP
FCS
Protocol
0021
IP datagrams
Protocol
C021
Link Control Data
Protocol
8021
Network Control Data

20. PPP framing Like ISO HDLC standard and use 0x7D as Escape Character
Replace 0x7E with 0x7D, 0x5E
Replace 0x7D with 0x7D, 0x5D
A byte value less than 0x20 e.g. 0x10 is transmitted with 0x7D, 0x21 7E 7D 01 1 1 1
ESC
ESC
ESC 7D 5E 7D 5D 7D 21 1 1 1 1 1 1

21. PPP advantage Support multiple protocols CRC for every frame
Dynamic negotiation of the IP address for each end
Link control protocol for negotiating data-link options

22. WAN Frame Format Summary
Link Control Protocol (LCP)
Code
identifier
Length
Data
PPP
Flag
Address
Control
Protocol
LCP
FCS
Cisco HDLC
Flag
Address
Control
Proprietary
Data
FCS
SDLC, LAPB, and Frame Relay
The graphic defines the data link frame formats for the services based on dedicate facilities.
The frame formats for LAPB and SDLC are very similar. With SDLC the address field always contains the address of the secondary station involved in the current communication. Because the primary station is always either the source or the destination of a communication, there is no need to include the primary station’s address in the frame.
Because PPP and other serial protocols were derived from SDLC, they have inherited some attributes that are not typically used. For example, the address field is assigned individual station addresses. PPP extends the basic SDLC frame by incorporating a protocol field. The protocol field identifies the protocol encapsulated in the information field of the frame.
The Link Control Protocol (LCP) that is used by the PPP, provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. LCP goes through several phases of operation: link establishment and configuration negotiation link quality determination network layer protocol configuration negotiation and link termination. LCP serves much the same function as the LLC in the LAN protocols.
The Cisco HDLC frame uses a proprietary type field that acts as a protocol field This makes it possible for multiple network layer protocols to share the same serial link.
LAPB uses a special coding of the address field to indicate whether the frame carries a command or a response. The control field provides further qualifications for command and response frames, and also indicates the frame format frame function, and the send/receive sequence number.
Flag
Address
Control
Data
FCS

23. Loopback Interface
channel for client and server on the same host use to communicate
Class A network ID 127 is reserved for the loopback interface
Most system assigns with the name localhost
Loopback interface appears as another link layer to the network layer

24. Summary
The physical layer provides access to the wires of an internetwork
The data link layer provides support for communications over several types of data links:
LAN (Ethernet/IEEE 802.3, Token Ring/IEEE 8025, FDDI)
Dedicated WAN (SDLC, HDLC, PPP, LAPB)
Switched WAN (X.25, Frame Relay, ISDN)